[PATCH 0/4] qemu_passt: Don't let passt fork off

[PATCH 0/4] qemu_passt: Don't let passt fork off

[Michal Privoznik]

Here are some cleanup, as promised here:

https://listman.redhat.com/archives/libvir-list/2023-February/237721.html

Now, there are still some patches missing.

Firstly, we still don't really capture error from passt. My suggestion
was to wait for socket to show up with errfd open. But active wait is
viewed as undesirable [1].

Secondly, Laine reported SELinux issues. Yeah, I don't see us setting
SELinux label on nor socket that passt and QEMU talk to each other, nor
on the log file. Speaking of which - we usually have per domain (or per
helper daemon instance even) log file, while for passt we have a global
one (/var/log/passt.log). I don't think that will fly if two or more
SELinux enabled domains want to use passt.

Thirdly, Stefano suggested a graceful shutdown for passt: have libvirt
connect to the socket and close it. Since we pass --one-off, this should
singal passt to exit. But I haven't implemented that because it's
redundant. We can't rely on passt quitting itself and thus use the big
gun (virPidFileForceCleanupPath()) at which point, the socket way is
just an optimization.

I might look into the first two, at some point. But not today.

1: https://listman.redhat.com/archives/libvir-list/2023-February/237663.html

Michal Prívozník (4):
  Revert "qemu: allow passt to self-daemonize"
  qemu_extdevice: Make qemuExtDevicesHasDevice() check def->nets
  qemu_passt: Report error when getting passt PID failed
  qemu_passt: Don't let passt fork off

 src/qemu/qemu_extdevice.c | 11 +++++++++++
 src/qemu/qemu_passt.c     | 15 ++++++++++-----
 2 files changed, 21 insertions(+), 5 deletions(-)

-- 
2.39.1

[PATCH 1/4] Revert "qemu: allow passt to self-daemonize"

[Michal Privoznik]

This reverts commit 0c4e716835eaf2a575bd063fde074c0fc7c4e4d4.

This patch was pushed by my mistake. Even though it got ACKed on
the list, I've raised couple of issues with it. They will be
fixed in next commits.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
---
 src/qemu/qemu_passt.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/src/qemu/qemu_passt.c b/src/qemu/qemu_passt.c
index f640a69c00..0f09bf3db8 100644
--- a/src/qemu/qemu_passt.c
+++ b/src/qemu/qemu_passt.c
@@ -141,23 +141,24 @@ qemuPasstStart(virDomainObj *vm,
     g_autofree char *passtSocketName = qemuPasstCreateSocketPath(vm, net);
     g_autoptr(virCommand) cmd = NULL;
     g_autofree char *pidfile = qemuPasstCreatePidFilename(vm, net);
-    g_autofree char *errbuf = NULL;
     char macaddr[VIR_MAC_STRING_BUFLEN];
     size_t i;
     pid_t pid = (pid_t) -1;
     int exitstatus = 0;
     int cmdret = 0;
+    VIR_AUTOCLOSE errfd = -1;
 
     cmd = virCommandNew(PASST);
 
     virCommandClearCaps(cmd);
-    virCommandSetErrorBuffer(cmd, &errbuf);
+    virCommandSetPidFile(cmd, pidfile);
+    virCommandSetErrorFD(cmd, &errfd);
+    virCommandDaemonize(cmd);
 
     virCommandAddArgList(cmd,
                          "--one-off",
                          "--socket", passtSocketName,
                          "--mac-addr", virMacAddrFormat(&net->mac, macaddr),
-                         "--pid", pidfile,
                          NULL);
 
     if (net->mtu) {
@@ -263,7 +264,7 @@ qemuPasstStart(virDomainObj *vm,
 
     if (cmdret < 0 || exitstatus != 0) {
         virReportError(VIR_ERR_INTERNAL_ERROR,
-                       _("Could not start 'passt': %s"), errbuf);
+                       _("Could not start 'passt'. exitstatus: %d"), exitstatus);
         goto error;
     }
 
-- 
@@ -141,23 +141,24 @@ qemuPasstStart(virDomainObj *vm,
     g_autofree char *passtSocketName = qemuPasstCreateSocketPath(vm, net);
     g_autoptr(virCommand) cmd = NULL;
     g_autofree char *pidfile = qemuPasstCreatePidFilename(vm, net);
-    g_autofree char *errbuf = NULL;
     char macaddr[VIR_MAC_STRING_BUFLEN];
     size_t i;
     pid_t pid = (pid_t) -1;
     int exitstatus = 0;
     int cmdret = 0;
+    VIR_AUTOCLOSE errfd = -1;
 
     cmd = virCommandNew(PASST);
 
     virCommandClearCaps(cmd);
-    virCommandSetErrorBuffer(cmd, &errbuf);
+    virCommandSetPidFile(cmd, pidfile);
+    virCommandSetErrorFD(cmd, &errfd);
+    virCommandDaemonize(cmd);
 
     virCommandAddArgList(cmd,
                          "--one-off",
                          "--socket", passtSocketName,
                          "--mac-addr", virMacAddrFormat(&net->mac, macaddr),
-                         "--pid", pidfile,
                          NULL);
 
     if (net->mtu) {
@@ -263,7 +264,7 @@ qemuPasstStart(virDomainObj *vm,
 
     if (cmdret < 0 || exitstatus != 0) {
         virReportError(VIR_ERR_INTERNAL_ERROR,
-                       _("Could not start 'passt': %s"), errbuf);
+                       _("Could not start 'passt'. exitstatus: %d"), exitstatus);
         goto error;
     }
 
-- 
2.39.1

Re: [PATCH 1/4] Revert "qemu: allow passt to self-daemonize"

[Laine Stump]

On 2/14/23 6:51 AM, Michal Privoznik wrote:
> This reverts commit 0c4e716835eaf2a575bd063fde074c0fc7c4e4d4.
> 
> This patch was pushed by my mistake. Even though it got ACKed on
> the list, I've raised couple of issues with it. They will be
> fixed in next commits.

I admire your optimism :-)

> 
> Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

Reviewed-by: Laine Stump <laine@redhat.com>

> ---
>   src/qemu/qemu_passt.c | 9 +++++----
>   1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/src/qemu/qemu_passt.c b/src/qemu/qemu_passt.c
> index f640a69c00..0f09bf3db8 100644
> --- a/src/qemu/qemu_passt.c
> +++ b/src/qemu/qemu_passt.c
> @@ -141,23 +141,24 @@ qemuPasstStart(virDomainObj *vm,
>       g_autofree char *passtSocketName = qemuPasstCreateSocketPath(vm, net);
>       g_autoptr(virCommand) cmd = NULL;
>       g_autofree char *pidfile = qemuPasstCreatePidFilename(vm, net);
> -    g_autofree char *errbuf = NULL;
>       char macaddr[VIR_MAC_STRING_BUFLEN];
>       size_t i;
>       pid_t pid = (pid_t) -1;
>       int exitstatus = 0;
>       int cmdret = 0;
> +    VIR_AUTOCLOSE errfd = -1;
>   
>       cmd = virCommandNew(PASST);
>   
>       virCommandClearCaps(cmd);
> -    virCommandSetErrorBuffer(cmd, &errbuf);
> +    virCommandSetPidFile(cmd, pidfile);
> +    virCommandSetErrorFD(cmd, &errfd);
> +    virCommandDaemonize(cmd);
>   
>       virCommandAddArgList(cmd,
>                            "--one-off",
>                            "--socket", passtSocketName,
>                            "--mac-addr", virMacAddrFormat(&net->mac, macaddr),
> -                         "--pid", pidfile,
>                            NULL);
>   
>       if (net->mtu) {
> @@ -263,7 +264,7 @@ qemuPasstStart(virDomainObj *vm,
>   
>       if (cmdret < 0 || exitstatus != 0) {
>           virReportError(VIR_ERR_INTERNAL_ERROR,
> -                       _("Could not start 'passt': %s"), errbuf);
> +                       _("Could not start 'passt'. exitstatus: %d"), exitstatus);
>           goto error;
>       }
>   

[PATCH 2/4] qemu_extdevice: Make qemuExtDevicesHasDevice() check def->nets

[Michal Privoznik]

