[PATCH] conf: Bind inbound ports with CAP_NET_BIND_SERVICE before isolate_user()

[Stefano Brivio <sbrivio@redhat.com>]

Even if CAP_NET_BIND_SERVICE is granted, we'll lose the capability in
the target user namespace as we isolate the process, which means
we're unable to bind to low ports at that point.

Bind inbound ports, and only those, before isolate_user(). Keep the
handling of outbound ports (for pasta mode only) after the setup of
the namespace, because that's where we'll bind them.

To this end, initialise the netlink socket for the init namespace
before isolate_user() as well, as we actually need to know the
addresses of the upstream interface before binding ports, in case
they're not explicitly passed by the user.

As we now call nl_sock_init() twice, checking its return code from
conf() twice looks a bit heavy: make it exit(), instead, as we
can't do much if we don't have netlink sockets.

While at it:

- move the v4_only && v6_only options check just after the first
  option processing loop, as this is more strictly related to
  option parsing proper

- update the man page, explaining that CAP_NET_BIND_SERVICE is
  *not* the preferred way to bind ports, because passt and pasta
  can be abused to allow other processes to make effective usage
  of it. Add a note about the recommended sysctl instead

Reported-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
---
This is based on David's patchset "Fixes and cleanups for capability
handling".

 conf.c    | 71 +++++++++++++++++++++++++++----------------------------
 netlink.c | 20 +++++++++-------
 netlink.h |  2 +-
 passt.1   | 45 +++++++++++++++++++++++++++++------
 tap.c     |  1 +
 5 files changed, 87 insertions(+), 52 deletions(-)

diff --git a/conf.c b/conf.c
index a22ef48..e1f42f1 100644
--- a/conf.c
+++ b/conf.c
@@ -1530,6 +1530,11 @@ void conf(struct ctx *c, int argc, char **argv)
 		}
 	} while (name != -1);
 
+	if (v4_only && v6_only) {
+		err("Options ipv4-only and ipv6-only are mutually exclusive");
+		usage(argv[0]);
+	}
+
 	ret = conf_ugid(runas, &uid, &gid);
 	if (ret)
 		usage(argv[0]);
@@ -1539,6 +1544,30 @@ void conf(struct ctx *c, int argc, char **argv)
 			     logfile, logsize);
 	}
 
+	nl_sock_init(c, false);
+	if (!v6_only)
+		c->ifi4 = conf_ip4(ifi, &c->ip4, c->mac);
+	if (!v4_only)
+		c->ifi6 = conf_ip6(ifi, &c->ip6, c->mac);
+	if (!c->ifi4 && !c->ifi6) {
+		err("External interface not usable");
+		exit(EXIT_FAILURE);
+	}
+
+	/* Inbound port options can be parsed now (after IPv4/IPv6 settings) */
+	optind = 1;
+	do {
+		struct port_fwd *fwd;
+
+		name = getopt_long(argc, argv, optstring, options, NULL);
+
+		if ((name == 't' && (fwd = &c->tcp.fwd_in)) ||
+		    (name == 'u' && (fwd = &c->udp.fwd_in.f))) {
+			if (!optarg || conf_ports(c, name, optarg, fwd))
+				usage(argv[0]);
+		}
+	} while (name != -1);
+
 	if (c->mode == MODE_PASTA) {
 		if (conf_pasta_ns(&netns_only, userns, netns,
 				  optind, argc, argv) < 0)
@@ -1561,50 +1590,20 @@ void conf(struct ctx *c, int argc, char **argv)
 		}
 	}
 
-	if (nl_sock_init(c)) {
-		err("Failed to get netlink socket");
-		exit(EXIT_FAILURE);
-	}
-
-	if (v4_only && v6_only) {
-		err("Options ipv4-only and ipv6-only are mutually exclusive");
-		usage(argv[0]);
-	}
-	if (!v6_only)
-		c->ifi4 = conf_ip4(ifi, &c->ip4, c->mac);
-	if (!v4_only)
-		c->ifi6 = conf_ip6(ifi, &c->ip6, c->mac);
-	if (!c->ifi4 && !c->ifi6) {
-		err("External interface not usable");
-		exit(EXIT_FAILURE);
-	}
+	if (c->mode == MODE_PASTA)
+		nl_sock_init(c, true);
 
-	/* Now we can process port configuration options */
+	/* ...and outbound port options now that namespaces are set up. */
 	optind = 1;
 	do {
-		struct port_fwd *fwd = NULL;
+		struct port_fwd *fwd;
 
 		name = getopt_long(argc, argv, optstring, options, NULL);
-		switch (name) {
-		case 't':
-		case 'u':
-		case 'T':
-		case 'U':
-			if (name == 't')
-				fwd = &c->tcp.fwd_in;
-			else if (name == 'T')
-				fwd = &c->tcp.fwd_out;
-			else if (name == 'u')
-				fwd = &c->udp.fwd_in.f;
-			else if (name == 'U')
-				fwd = &c->udp.fwd_out.f;
 
