Re: [PATCH 01/17] exec: Move unshare_files to fix posix file locking during exec

Christian Brauner
2020-08-18T10:04:32Z

On Mon, Aug 17, 2020 at 05:04:09PM -0500, Eric W. Biederman wrote:
> Many moons ago the binfmts were doing some very questionable things
> with file descriptors and an unsharing of the file descriptor table
> was added to make things better[1][2]. The helper steal_files was
> added to avoid breaking the userspace programs[3][4][6].
>
> Unfortunately it turned out that steal_locks did not work for network
> file systems[5], so it was removed to see if anyone would
> complain[7][8]. It was thought at the time that NPTL would not be
> affected as the unshare_files happened after the other threads were
> killed[8]. Unfortunately because there was an unshare_files in
> binfmt_elf.c before the threads were killed this analysis was
> incorrect.
>
> This unshare_files in binfmt_elf.c resulted in the unshares_files
> happening whenever threads were present. Which led to unshare_files
> being moved to the start of do_execve[9].
>
> Later the problems were rediscovered and suggested approach was to
> readd steal_locks under a different name[10]. I happened to be
> reviewing patches and I noticed that this approach was a step
> backwards[11].
>
> I proposed simply moving unshare_files[12] and it was pointed
> out that moving unshare_files without auditing the code was
> also unsafe[13].
>
> There were then several attempts to solve this[14][15][16] and I even
> posted this set of changes[17]. Unfortunately because auditing all of
> execve is time consuming this change did not make it in at the time.
>
> Well now that I am cleaning up exec I have made the time to read
> through all of the binfmts and the only playing with file descriptors
> is either the security modules closing them in
> security_bprm_committing_creds or is in the generic code in fs/exec.c.
> None of it happens before begin_new_exec is called.
>
> So move unshare_files into begin_new_exec, after the point of no
> return. If memory is very very very low and the application calling
> exec is sharing file descriptor tables between processes we might fail
> past the point of no return. Which is unfortunate but no different
> than any of the other places where we allocate memory after the point
> of no return.
>
> This movement allows another process that shares the file table, or
> another thread of the same process and that closes files or changes
> their close on exec behavior and races with execve to cause some
> unexpected things to happen. There is only one time of check to time

It seems to only make the already existing race window wider by moving
it from bprm_execve() to begin_new_exec() which isn't great but probably
ok since done for a good reason.

> of use race and it is just there so that execve fails instead of
> an interpreter failing when it tries to open the file it is supposed
> to be interpreting. Failing later if userspace is being silly is
> not a problem.
>
> With this change it the following discription from the removal
> of steal_locks[8] finally becomes true.
>
> Apps using NPTL are not affected, since all other threads are killed before
> execve.
>
> Apps using LinuxThreads are only affected if they
>
> - have multiple threads during exec (LinuxThreads doesn't kill other
> threads, the app may do it with pthread_kill_other_threads_np())
> - rely on POSIX locks being inherited across exec
>
> Both conditions are documented, but not their interaction.
>
> Apps using clone() natively are affected if they
>
> - use clone(CLONE_FILES)
> - rely on POSIX locks being inherited across exec
>
> I have investigated some paths to make it possible to solve this
> without moving unshare_files but they all look more complicated[18].
>
> Reported-by: Daniel P. Berrangé <berrange@redhat.com>
> Reported-by: Jeff Layton <jlayton@redhat.com>
> History-tree: git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
> [1] 02cda956de0b ("[PATCH] unshare_files"
> [2] 04e9bcb4d106 ("[PATCH] use new unshare_files helper")
> [3] 088f5d7244de ("[PATCH] add steal_locks helper")
> [4] 02c541ec8ffa ("[PATCH] use new steal_locks helper")
> [5] https://lkml.kernel.org/r/E1FLIlF-0007zR-00@dorka.pomaz.szeredi.hu
> [6] https://lkml.kernel.org/r/0060321191605.GB15997@sorel.sous-sol.org
> [7] https://lkml.kernel.org/r/E1FLwjC-0000kJ-00@dorka.pomaz.szeredi.hu
> [8] c89681ed7d0e ("[PATCH] remove steal_locks()")
> [9] fd8328be874f ("[PATCH] sanitize handling of shared descriptor tables in failing execve()")
> [10] https://lkml.kernel.org/r/20180317142520.30520-1-jlayton@kernel.org
> [11] https://lkml.kernel.org/r/87r2nwqk73.fsf@xmission.com
> [12] https://lkml.kernel.org/r/87bmfgvg8w.fsf@xmission.com
> [13] https://lkml.kernel.org/r/20180322111424.GE30522@ZenIV.linux.org.uk
> [14] https://lkml.kernel.org/r/20180827174722.3723-1-jlayton@kernel.org
> [15] https://lkml.kernel.org/r/20180830172423.21964-1-jlayton@kernel.org
> [16] https://lkml.kernel.org/r/20180914105310.6454-1-jlayton@kernel.org
> [17] https://lkml.kernel.org/r/87a7ohs5ow.fsf@xmission.com
> [18] https://lkml.kernel.org/r/87pn8c1uj6.fsf_-_@x220.int.ebiederm.org
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> ---

Slightly scary change but it solves a problem.
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>