We can have external helper processes running for domain
<interface/> too (e.g. slirp or passt). But this is not reflected
in qemuExtDevicesHasDevice() which simply ignores these.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
---
 src/qemu/qemu_extdevice.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/src/qemu/qemu_extdevice.c b/src/qemu/qemu_extdevice.c
index fdefe59215..47e97f3565 100644
--- a/src/qemu/qemu_extdevice.c
+++ b/src/qemu/qemu_extdevice.c
@@ -296,6 +296,17 @@ qemuExtDevicesHasDevice(virDomainDef *def)
             return true;
     }
 
+    for (i = 0; i < def->nnets; i++) {
+        virDomainNetDef *net = def->nets[i];
+
+        if (QEMU_DOMAIN_NETWORK_PRIVATE(net)->slirp)
+            return true;
+
+        if (net->type == VIR_DOMAIN_NET_TYPE_USER &&
+            net->backend.type == VIR_DOMAIN_NET_BACKEND_PASST)
+            return true;
+    }
+
     for (i = 0; i < def->ntpms; i++) {
         if (def->tpms[i]->type == VIR_DOMAIN_TPM_TYPE_EMULATOR)
             return true;
-- 
@@ -296,6 +296,17 @@ qemuExtDevicesHasDevice(virDomainDef *def)
             return true;
     }
 
+    for (i = 0; i < def->nnets; i++) {
+        virDomainNetDef *net = def->nets[i];
+
+        if (QEMU_DOMAIN_NETWORK_PRIVATE(net)->slirp)
+            return true;
+
+        if (net->type == VIR_DOMAIN_NET_TYPE_USER &&
+            net->backend.type == VIR_DOMAIN_NET_BACKEND_PASST)
+            return true;
+    }
+
     for (i = 0; i < def->ntpms; i++) {
         if (def->tpms[i]->type == VIR_DOMAIN_TPM_TYPE_EMULATOR)
             return true;
-- 
2.39.1

Re: [PATCH 2/4] qemu_extdevice: Make qemuExtDevicesHasDevice() check def->nets

[Laine Stump]

On 2/14/23 6:51 AM, Michal Privoznik wrote:
> We can have external helper processes running for domain
> <interface/> too (e.g. slirp or passt). But this is not reflected
> in qemuExtDevicesHasDevice() which simply ignores these.

The slirp-helper patches missed adding the check in this (oddly-named) 
function (even while adding in the correct hunk to 
qemuExtDevicesSetupCroup()) probably because it wasn't really obvious 
without reading/interpreting/understanding all the code in two separate 
files that it was needed; my passt patches missed adding the check in 
this function because I was following the pattern of what was done for 
slirp, and slirp hadn't touched this function (nor had it touched the 
function that calls both of these functions, 
qemuSetupCgroupForExtDevices(), which is in another file). It's 
reasonable to think that some future person may also not notice 
qemuExtDevicesHasDevice(), and believe that they only need to modify 
qemuExtDevicesSetupCgroup().

Anyway, my point is that I think this could be avoided by adding a 
comment in qemuExtDevicesSetupCgroup() that points out it is only called 
if qemuExtDevicesHasDevice() returns true, and so any addition to 
qemuExtDevicesSetupCgroup() should have a corresponding addition to 
qemuExtDevicesHasDevice(). It's too late at night / early in the morning 
for my brain to compose a concise sentence to this effect, but it would 
make me happy if you added one before pushing.

> 
> Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

Reviewed-by: Laine Stump <laine@redhat.com>

> ---
>   src/qemu/qemu_extdevice.c | 11 +++++++++++
>   1 file changed, 11 insertions(+)
> 
> diff --git a/src/qemu/qemu_extdevice.c b/src/qemu/qemu_extdevice.c
> index fdefe59215..47e97f3565 100644
> --- a/src/qemu/qemu_extdevice.c
> +++ b/src/qemu/qemu_extdevice.c
> @@ -296,6 +296,17 @@ qemuExtDevicesHasDevice(virDomainDef *def)
>               return true;
>       }
>   
> +    for (i = 0; i < def->nnets; i++) {
> +        virDomainNetDef *net = def->nets[i];
> +
> +        if (QEMU_DOMAIN_NETWORK_PRIVATE(net)->slirp)
> +            return true;
> +
> +        if (net->type == VIR_DOMAIN_NET_TYPE_USER &&
> +            net->backend.type == VIR_DOMAIN_NET_BACKEND_PASST)
> +            return true;
> +    }
> +
>       for (i = 0; i < def->ntpms; i++) {
>           if (def->tpms[i]->type == VIR_DOMAIN_TPM_TYPE_EMULATOR)
>               return true;

Re: [PATCH 2/4] qemu_extdevice: Make qemuExtDevicesHasDevice() check def->nets

[Michal Prívozník]

On 2/15/23 08:22, Laine Stump wrote:
> On 2/14/23 6:51 AM, Michal Privoznik wrote:
>> We can have external helper processes running for domain
>> <interface/> too (e.g. slirp or passt). But this is not reflected
>> in qemuExtDevicesHasDevice() which simply ignores these.
> 
> The slirp-helper patches missed adding the check in this (oddly-named)
> function (even while adding in the correct hunk to
> qemuExtDevicesSetupCroup()) probably because it wasn't really obvious
> without reading/interpreting/understanding all the code in two separate
> files that it was needed; my passt patches missed adding the check in
> this function because I was following the pattern of what was done for
> slirp, and slirp hadn't touched this function (nor had it touched the
> function that calls both of these functions,
> qemuSetupCgroupForExtDevices(), which is in another file). It's
> reasonable to think that some future person may also not notice
> qemuExtDevicesHasDevice(), and believe that they only need to modify
> qemuExtDevicesSetupCgroup().
> 
> Anyway, my point is that I think this could be avoided by adding a
> comment in qemuExtDevicesSetupCgroup() that points out it is only called
> if qemuExtDevicesHasDevice() returns true, and so any addition to
> qemuExtDevicesSetupCgroup() should have a corresponding addition to
> qemuExtDevicesHasDevice(). It's too late at night / early in the morning
> for my brain to compose a concise sentence to this effect, but it would
> make me happy if you added one before pushing.

Sounds good, but I'd rather do that in a follow up patch, as it's a
different semantic change than this commit.


Done here:

https://listman.redhat.com/archives/libvir-list/2023-February/237863.html

> 
>>
>> Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
> 
> Reviewed-by: Laine Stump <laine@redhat.com>

Michal

[PATCH 3/4] qemu_passt: Report error when getting passt PID failed

[Michal Privoznik]

If qemuPasstGetPid() fails, or the passt's PID is -1 then
qemuPasstSetupCgroup() returns early without any error message
set. Report an appropriate error.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
---
 src/qemu/qemu_passt.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/qemu/qemu_passt.c b/src/qemu/qemu_passt.c
index 0f09bf3db8..78830fdc26 100644
--- a/src/qemu/qemu_passt.c
+++ b/src/qemu/qemu_passt.c
@@ -125,8 +125,11 @@ qemuPasstSetupCgroup(virDomainObj *vm,
 {
     pid_t pid = (pid_t) -1;
 
-    if (qemuPasstGetPid(vm, net, &pid) < 0 || pid <= 0)
+    if (qemuPasstGetPid(vm, net, &pid) < 0 || pid <= 0) {
+        virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
+                       _("Could not get process ID of passt"));
         return -1;
+    }
 
     return virCgroupAddProcess(cgroup, pid);
 }
-- 
@@ -125,8 +125,11 @@ qemuPasstSetupCgroup(virDomainObj *vm,
 {
     pid_t pid = (pid_t) -1;
 
-    if (qemuPasstGetPid(vm, net, &pid) < 0 || pid <= 0)
+    if (qemuPasstGetPid(vm, net, &pid) < 0 || pid <= 0) {
+        virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
+                       _("Could not get process ID of passt"));
         return -1;
+    }
 
     return virCgroupAddProcess(cgroup, pid);
 }
-- 
2.39.1

Re: [PATCH 3/4] qemu_passt: Report error when getting passt PID failed

[Laine Stump]