+		if ((name == 'T' && (fwd = &c->tcp.fwd_out)) ||
+		    (name == 'U' && (fwd = &c->udp.fwd_out.f))) {
 			if (!optarg || conf_ports(c, name, optarg, fwd))
 				usage(argv[0]);
-
-			break;
-		default:
-			break;
 		}
 	} while (name != -1);
 
diff --git a/netlink.c b/netlink.c
index 6e5a96b..3ee4d42 100644
--- a/netlink.c
+++ b/netlink.c
@@ -19,6 +19,7 @@
 #include <sys/types.h>
 #include <limits.h>
 #include <stdlib.h>
+#include <stdbool.h>
 #include <stdint.h>
 #include <unistd.h>
 #include <arpa/inet.h>
@@ -71,25 +72,28 @@ ns:
 }
 
 /**
- * nl_sock_init() - Call nl_sock_init_do() and check for failures
+ * nl_sock_init() - Call nl_sock_init_do(), won't return on failure
  * @c:		Execution context
- *
- * Return: -EIO if sockets couldn't be set up, 0 otherwise
+ * @ns:		Get socket in namespace, not in init
  */
-int nl_sock_init(const struct ctx *c)
+void nl_sock_init(const struct ctx *c, bool ns)
 {
-	if (c->mode == MODE_PASTA) {
+	if (c->mode == MODE_PASTA && ns) {
 		NS_CALL(nl_sock_init_do, c);
 		if (nl_sock_ns == -1)
-			return -EIO;
+			goto fail;
 	} else {
 		nl_sock_init_do(NULL);
 	}
 
 	if (nl_sock == -1)
-		return -EIO;
+		goto fail;
 
-	return 0;
+	return;
+
+fail:
+	err("Failed to get netlink socket");
+	exit(EXIT_FAILURE);
 }
 
 /**
diff --git a/netlink.h b/netlink.h
index 5ce5037..3c991cc 100644
--- a/netlink.h
+++ b/netlink.h
@@ -6,7 +6,7 @@
 #ifndef NETLINK_H
 #define NETLINK_H
 
-int nl_sock_init(const struct ctx *c);
+void nl_sock_init(const struct ctx *c, bool ns);
 unsigned int nl_get_ext_if(sa_family_t af);
 void nl_route(int ns, unsigned int ifi, sa_family_t af, void *gw);
 void nl_addr(int ns, unsigned int ifi, sa_family_t af,
diff --git a/passt.1 b/passt.1
index 7d113f2..2cdba5d 100644
--- a/passt.1
+++ b/passt.1
@@ -771,14 +771,45 @@ possible to bind sockets to foreign addresses.
 
 .SS Binding to low numbered ports (well-known or system ports, up to 1023)
 
-If the port forwarding configuration requires binding to port numbers lower than
-1024, \fBpasst\fR and \fBpasta\fR will try to bind to them, but will fail if not
-running as root, or without the \fICAP_NET_BIND_SERVICE\fR Linux capability, see
-\fBservices\fR(5) and \fBcapabilities\fR(7). To grant the
-\fICAP_NET_BIND_SERVICE\fR capability to passt, you can issue, as root:
+If the port forwarding configuration requires binding to ports with numbers
+lower than 1024, \fBpasst\fR and \fBpasta\fR will try to bind to them, but will
+fail, unless, either:
+
+.IP \(bu 2
+the \fIsys.net.ipv4.ip_unprivileged_port_start\fR sysctl is set to the number
+of the lowest port \fBpasst\fR and \fBpasta\fR need. For example, as root:
+
+.nf
+	sysctl -w net.ipv4.ip_unprivileged_port_start=443
+.fi
+
+\fBNote\fR: this is the recommended way of enabling \fBpasst\fR and \fBpasta\fR
+to bind to ports with numbers below 1024.
+
+.IP \(bu
+or the \fICAP_NET_BIND_SERVICE\fR Linux capability is granted, see
+\fBservices\fR(5) and \fBcapabilities\fR(7).
+
+This is, in general, \fBnot the recommended way\fR, because \fBpasst\fR and
+\fBpasta\fR might be used as vector to effectively use this capability from
+another process.
+
+However, if your environment is sufficiently controlled by an LSM (Linux
+Security Module) such as \fIAppArmor\fR, \fISELinux\fR, \fISmack\fR or
+\fITOMOYO\fR, and no other processes can interact in such a way in virtue of
+this, granting this capability to \fBpasst\fR and \fBpasta\fR only can
+effectively prevent other processes from utilising it.
+
+Note that this will not work for automatic detection and forwarding of ports
+with \fBpasta\fR, because \fBpasta\fR will relinquish this capability at
+runtime.
+
+To grant this capability, you can issue, as root:
+
+.nf
+	setcap 'cap_net_bind_service=+ep' $(which passt)
+.fi
 
-.RS
-setcap 'cap_net_bind_service=+ep' $(which passt)
 .RE
 
 .SS ICMP/ICMPv6 Echo sockets
diff --git a/tap.c b/tap.c
index 77e513c..8b6d9bc 100644
--- a/tap.c
+++ b/tap.c
@@ -30,6 +30,7 @@
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <sys/uio.h>
+#include <stdbool.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <netinet/ip.h>
-- 
@@ -30,6 +30,7 @@
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <sys/uio.h>
+#include <stdbool.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <netinet/ip.h>
-- 
2.35.1