[PATCH 1/2] audit: Fix use after free in audit_remove_watch_rule()

[Paul Moore]

On Tue, Aug 15, 2017 at 7:00 AM, Jan Kara <jack at suse.cz> wrote:
> audit_remove_watch_rule() drops watch's reference to parent but then
> continues to work with it. That is not safe as parent can get freed once
> we drop our reference. The following is a trivial reproducer:
>
> mount -o loop image /mnt
> touch /mnt/file
> auditctl -w /mnt/file -p wax
> umount /mnt
> auditctl -D
> <crash in fsnotify_destroy_mark()>
>
> Grab our own reference in audit_remove_watch_rule() earlier to make sure
> mark does not get freed under us.
>
> CC: stable at vger.kernel.org
> Reported-by: Tony Jones <tonyj at suse.de>
> Signed-off-by: Jan Kara <jack at suse.cz>
> ---
>  kernel/audit_watch.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)

This looks good to me, thanks for the fix.  I'm going to merge this
into the audit/stable-4.13 branch and assuming it passes the audit
tests I'll send it up to Linus later this week.

> diff --git a/kernel/audit_watch.c b/kernel/audit_watch.c
> index 62d686d96581..ed748ee40029 100644
> --- a/kernel/audit_watch.c
> +++ b/kernel/audit_watch.c
> @@ -457,13 +457,15 @@ void audit_remove_watch_rule(struct audit_krule *krule)
>         list_del(&krule->rlist);
>
>         if (list_empty(&watch->rules)) {
> +               /*
> +                * audit_remove_watch() drops our reference to 'parent' which
> +                * can get freed. Grab our own reference to be safe.
> +                */
> +               audit_get_parent(parent);
>                 audit_remove_watch(watch);
> -
> -               if (list_empty(&parent->watches)) {
> -                       audit_get_parent(parent);
> +               if (list_empty(&parent->watches))
>                         fsnotify_destroy_mark(&parent->mark, audit_watch_group);
> -                       audit_put_parent(parent);
> -               }
> +               audit_put_parent(parent);
>         }
>  }
>
> --
> 2.12.3

-- 
paul moore
www.paul-moore.com