On 2/14/23 6:51 AM, Michal Privoznik wrote:
> If qemuPasstGetPid() fails, or the passt's PID is -1 then
> qemuPasstSetupCgroup() returns early without any error message
> set. Report an appropriate error.
> 
> Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

Reviewed-by: Laine Stump <laine@redhat.com>

> ---
>   src/qemu/qemu_passt.c | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/src/qemu/qemu_passt.c b/src/qemu/qemu_passt.c
> index 0f09bf3db8..78830fdc26 100644
> --- a/src/qemu/qemu_passt.c
> +++ b/src/qemu/qemu_passt.c
> @@ -125,8 +125,11 @@ qemuPasstSetupCgroup(virDomainObj *vm,
>   {
>       pid_t pid = (pid_t) -1;
>   
> -    if (qemuPasstGetPid(vm, net, &pid) < 0 || pid <= 0)
> +    if (qemuPasstGetPid(vm, net, &pid) < 0 || pid <= 0) {
> +        virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
> +                       _("Could not get process ID of passt"));
>           return -1;
> +    }
>   
>       return virCgroupAddProcess(cgroup, pid);
>   }

[PATCH 4/4] qemu_passt: Don't let passt fork off

[Michal Privoznik]

When passt starts it tries to do some security measures to
restrict itself. For instance, it creates its own namespaces,
umounts basically everything, drops capabilities, forks off to
further restrict itself (the child is where all interesting work
takes place now). This is sound, except it's causing two
problems:

1) the PID file FD, which we leak into the passt process, gets
   closed (and thus our virPidFile*() helpers see unlocked PID
   file, which makes them think the process is gone),

2) the PID file no longer reflects true PID of the process.

Worse, the child calls setsid() so we can't even kill the whole
process group. I mean, we can but it won't be any good.

Fortunately, passt has '--foreground' argument, which causes it
to undergo the same security measures but without forking off the
child. This in turn means, that the PID file FD won't get closed
and the PID file itself contains the correct PID.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
---
 src/qemu/qemu_passt.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/qemu/qemu_passt.c b/src/qemu/qemu_passt.c
index 78830fdc26..441cfe87e8 100644
--- a/src/qemu/qemu_passt.c
+++ b/src/qemu/qemu_passt.c
@@ -159,6 +159,7 @@ qemuPasstStart(virDomainObj *vm,
     virCommandDaemonize(cmd);
 
     virCommandAddArgList(cmd,
+                         "--foreground",
                          "--one-off",
                          "--socket", passtSocketName,
                          "--mac-addr", virMacAddrFormat(&net->mac, macaddr),
-- 
@@ -159,6 +159,7 @@ qemuPasstStart(virDomainObj *vm,
     virCommandDaemonize(cmd);
 
     virCommandAddArgList(cmd,
+                         "--foreground",
                          "--one-off",
                          "--socket", passtSocketName,
                          "--mac-addr", virMacAddrFormat(&net->mac, macaddr),
-- 
2.39.1

Re: [PATCH 4/4] qemu_passt: Don't let passt fork off

[Stefano Brivio]

On Tue, 14 Feb 2023 12:51:22 +0100
Michal Privoznik <mprivozn@redhat.com> wrote:

> When passt starts it tries to do some security measures to
> restrict itself. For instance, it creates its own namespaces,
> umounts basically everything, drops capabilities, forks off to
> further restrict itself (the child is where all interesting work
> takes place now). This is sound, except it's causing two
> problems:
> 
> 1) the PID file FD, which we leak into the passt process, gets
>    closed (and thus our virPidFile*() helpers see unlocked PID
>    file, which makes them think the process is gone),

I didn't realise this was the case, but giving passt write (unless I'm
missing something) access to a file created by libvirtd doesn't look
desirable to me.

> 2) the PID file no longer reflects true PID of the process.
> 
> Worse, the child calls setsid() so we can't even kill the whole
> process group. I mean, we can but it won't be any good.
> 
> Fortunately, passt has '--foreground' argument, which causes it
> to undergo the same security measures but without forking off the
> child.

They're not the same -- unfortunately they can't be, because, on Linux,
you can't change the PID of an existing process, so there's no way to
enter a new PID namespace without clone().

If passt remains in the same PID namespace, it's still able to see PIDs
of other processes, which is not desirable from a security perspective.

Again from a security perspective, this is probably a small impact, so
I guess it's fine if there's no other way around it. But I see a lot of
ways around it...

-- 
Stefano

Re: [PATCH 4/4] qemu_passt: Don't let passt fork off

[Michal Prívozník]

On 2/14/23 14:02, Stefano Brivio wrote:
> On Tue, 14 Feb 2023 12:51:22 +0100
> Michal Privoznik <mprivozn@redhat.com> wrote:
> 
>> When passt starts it tries to do some security measures to
>> restrict itself. For instance, it creates its own namespaces,
>> umounts basically everything, drops capabilities, forks off to
>> further restrict itself (the child is where all interesting work
>> takes place now). This is sound, except it's causing two
>> problems:
>>
>> 1) the PID file FD, which we leak into the passt process, gets
>>    closed (and thus our virPidFile*() helpers see unlocked PID
>>    file, which makes them think the process is gone),
> 
> I didn't realise this was the case, but giving passt write (unless I'm
> missing something) access to a file created by libvirtd doesn't look
> desirable to me.

That's not what's happening here. In fact, the opposite. We let libvirt
create the file, lock and write it. Then the file's FD is leaked into
passt process. This way, we can learn whether the PID file is still
valid: if we just try to lock it and:

a) fail, then the file is still locked, i.e. passt is still running,
b) succeed, then the file is no longer locket by passt, i.e. it's no
longer running.

Thing is, passt doesn't even know about the FD (unless it starts closing
FDs "randomly", e.g. via closefrom(3) or an alternative). This is the
reason we can't use passt's --pid, because it writes the PID file
without any locking dance. Therefore, libvirt can't know whether the PID
file is still valid. If it did just read the file and killed PID from
there it might kill a random process that was unfortunate to be assigned
the same PID.

BTW, there are two other issues with PID files in passt:
1) no locking means, that the file is very easily overwritten:

 $ passt --pid /tmp/passt.pid 2>/dev/null & \
   passt --pid /tmp/passt.pid 2>/dev/null; \
   pgrep passt ; cat /tmp/passt.pid
[1] 21029
[1]+  Done                    passt --pid /tmp/passt.pid 2> /dev/null
21034
21035
21035

2) with --daemonize, only the PID of the parent process is written which
is pretty much useless, as the parent quits shortly after clone().

Now, libvirt's virPidFile* machinery (src/util/virpidfile.c) ensures
that case 1) won't happen (yes, we have internal APIs that read pid
files unlocked, but those are not used from this code we're talking about).

And this patch tries to fix the case 2) by instructing passt to not
clone(). If it means that it can't use PID namespace, then so be it.
It's better to not let processes behind.

> 
>> 2) the PID file no longer reflects true PID of the process.
>>
>> Worse, the child calls setsid() so we can't even kill the whole
>> process group. I mean, we can but it won't be any good.
>>
>> Fortunately, passt has '--foreground' argument, which causes it
>> to undergo the same security measures but without forking off the
>> child.
> 
> They're not the same -- unfortunately they can't be, because, on Linux,
> you can't change the PID of an existing process, so there's no way to
> enter a new PID namespace without clone().
> 
> If passt remains in the same PID namespace, it's still able to see PIDs
> of other processes, which is not desirable from a security perspective.

I understand that, but it's already confined in plenty other ways. It's
way worse to leave a process running than being able to see other PIDs.

> 
> Again from a security perspective, this is probably a small impact, so
> I guess it's fine if there's no other way around it. But I see a lot of
> ways around it...
> 

Is there? If we have a helper process that fork()-s then the only way we
can 'track' its PIDs (and kill them) is by placing them into a CGroup.
Until we do that we have to trust helper processes to behave properly.
But that's something I'd like to avoid.

