Re: [PATCH v2] passt, util: Close any open file that the parent might have leaked

[Stefano Brivio <sbrivio@redhat.com>]

On Wed, 7 Aug 2024 11:34:34 +0200
Paul Holzinger <pholzing@redhat.com> wrote:

> On 07/08/2024 10:27, Stefano Brivio wrote:
> > If a parent accidentally or due to implementation reasons leaks any
> > open file, we don't want to have access to them, except for the file
> > passed via --fd, if any.
> >
> > This is the case for Podman when Podman's parent leaks files into
> > Podman: it's not practical for Podman to close unrelated files before
> > starting pasta, as reported by Paul.
> >
> > Use close_range(2) to close all open files except for standard streams
> > and the one from --fd.
> >
> > Given that parts of conf() depend on other files to be already opened,
> > such as the epoll file descriptor, we can't easily defer this to a
> > more convenient point, where --fd was already parsed. Introduce a
> > minimal, duplicate version of --fd parsing to keep this simple.
> >
> > Suggested-by: Paul Holzinger <pholzing@redhat.com>
> > Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
> > ---
> > v2: Move call to close_open_files() to isolate_initial()
> >
> >   conf.c      |  1 +
> >   isolation.c | 12 +++++++++---
> >   isolation.h |  2 +-
> >   passt.c     |  2 +-
> >   util.c      | 36 ++++++++++++++++++++++++++++++++++++
> >   util.h      |  1 +
> >   6 files changed, 49 insertions(+), 5 deletions(-)
> >
> > diff --git a/conf.c b/conf.c
> > index 14d8ece..89f5b3d 100644
> > --- a/conf.c
> > +++ b/conf.c
> > @@ -1260,6 +1260,7 @@ void conf(struct ctx *c, int argc, char **argv)
> >   	c->tcp.fwd_in.mode = c->tcp.fwd_out.mode = FWD_UNSET;
> >   	c->udp.fwd_in.mode = c->udp.fwd_out.mode = FWD_UNSET;
> >   
> > +	optind = 1;
> >   	do {
> >   		name = getopt_long(argc, argv, optstring, options, NULL);
> >   
> > diff --git a/isolation.c b/isolation.c
> > index 4956d7e..45fba1e 100644
> > --- a/isolation.c
> > +++ b/isolation.c
> > @@ -29,7 +29,8 @@
> >    *
> >    * Executed immediately after startup, drops capabilities we don't
> >    * need at any point during execution (or which we gain back when we
> > - * need by joining other namespaces).
> > + * need by joining other namespaces), and closes any leaked file we
> > + * might have inherited from the parent process.
> >    *
> >    * 2. isolate_user()
> >    * =================
> > @@ -166,14 +167,17 @@ static void clamp_caps(void)
> >   }
> >   
> >   /**
> > - * isolate_initial() - Early, config independent self isolation
> > + * isolate_initial() - Early, mostly config independent self isolation
> > + * @argc:	Argument count
> > + * @argv:	Command line options: only --fd (if present) is relevant here
> >    *
> >    * Should:
> >    *  - drop unneeded capabilities
> > + *  - close all open files except for standard streams and the one from --fd
> >    * Musn't:
> >    *  - remove filesytem access (we need to access files during setup)
> >    */
> > -void isolate_initial(void)
> > +void isolate_initial(int argc, char **argv)
> >   {
> >   	uint64_t keep;
> >   
> > @@ -207,6 +211,8 @@ void isolate_initial(void)
> >   		keep |= BIT(CAP_SETFCAP) | BIT(CAP_SYS_PTRACE);
> >   
> >   	drop_caps_ep_except(keep);
> > +
> > +	close_open_files(argc, argv);
> >   }
> >   
> >   /**
> > diff --git a/isolation.h b/isolation.h
> > index 846b2af..80bb68d 100644
> > --- a/isolation.h
> > +++ b/isolation.h
> > @@ -7,7 +7,7 @@
> >   #ifndef ISOLATION_H
> >   #define ISOLATION_H
> >   
> > -void isolate_initial(void);
> > +void isolate_initial(int argc, char **argv);
> >   void isolate_user(uid_t uid, gid_t gid, bool use_userns, const char *userns,
> >   		  enum passt_modes mode);
> >   int isolate_prefork(const struct ctx *c);
> > diff --git a/passt.c b/passt.c
> > index ea5bece..4b3c306 100644
> > --- a/passt.c
> > +++ b/passt.c
> > @@ -211,7 +211,7 @@ int main(int argc, char **argv)
> >   
> >   	arch_avx2_exec(argv);
> >   
> > -	isolate_initial();
> > +	isolate_initial(argc, argv);
> >   
> >   	c.pasta_netns_fd = c.fd_tap = c.pidfile_fd = -1;
> >   
> > diff --git a/util.c b/util.c
> > index 07fb21c..bd65b5a 100644
> > --- a/util.c
> > +++ b/util.c
> > @@ -26,6 +26,7 @@
> >   #include <errno.h>
> >   #include <stdbool.h>
> >   #include <linux/errqueue.h>
> > +#include <getopt.h>
> >   
> >   #include "util.h"
> >   #include "iov.h"
> > @@ -694,3 +695,38 @@ const char *str_ee_origin(const struct sock_extended_err *ee)
> >   
> >   	return "<invalid>";
> >   }
> > +
> > +/**
> > + * close_open_files() - Close leaked files, but not --fd, stdin, stdout, stderr
> > + * @argc:	Argument count
> > + * @argv:	Command line options, as we need to skip any file given via --fd
> > + */
> > +void close_open_files(int argc, char **argv)
> > +{
> > +	const struct option optfd[] = { { "fd", required_argument, NULL, 'F' },
> > +					{ 0 },
> > +				      };
> > +	long fd = -1;
> > +	int name;
> > +
> > +	do {
> > +		name = getopt_long(argc, argv, ":F", optfd, NULL);
> > +
> > +		if (name == 'F') {
> > +			errno = 0;
> > +			fd = strtol(optarg, NULL, 0);
> > +
> > +			if (fd < 0 || errno)
> > +				die("Invalid --fd: %s", optarg);
> > +		}
> > +	} while (name != -1);
> > +
> > +	if (fd == -1) {
> > +		if (close_range(3,	~0U,	CLOSE_RANGE_UNSHARE))
> > +			die_perror("Failed to close files leaked by parent");
> > +	} else {
> > +		if (close_range(3,	fd - 1,	CLOSE_RANGE_UNSHARE) ||
> > +		    close_range(fd + 1,	~0U,	CLOSE_RANGE_UNSHARE))
> > +			die_perror("Failed to close files leaked by parent");  
> 
> This will fail for fd == 3 as the first range is 3 - 2 in that case 
> which causes EINVAL which could be a common choice for the extra passed fd.

Whoops.

> On the other end of the range it is the same issue, you seem to parse 
> the fd as long so allows values > 4294967295 which then overflows when 
> passed to close_range as uint which causes issues and even allows us to 
> close stderr streams. Less likely that users would pass such a number 
> but no reason to allow it in the first place.

Of course. Nice catch as well. Thanks.

> $ strace -e close_range ./pasta --config-net --fd 4294967296 echo test
> close_range(3, 4294967295, CLOSE_RANGE_UNSHARE) = 0
> close_range(1, 4294967295, CLOSE_RANGE_UNSHARE) = 0

Hey, that's a cool feature! So much easier to type than >/dev/null.
Sending v3.

-- 
Stefano