Michal

Re: [PATCH 4/4] qemu_passt: Don't let passt fork off

[Stefano Brivio]

On Tue, 14 Feb 2023 16:30:17 +0100
Michal Prívozník <mprivozn@redhat.com> wrote:

> On 2/14/23 14:02, Stefano Brivio wrote:
> > On Tue, 14 Feb 2023 12:51:22 +0100
> > Michal Privoznik <mprivozn@redhat.com> wrote:
> >   
> >> When passt starts it tries to do some security measures to
> >> restrict itself. For instance, it creates its own namespaces,
> >> umounts basically everything, drops capabilities, forks off to
> >> further restrict itself (the child is where all interesting work
> >> takes place now). This is sound, except it's causing two
> >> problems:
> >>
> >> 1) the PID file FD, which we leak into the passt process, gets
> >>    closed (and thus our virPidFile*() helpers see unlocked PID
> >>    file, which makes them think the process is gone),  
> > 
> > I didn't realise this was the case, but giving passt write (unless I'm
> > missing something) access to a file created by libvirtd doesn't look
> > desirable to me.  
> 
> That's not what's happening here. In fact, the opposite. We let libvirt
> create the file, lock and write it.

Okay, but the outcome is exactly what I'm saying. You're using advisory
locking, what prevents passt from writing (a lot of bytes) to it? I
just followed the path from open() to fcntl(), maybe I'm missing
something.

> Then the file's FD is leaked into
> passt process. This way, we can learn whether the PID file is still
> valid: if we just try to lock it and:
> 
> a) fail, then the file is still locked, i.e. passt is still running,
> b) succeed, then the file is no longer locket by passt, i.e. it's no
> longer running.
> 
> Thing is, passt doesn't even know about the FD (unless it starts closing
> FDs "randomly", e.g. via closefrom(3) or an alternative).

But it can also use it for write(2), as long as the attacker knows
(quite obvious information) that libvirt is starting passt. The number
of that file descriptor is deterministic, too.

> This is the
> reason we can't use passt's --pid, because it writes the PID file
> without any locking dance. Therefore, libvirt can't know whether the PID
> file is still valid. If it did just read the file and killed PID from
> there it might kill a random process that was unfortunate to be assigned
> the same PID.

I don't think that's enough, passt could pass a copy of that file
descriptor to another process (via e.g. SCM_RIGHTS), which then locks
it. It's enough to cover the unfortunate random process, but not
something done intentionally.

That's the reason why pidfd_open(2) and CLONE_PIDFD were introduced.

> BTW, there are two other issues with PID files in passt:
> 1) no locking means, that the file is very easily overwritten:
> 
>  $ passt --pid /tmp/passt.pid 2>/dev/null & \
>    passt --pid /tmp/passt.pid 2>/dev/null; \
>    pgrep passt ; cat /tmp/passt.pid
> [1] 21029
> [1]+  Done                    passt --pid /tmp/passt.pid 2> /dev/null
> 21034
> 21035
> 21035

It can be overwritten even with advisory locking, and mandatory locking
isn't implemented in recent kernels. I'm not aware of any way to
prevent this (and I wouldn't want to give the illusion that this is a
"secure" mechanism, either).

> 2) with --daemonize, only the PID of the parent process is written which
> is pretty much useless, as the parent quits shortly after clone().

No, that's the PID of the child (of course!), see passt's __daemon()
(util.c). It's written by the parent, sure, because the child doesn't
know its own "real" PID.

> Now, libvirt's virPidFile* machinery (src/util/virpidfile.c) ensures
> that case 1) won't happen (yes, we have internal APIs that read pid
> files unlocked, but those are not used from this code we're talking about).
> 
> And this patch tries to fix the case 2) by instructing passt to not
> clone(). If it means that it can't use PID namespace, then so be it.
> It's better to not let processes behind.

Even though 2) is nothing you need to fix, I'd still suggest to *not*
use that option *for this purpose*, because it's prone to races.

> >> 2) the PID file no longer reflects true PID of the process.
> >>
> >> Worse, the child calls setsid() so we can't even kill the whole
> >> process group. I mean, we can but it won't be any good.
> >>
> >> Fortunately, passt has '--foreground' argument, which causes it
> >> to undergo the same security measures but without forking off the
> >> child.  
> > 
> > They're not the same -- unfortunately they can't be, because, on Linux,
> > you can't change the PID of an existing process, so there's no way to
> > enter a new PID namespace without clone().
> > 
> > If passt remains in the same PID namespace, it's still able to see PIDs
> > of other processes, which is not desirable from a security perspective.  
> 
> I understand that, but it's already confined in plenty other ways. It's
> way worse to leave a process running than being able to see other PIDs.

Not if somebody manages to find a creative way to attack other
processes by arbitrary code execution in passt. In any case, there's no
need to leave any process running.

> > Again from a security perspective, this is probably a small impact, so
> > I guess it's fine if there's no other way around it. But I see a lot of
> > ways around it...
> 
> Is there?

Yes, see my email on the other thread, archived at:

  https://listman.redhat.com/archives/libvir-list/2023-February/237741.html

That is: just connect() and close() the socket if qemu doesn't do it.
That's not more or less reliable than expecting passt to do the right
thing when you give --foreground.

> If we have a helper process that fork()-s then the only way we
> can 'track' its PIDs (and kill them) is by placing them into a CGroup.
> Until we do that we have to trust helper processes to behave properly.
> But that's something I'd like to avoid.

By the way, you need in any case to rely on --one-off as qemu
terminates, because passt should run even without libvirtd running, as
long as the guest is connected (for details about this reasoning, see
https://bugzilla.redhat.com/show_bug.cgi?id=1597326).

-- 
Stefano

Re: [PATCH 4/4] qemu_passt: Don't let passt fork off

[Laine Stump]

On 2/14/23 8:02 AM, Stefano Brivio wrote:
> On Tue, 14 Feb 2023 12:51:22 +0100
> Michal Privoznik <mprivozn@redhat.com> wrote:
> 
>> When passt starts it tries to do some security measures to
>> restrict itself. For instance, it creates its own namespaces,
>> umounts basically everything, drops capabilities, forks off to
>> further restrict itself (the child is where all interesting work
>> takes place now). This is sound, except it's causing two
>> problems:
>>
>> 1) the PID file FD, which we leak into the passt process, gets
>>     closed (and thus our virPidFile*() helpers see unlocked PID
>>     file, which makes them think the process is gone),
> 
> I didn't realise this was the case, but giving passt write (unless I'm
> missing something) access to a file created by libvirtd doesn't look
> desirable to me.

> 
>> 2) the PID file no longer reflects true PID of the process.
>>
>> Worse, the child calls setsid() so we can't even kill the whole
>> process group. I mean, we can but it won't be any good.

I think that (incorrect PID in the pidfile) is  happening because Michal 
is using the original version of my patches that were pushed - I had 
mimicked the behavior of slirp, where libvirt deamonizes the new 
process. If that process then daemonizes itself, we have some sort of 
"double daemon"; libvirt has saved off the pid of what it thinks is 
going to be the final process, but then that process further forks and 
exits from the process whose pid libvirt saved. But because passt was 
cleaning up after itself I hadn't noticed the discrepancy in pids when 
testing.

Without going into all the details of the pidfile and locking and etc, I 
just want to say that if we can fork/exec dnsmasq and let it daemonize 
itself and create its own pidfile, then certainly we can do the same 
thing for passt. (and if there's a fundamental problem, then it's a 
fundamental problem for dnsmasq as well).

>>
>> Fortunately, passt has '--foreground' argument, which causes it
>> to undergo the same security measures but without forking off the
>> child.

But if we do --foreground in combination with calling 
virCommandDaemonize(), then won't we still have the problem that libvirt 
won't know whether or not passt has failed to start (not unless we want 
to put in a bunch of gorp to continue grabbing stderr while watching to 
see if the passt process has exited, etc. It's going to take some 
convincing for me to think we should run passt with --foreground rather 
than letting it daemonize itself.

> 
> They're not the same -- unfortunately they can't be, because, on Linux,
> you can't change the PID of an existing process, so there's no way to
> enter a new PID namespace without clone().
> 
> If passt remains in the same PID namespace, it's still able to see PIDs
> of other processes, which is not desirable from a security perspective.
> 
> Again from a security perspective, this is probably a small impact, so
> I guess it's fine if there's no other way around it. But I see a lot of
> ways around it...
> 

Re: [PATCH 4/4] qemu_passt: Don't let passt fork off

[Michal Prívozník]

On 2/15/23 08:50, Laine Stump wrote:
> On 2/14/23 8:02 AM, Stefano Brivio wrote:
>> On Tue, 14 Feb 2023 12:51:22 +0100
>> Michal Privoznik <mprivozn@redhat.com> wrote:
>>
>>> When passt starts it tries to do some security measures to
>>> restrict itself. For instance, it creates its own namespaces,
>>> umounts basically everything, drops capabilities, forks off to
>>> further restrict itself (the child is where all interesting work
>>> takes place now). This is sound, except it's causing two
>>> problems:
>>>
>>> 1) the PID file FD, which we leak into the passt process, gets
>>>     closed (and thus our virPidFile*() helpers see unlocked PID
>>>     file, which makes them think the process is gone),
>>
>> I didn't realise this was the case, but giving passt write (unless I'm
>> missing something) access to a file created by libvirtd doesn't look
>> desirable to me.
> 
>>
>>> 2) the PID file no longer reflects true PID of the process.
>>>
>>> Worse, the child calls setsid() so we can't even kill the whole
>>> process group. I mean, we can but it won't be any good.
> 
> I think that (incorrect PID in the pidfile) is  happening because Michal
> is using the original version of my patches that were pushed - I had
> mimicked the behavior of slirp, where libvirt deamonizes the new
> process. If that process then daemonizes itself, we have some sort of
> "double daemon"; libvirt has saved off the pid of what it thinks is
> going to be the final process, but then that process further forks and
> exits from the process whose pid libvirt saved. But because passt was
> cleaning up after itself I hadn't noticed the discrepancy in pids when
> testing.
> 
> Without going into all the details of the pidfile and locking and etc, I
> just want to say that if we can fork/exec dnsmasq and let it daemonize
> itself and create its own pidfile, then certainly we can do the same
> thing for passt. (and if there's a fundamental problem, then it's a
> fundamental problem for dnsmasq as well).

Alright. I think I have a solution that would please everybody involved.
I'll post it tomorrow though. I need to test it thoroughly. We would be
able to get passt's PID (which is needed not only for killing it, but
also for CGroup placement), NOT use --foreground and still pass errors
from it to users (that is unless logfile was specified, because
unfortunately, --log-file and --stderr are mutually exclusive).

Michal

Re: [PATCH 4/4] qemu_passt: Don't let passt fork off

[Laine Stump]

On 2/15/23 12:04 PM, Michal Prívozník wrote:
> On 2/15/23 08:50, Laine Stump wrote:
>> On 2/14/23 8:02 AM, Stefano Brivio wrote:
>>> On Tue, 14 Feb 2023 12:51:22 +0100
>>> Michal Privoznik <mprivozn@redhat.com> wrote:
>>>
>>>> When passt starts it tries to do some security measures to
>>>> restrict itself. For instance, it creates its own namespaces,
>>>> umounts basically everything, drops capabilities, forks off to
>>>> further restrict itself (the child is where all interesting work
>>>> takes place now). This is sound, except it's causing two
>>>> problems:
>>>>
>>>> 1) the PID file FD, which we leak into the passt process, gets
>>>>      closed (and thus our virPidFile*() helpers see unlocked PID
>>>>      file, which makes them think the process is gone),
>>>
>>> I didn't realise this was the case, but giving passt write (unless I'm
>>> missing something) access to a file created by libvirtd doesn't look
>>> desirable to me.
>>
>>>
>>>> 2) the PID file no longer reflects true PID of the process.
>>>>
>>>> Worse, the child calls setsid() so we can't even kill the whole
>>>> process group. I mean, we can but it won't be any good.
>>
>> I think that (incorrect PID in the pidfile) is  happening because Michal
>> is using the original version of my patches that were pushed - I had
>> mimicked the behavior of slirp, where libvirt deamonizes the new
>> process. If that process then daemonizes itself, we have some sort of
>> "double daemon"; libvirt has saved off the pid of what it thinks is
>> going to be the final process, but then that process further forks and
>> exits from the process whose pid libvirt saved. But because passt was
>> cleaning up after itself I hadn't noticed the discrepancy in pids when
>> testing.
>>
>> Without going into all the details of the pidfile and locking and etc, I
>> just want to say that if we can fork/exec dnsmasq and let it daemonize
>> itself and create its own pidfile, then certainly we can do the same
>> thing for passt. (and if there's a fundamental problem, then it's a
>> fundamental problem for dnsmasq as well).
> 
> Alright. I think I have a solution that would please everybody involved.
> I'll post it tomorrow though. I need to test it thoroughly. We would be
> able to get passt's PID (which is needed not only for killing it, but
> also for CGroup placement), NOT use --foreground and still pass errors
> from it to users (that is unless logfile was specified, because
> unfortunately, --log-file and --stderr are mutually exclusive).

This last item is fixed by patches I have pending to passt itself.

Re: [PATCH 4/4] qemu_passt: Don't let passt fork off

[Stefano Brivio]

On Wed, 15 Feb 2023 18:04:56 +0100
Michal Prívozník <mprivozn@redhat.com> wrote:

> On 2/15/23 08:50, Laine Stump wrote:
> > On 2/14/23 8:02 AM, Stefano Brivio wrote:  
> >> On Tue, 14 Feb 2023 12:51:22 +0100
> >> Michal Privoznik <mprivozn@redhat.com> wrote:
> >>  
> >>> When passt starts it tries to do some security measures to
> >>> restrict itself. For instance, it creates its own namespaces,
> >>> umounts basically everything, drops capabilities, forks off to
> >>> further restrict itself (the child is where all interesting work
> >>> takes place now). This is sound, except it's causing two
> >>> problems:
> >>>
> >>> 1) the PID file FD, which we leak into the passt process, gets
> >>>     closed (and thus our virPidFile*() helpers see unlocked PID
> >>>     file, which makes them think the process is gone),  
> >>
> >> I didn't realise this was the case, but giving passt write (unless I'm
> >> missing something) access to a file created by libvirtd doesn't look
> >> desirable to me.  
> >   
> >>  
> >>> 2) the PID file no longer reflects true PID of the process.
> >>>
> >>> Worse, the child calls setsid() so we can't even kill the whole
> >>> process group. I mean, we can but it won't be any good.  
> > 
> > I think that (incorrect PID in the pidfile) is  happening because Michal
> > is using the original version of my patches that were pushed - I had
> > mimicked the behavior of slirp, where libvirt deamonizes the new
> > process. If that process then daemonizes itself, we have some sort of
> > "double daemon"; libvirt has saved off the pid of what it thinks is
> > going to be the final process, but then that process further forks and
> > exits from the process whose pid libvirt saved. But because passt was
> > cleaning up after itself I hadn't noticed the discrepancy in pids when
> > testing.
> > 
> > Without going into all the details of the pidfile and locking and etc, I
> > just want to say that if we can fork/exec dnsmasq and let it daemonize
> > itself and create its own pidfile, then certainly we can do the same
> > thing for passt. (and if there's a fundamental problem, then it's a
> > fundamental problem for dnsmasq as well).  
> 
> Alright. I think I have a solution that would please everybody involved.
> I'll post it tomorrow though. I need to test it thoroughly. We would be
> able to get passt's PID (which is needed not only for killing it, but
> also for CGroup placement), NOT use --foreground and still pass errors
> from it to users (that is unless logfile was specified, because
> unfortunately, --log-file and --stderr are mutually exclusive).

That doesn't need to be the case (--log-file and --stderr being
mutually exclusive)... if you have a use case for it, let's change that
in passt. I just wanted to keep it simple for users ("give a log file,
and be sure it won't spam").

Also mind that Laine's series:
  https://archives.passt.top/passt-dev/20230215082437.110151-1-laine@redhat.com/

*should* already cover all the cases where libvirt is interested in
relaying "early" errors back to the user.

By the way, the one below is pretty much the patch I would have proposed
for libvirt. I prepared it earlier today and didn't have a chance to
test it yet, it's compile-tested only, and doesn't take cgroups into
account (which, it seems, is needed no matter the lifecycle).

So I'm sharing it here as reference (that's how simple I wanted it to
be -- minus cgroups), or if it's convenient for you to copy and paste
something.

---

diff --git a/src/qemu/qemu_extdevice.c b/src/qemu/qemu_extdevice.c
index fdefe59215..23d25c134a 100644
--- a/src/qemu/qemu_extdevice.c
+++ b/src/qemu/qemu_extdevice.c
@@ -337,12 +337,6 @@ qemuExtDevicesSetupCgroup(virQEMUDriver *driver,
 
         if (slirp && qemuSlirpSetupCgroup(slirp, cgroup) < 0)
             return -1;
-
-        if (net->type == VIR_DOMAIN_NET_TYPE_USER &&
-            net->backend.type == VIR_DOMAIN_NET_BACKEND_PASST &&
-            qemuPasstSetupCgroup(vm, net, cgroup) < 0) {
-            return -1;
-        }
     }
 
     for (i = 0; i < def->ntpms; i++) {
diff --git a/src/qemu/qemu_passt.c b/src/qemu/qemu_passt.c
index f640a69c00..2327b3e25e 100644
--- a/src/qemu/qemu_passt.c
+++ b/src/qemu/qemu_passt.c
@@ -28,29 +28,12 @@
 #include "virerror.h"
 #include "virjson.h"
 #include "virlog.h"
-#include "virpidfile.h"
 
 #define VIR_FROM_THIS VIR_FROM_NONE
 
 VIR_LOG_INIT("qemu.passt");
 
 
-static char *
-qemuPasstCreatePidFilename(virDomainObj *vm,
-                           virDomainNetDef *net)
-{
-    qemuDomainObjPrivate *priv = vm->privateData;
-    virQEMUDriver *driver = priv->driver;
-    g_autofree char *shortName = virDomainDefGetShortName(vm->def);
-    g_autoptr(virQEMUDriverConfig) cfg = virQEMUDriverGetConfig(driver);
-    g_autofree char *name = NULL;
-
-    name = g_strdup_printf("%s-%s-passt", shortName, net->info.alias);
-
-    return virPidFileBuildPath(cfg->passtStateDir, name);
-}
-
-
 static char *
 qemuPasstCreateSocketPath(virDomainObj *vm,
                           virDomainNetDef *net)
@@ -65,17 +48,6 @@ qemuPasstCreateSocketPath(virDomainObj *vm,
 }
 
 
-static int
-qemuPasstGetPid(virDomainObj *vm,
-                virDomainNetDef *net,
-                pid_t *pid)
-{
-    g_autofree char *pidfile = qemuPasstCreatePidFilename(vm, net);
-
-    return virPidFileReadPathIfLocked(pidfile, pid);
-}
-
-
 int
 qemuPasstAddNetProps(virDomainObj *vm,
                      virDomainNetDef *net,
@@ -106,29 +78,32 @@ void
 qemuPasstStop(virDomainObj *vm,
               virDomainNetDef *net)
 {
-    g_autofree char *pidfile = qemuPasstCreatePidFilename(vm, net);
-    virErrorPtr orig_err;
-
-    virErrorPreserveLast(&orig_err);
-
-    if (virPidFileForceCleanupPath(pidfile) < 0)
-        VIR_WARN("Unable to kill passt process");
-
-    virErrorRestore(&orig_err);
-}
-
+    g_autofree char *passtSocketName = qemuPasstCreateSocketPath(vm, net);
+    struct sockaddr_un addr = { .sun_family = AF_UNIX };
+    int fd;
+
+    fd = socket(AF_UNIX, SOCK_STREAM, 0);
+    if (fd < 0) {
+        virReportError(errno,
+                       "%s", _("Unable to open socket to connect to passt"));
+        return;
+    }
 
-int
-qemuPasstSetupCgroup(virDomainObj *vm,
-                     virDomainNetDef *net,
-                     virCgroup *cgroup)
-{
-    pid_t pid = (pid_t) -1;
+    if (virStrcpyStatic(addr.sun_path, passtSocketName) < 0) {
+        virReportError(VIR_ERR_INTERNAL_ERROR,
+                       _("Socket path %s too big for destination"),
+                       passtSocketName);
+        goto out;
+    }
 
-    if (qemuPasstGetPid(vm, net, &pid) < 0 || pid <= 0)
-        return -1;
+    if (connect(fd, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
+        if (errno != ECONNREFUSED && errno != ENOENT)
+            virReportError(errno,
+                       "%s", _("Unable to connect to passt to terminate it"));
+    }
 
-    return virCgroupAddProcess(cgroup, pid);
+ out:
+    VIR_FORCE_CLOSE(fd);
 }
 
 
@@ -140,13 +115,9 @@ qemuPasstStart(virDomainObj *vm,
     virQEMUDriver *driver = priv->driver;
     g_autofree char *passtSocketName = qemuPasstCreateSocketPath(vm, net);
     g_autoptr(virCommand) cmd = NULL;
-    g_autofree char *pidfile = qemuPasstCreatePidFilename(vm, net);
     g_autofree char *errbuf = NULL;
     char macaddr[VIR_MAC_STRING_BUFLEN];
     size_t i;
-    pid_t pid = (pid_t) -1;
-    int exitstatus = 0;
-    int cmdret = 0;
 
     cmd = virCommandNew(PASST);
 
@@ -157,7 +128,6 @@ qemuPasstStart(virDomainObj *vm,
                          "--one-off",
                          "--socket", passtSocketName,
                          "--mac-addr", virMacAddrFormat(&net->mac, macaddr),
-                         "--pid", pidfile,
                          NULL);
 
     if (net->mtu) {
@@ -254,26 +224,15 @@ qemuPasstStart(virDomainObj *vm,
         virCommandAddArg(cmd, virBufferCurrentContent(&buf));
     }
 
-
     if (qemuExtDeviceLogCommand(driver, vm, cmd, "passt") < 0)
         return -1;
 
-    if (qemuSecurityCommandRun(driver, vm, cmd, -1, -1, &exitstatus, &cmdret) < 0)
-        goto error;
-
-    if (cmdret < 0 || exitstatus != 0) {
+    /* passt forks once it's ready, terminates on connection closure */
+    if (virCommandRun(cmd, NULL) < 0) {
         virReportError(VIR_ERR_INTERNAL_ERROR,
                        _("Could not start 'passt': %s"), errbuf);
-        goto error;
+        return -1;
     }
 
     return 0;
-
- error:
-    ignore_value(virPidFileReadPathIfLocked(pidfile, &pid));
-    if (pid != -1)
-        virProcessKillPainfully(pid, true);
-    unlink(pidfile);
-
-    return -1;
 }

-- 
@@ -28,29 +28,12 @@
 #include "virerror.h"
 #include "virjson.h"
 #include "virlog.h"
-#include "virpidfile.h"
 
 #define VIR_FROM_THIS VIR_FROM_NONE
 
 VIR_LOG_INIT("qemu.passt");
 
 
-static char *
-qemuPasstCreatePidFilename(virDomainObj *vm,
-                           virDomainNetDef *net)
-{
-    qemuDomainObjPrivate *priv = vm->privateData;
-    virQEMUDriver *driver = priv->driver;
-    g_autofree char *shortName = virDomainDefGetShortName(vm->def);
-    g_autoptr(virQEMUDriverConfig) cfg = virQEMUDriverGetConfig(driver);
-    g_autofree char *name = NULL;
-
-    name = g_strdup_printf("%s-%s-passt", shortName, net->info.alias);
-
-    return virPidFileBuildPath(cfg->passtStateDir, name);
-}
-
-
 static char *
 qemuPasstCreateSocketPath(virDomainObj *vm,
                           virDomainNetDef *net)
@@ -65,17 +48,6 @@ qemuPasstCreateSocketPath(virDomainObj *vm,
 }
 
 
-static int
-qemuPasstGetPid(virDomainObj *vm,
-                virDomainNetDef *net,
-                pid_t *pid)
-{
-    g_autofree char *pidfile = qemuPasstCreatePidFilename(vm, net);
-
-    return virPidFileReadPathIfLocked(pidfile, pid);
-}
-
-
 int
 qemuPasstAddNetProps(virDomainObj *vm,
                      virDomainNetDef *net,
@@ -106,29 +78,32 @@ void
 qemuPasstStop(virDomainObj *vm,
               virDomainNetDef *net)
 {
-    g_autofree char *pidfile = qemuPasstCreatePidFilename(vm, net);
-    virErrorPtr orig_err;
-
-    virErrorPreserveLast(&orig_err);
-
-    if (virPidFileForceCleanupPath(pidfile) < 0)
-        VIR_WARN("Unable to kill passt process");
-
-    virErrorRestore(&orig_err);
-}
-
+    g_autofree char *passtSocketName = qemuPasstCreateSocketPath(vm, net);
+    struct sockaddr_un addr = { .sun_family = AF_UNIX };
+    int fd;
+
+    fd = socket(AF_UNIX, SOCK_STREAM, 0);
+    if (fd < 0) {
+        virReportError(errno,
+                       "%s", _("Unable to open socket to connect to passt"));
+        return;
+    }
 
-int
-qemuPasstSetupCgroup(virDomainObj *vm,
-                     virDomainNetDef *net,
-                     virCgroup *cgroup)
-{
-    pid_t pid = (pid_t) -1;
+    if (virStrcpyStatic(addr.sun_path, passtSocketName) < 0) {
+        virReportError(VIR_ERR_INTERNAL_ERROR,
+                       _("Socket path %s too big for destination"),
+                       passtSocketName);
+        goto out;
+    }
 
-    if (qemuPasstGetPid(vm, net, &pid) < 0 || pid <= 0)
-        return -1;
+    if (connect(fd, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
+        if (errno != ECONNREFUSED && errno != ENOENT)
+            virReportError(errno,
+                       "%s", _("Unable to connect to passt to terminate it"));
+    }
 
-    return virCgroupAddProcess(cgroup, pid);
+ out:
+    VIR_FORCE_CLOSE(fd);
 }
 
 
@@ -140,13 +115,9 @@ qemuPasstStart(virDomainObj *vm,
     virQEMUDriver *driver = priv->driver;
     g_autofree char *passtSocketName = qemuPasstCreateSocketPath(vm, net);
     g_autoptr(virCommand) cmd = NULL;
-    g_autofree char *pidfile = qemuPasstCreatePidFilename(vm, net);
     g_autofree char *errbuf = NULL;
     char macaddr[VIR_MAC_STRING_BUFLEN];
     size_t i;
-    pid_t pid = (pid_t) -1;
-    int exitstatus = 0;
-    int cmdret = 0;
 
     cmd = virCommandNew(PASST);
 
@@ -157,7 +128,6 @@ qemuPasstStart(virDomainObj *vm,
                          "--one-off",
                          "--socket", passtSocketName,
                          "--mac-addr", virMacAddrFormat(&net->mac, macaddr),
-                         "--pid", pidfile,
                          NULL);
 
     if (net->mtu) {
@@ -254,26 +224,15 @@ qemuPasstStart(virDomainObj *vm,
         virCommandAddArg(cmd, virBufferCurrentContent(&buf));
     }
 
-
     if (qemuExtDeviceLogCommand(driver, vm, cmd, "passt") < 0)
         return -1;
 
-    if (qemuSecurityCommandRun(driver, vm, cmd, -1, -1, &exitstatus, &cmdret) < 0)
-        goto error;
-
-    if (cmdret < 0 || exitstatus != 0) {
+    /* passt forks once it's ready, terminates on connection closure */
+    if (virCommandRun(cmd, NULL) < 0) {
         virReportError(VIR_ERR_INTERNAL_ERROR,
                        _("Could not start 'passt': %s"), errbuf);
-        goto error;
+        return -1;
     }
 
     return 0;
-
- error:
-    ignore_value(virPidFileReadPathIfLocked(pidfile, &pid));
-    if (pid != -1)
-        virProcessKillPainfully(pid, true);
-    unlink(pidfile);
-
-    return -1;
 }

-- 
Stefano

Re: [PATCH 4/4] qemu_passt: Don't let passt fork off

[Michal Prívozník]

On 2/15/23 19:30, Stefano Brivio wrote:
> On Wed, 15 Feb 2023 18:04:56 +0100
> Michal Prívozník <mprivozn@redhat.com> wrote:
> 
>> On 2/15/23 08:50, Laine Stump wrote:
>>> On 2/14/23 8:02 AM, Stefano Brivio wrote:  
>>>> On Tue, 14 Feb 2023 12:51:22 +0100
>>>> Michal Privoznik <mprivozn@redhat.com> wrote:
>>>>  
>>>>> When passt starts it tries to do some security measures to
>>>>> restrict itself. For instance, it creates its own namespaces,
>>>>> umounts basically everything, drops capabilities, forks off to
>>>>> further restrict itself (the child is where all interesting work
>>>>> takes place now). This is sound, except it's causing two
>>>>> problems:
>>>>>
>>>>> 1) the PID file FD, which we leak into the passt process, gets
>>>>>     closed (and thus our virPidFile*() helpers see unlocked PID
>>>>>     file, which makes them think the process is gone),  
>>>>
>>>> I didn't realise this was the case, but giving passt write (unless I'm
>>>> missing something) access to a file created by libvirtd doesn't look
>>>> desirable to me.  
>>>   
>>>>  
>>>>> 2) the PID file no longer reflects true PID of the process.
>>>>>
>>>>> Worse, the child calls setsid() so we can't even kill the whole
>>>>> process group. I mean, we can but it won't be any good.  
>>>
>>> I think that (incorrect PID in the pidfile) is  happening because Michal
>>> is using the original version of my patches that were pushed - I had
>>> mimicked the behavior of slirp, where libvirt deamonizes the new
>>> process. If that process then daemonizes itself, we have some sort of
>>> "double daemon"; libvirt has saved off the pid of what it thinks is
>>> going to be the final process, but then that process further forks and
>>> exits from the process whose pid libvirt saved. But because passt was
>>> cleaning up after itself I hadn't noticed the discrepancy in pids when
>>> testing.
>>>
>>> Without going into all the details of the pidfile and locking and etc, I
>>> just want to say that if we can fork/exec dnsmasq and let it daemonize
>>> itself and create its own pidfile, then certainly we can do the same
>>> thing for passt. (and if there's a fundamental problem, then it's a
>>> fundamental problem for dnsmasq as well).  
>>
>> Alright. I think I have a solution that would please everybody involved.
>> I'll post it tomorrow though. I need to test it thoroughly. We would be
>> able to get passt's PID (which is needed not only for killing it, but
>> also for CGroup placement), NOT use --foreground and still pass errors
>> from it to users (that is unless logfile was specified, because
>> unfortunately, --log-file and --stderr are mutually exclusive).
> 
> That doesn't need to be the case (--log-file and --stderr being
> mutually exclusive)... if you have a use case for it, let's change that
> in passt. I just wanted to keep it simple for users ("give a log file,
> and be sure it won't spam").
> 
> Also mind that Laine's series:
>   https://archives.passt.top/passt-dev/20230215082437.110151-1-laine@redhat.com/

Thanks, this looks exactly like what we need. So for now I can just pass
--stderr if there's no --log-file, to deal with those "releases" that
don't have those patches merged yet.

> 
> *should* already cover all the cases where libvirt is interested in
> relaying "early" errors back to the user.
> 
> By the way, the one below is pretty much the patch I would have proposed
> for libvirt. I prepared it earlier today and didn't have a chance to
> test it yet, it's compile-tested only, and doesn't take cgroups into
> account (which, it seems, is needed no matter the lifecycle).
> 
> So I'm sharing it here as reference (that's how simple I wanted it to
> be -- minus cgroups), or if it's convenient for you to copy and paste
> something.
> 

This effectively disables placing passt into the CGroup set up for
emulator thread. And I don't think we want that. Firstly, it makes
statistics gathering report incorrect values. Secondly, these helper
processes are "implementation detail" - I mean, users don't really care
(from accounting POV) whether a task runs in emulator thread inside of
QEMU or in a separate process. It's still an emulation and as such
should be accounted for. And also, on NUMA machines we definitely want
to place passt as close to the emulator as possible (i.e. if emulator
thread is pinned than helper processes should be pinned too).

Furthermore, it enhances security, because libvirt sets up devices
controller in such way, that only devices from domain XML are allowed
and everything else is forbidden.

I could go on with other controllers but I believe you get the picture.
Now true, for qemu:///session we don't set any CGroups as we lack the
permissions to do so [1], and this is probably the target audience for
this feature anyway, but for qemu:///system (when running
libvirtd/virtqemud as root) we do set up CGroups and MUST place helper
processes into them. I mean, if we are concerned about security (just
look at the discussion about --foreground), then CGroups are definitely
step in the right direction.

1: and even this might change in the future as there are some plans to
let a privileged component create the CGroup for us (e.g. systemd).

Michal

Re: [PATCH 4/4] qemu_passt: Don't let passt fork off

[Peter Krempa]

On Thu, Feb 16, 2023 at 09:52:27 +0100, Michal Prívozník wrote:
> On 2/15/23 19:30, Stefano Brivio wrote:
> > On Wed, 15 Feb 2023 18:04:56 +0100
> > Michal Prívozník <mprivozn@redhat.com> wrote:
> >> On 2/15/23 08:50, Laine Stump wrote:

[...]

> > *should* already cover all the cases where libvirt is interested in
> > relaying "early" errors back to the user.
> > 
> > By the way, the one below is pretty much the patch I would have proposed
> > for libvirt. I prepared it earlier today and didn't have a chance to
> > test it yet, it's compile-tested only, and doesn't take cgroups into
> > account (which, it seems, is needed no matter the lifecycle).
> > 
> > So I'm sharing it here as reference (that's how simple I wanted it to
> > be -- minus cgroups), or if it's convenient for you to copy and paste
> > something.
> > 
> 
> This effectively disables placing passt into the CGroup set up for
> emulator thread. And I don't think we want that. Firstly, it makes
> statistics gathering report incorrect values. Secondly, these helper
> processes are "implementation detail" - I mean, users don't really care
> (from accounting POV) whether a task runs in emulator thread inside of
> QEMU or in a separate process. It's still an emulation and as such
> should be accounted for. And also, on NUMA machines we definitely want
> to place passt as close to the emulator as possible (i.e. if emulator
> thread is pinned than helper processes should be pinned too).

Just a side note. We are already at the point where we need
infrastructure to pin/cgroup-place helper processes explicitly similarly
to how we do this for the emulator thread.

Originally the helper processes (e.g. the pr-helper) didn't do much so
it didn't matter much where we placed it.

With processes which do heavy lifting such as network communication or
in case of the qemu-storage-daemon I'm going to implent we are in the
ream of CPU hungry processes where it starts to make sense to dedicate
CPU to it or separate it from the rest.

The approach we pick should be generic enough so that we don't have to
keep re-doing it for each helper process in the future.

I plan to address that with the QSD work, but that will take some time.
If you end up dealing with this sooner, please consider other helper
daemons too.

Re: [PATCH 4/4] qemu_passt: Don't let passt fork off

[Stefano Brivio]

On Thu, 16 Feb 2023 09:52:27 +0100
Michal Prívozník <mprivozn@redhat.com> wrote:

> On 2/15/23 19:30, Stefano Brivio wrote:
> > On Wed, 15 Feb 2023 18:04:56 +0100
> > Michal Prívozník <mprivozn@redhat.com> wrote:
> >   
> >> On 2/15/23 08:50, Laine Stump wrote:  
> >>> On 2/14/23 8:02 AM, Stefano Brivio wrote:    
> >>>> On Tue, 14 Feb 2023 12:51:22 +0100
> >>>> Michal Privoznik <mprivozn@redhat.com> wrote:
> >>>>    
> >>>>> When passt starts it tries to do some security measures to
> >>>>> restrict itself. For instance, it creates its own namespaces,
> >>>>> umounts basically everything, drops capabilities, forks off to
> >>>>> further restrict itself (the child is where all interesting work
> >>>>> takes place now). This is sound, except it's causing two
> >>>>> problems:
> >>>>>
> >>>>> 1) the PID file FD, which we leak into the passt process, gets
> >>>>>     closed (and thus our virPidFile*() helpers see unlocked PID
> >>>>>     file, which makes them think the process is gone),    
> >>>>
> >>>> I didn't realise this was the case, but giving passt write (unless I'm
> >>>> missing something) access to a file created by libvirtd doesn't look
> >>>> desirable to me.    
> >>>     
> >>>>    
> >>>>> 2) the PID file no longer reflects true PID of the process.
> >>>>>
> >>>>> Worse, the child calls setsid() so we can't even kill the whole
> >>>>> process group. I mean, we can but it won't be any good.    
> >>>
> >>> I think that (incorrect PID in the pidfile) is  happening because Michal
> >>> is using the original version of my patches that were pushed - I had
> >>> mimicked the behavior of slirp, where libvirt deamonizes the new
> >>> process. If that process then daemonizes itself, we have some sort of
> >>> "double daemon"; libvirt has saved off the pid of what it thinks is
> >>> going to be the final process, but then that process further forks and
> >>> exits from the process whose pid libvirt saved. But because passt was
> >>> cleaning up after itself I hadn't noticed the discrepancy in pids when
> >>> testing.
> >>>
> >>> Without going into all the details of the pidfile and locking and etc, I
> >>> just want to say that if we can fork/exec dnsmasq and let it daemonize
> >>> itself and create its own pidfile, then certainly we can do the same
> >>> thing for passt. (and if there's a fundamental problem, then it's a
> >>> fundamental problem for dnsmasq as well).    
> >>
> >> Alright. I think I have a solution that would please everybody involved.
> >> I'll post it tomorrow though. I need to test it thoroughly. We would be
> >> able to get passt's PID (which is needed not only for killing it, but
> >> also for CGroup placement), NOT use --foreground and still pass errors
> >> from it to users (that is unless logfile was specified, because
> >> unfortunately, --log-file and --stderr are mutually exclusive).  
> > 
> > That doesn't need to be the case (--log-file and --stderr being
> > mutually exclusive)... if you have a use case for it, let's change that
> > in passt. I just wanted to keep it simple for users ("give a log file,
> > and be sure it won't spam").
> > 
> > Also mind that Laine's series:
> >   https://archives.passt.top/passt-dev/20230215082437.110151-1-laine@redhat.com/  
> 
> Thanks, this looks exactly like what we need. So for now I can just pass
> --stderr if there's no --log-file, to deal with those "releases" that
> don't have those patches merged yet.

I wouldn't even bother with that, the user base (especially with
libvirt) is small enough that we can be quite confident that 100% of the
users will upgrade as soon as a new release (most likely coming this
week) includes them. I'd suggest to keep it simple, because you really
can.

Those are actual releases. Their naming just doesn't follow "semantic
versioning".

> > *should* already cover all the cases where libvirt is interested in
> > relaying "early" errors back to the user.
> > 
> > By the way, the one below is pretty much the patch I would have proposed
> > for libvirt. I prepared it earlier today and didn't have a chance to
> > test it yet, it's compile-tested only, and doesn't take cgroups into
> > account (which, it seems, is needed no matter the lifecycle).
> > 
> > So I'm sharing it here as reference (that's how simple I wanted it to
> > be -- minus cgroups), or if it's convenient for you to copy and paste
> > something.
> 
> This effectively disables placing passt into the CGroup set up for
> emulator thread. And I don't think we want that. Firstly, it makes
> statistics gathering report incorrect values. Secondly, these helper
> processes are "implementation detail" - I mean, users don't really care
> (from accounting POV) whether a task runs in emulator thread inside of
> QEMU or in a separate process. It's still an emulation and as such
> should be accounted for. And also, on NUMA machines we definitely want
> to place passt as close to the emulator as possible (i.e. if emulator
> thread is pinned than helper processes should be pinned too).

Yes, definitely, I see now -- I thought, earlier, that cgroups were just
used to handle lifecycles at the moment.

> [...]

-- 
Stefano