Re: [PATCH] apparmor: fix NULL ctx->peer derefs in unix socket ctx updates

[Salvatore Bonaccorso <carnil@debian.org>]

Hi Maxime,

On Mon, Aug 31, 2026 at 10:25:07PM +0200, Aurelien Jarno wrote:
> Hi,
> 
> On 2026-08-24 23:32, Aurelien Jarno wrote:
> > Hi Maxime,
> > 
> > On 2026-08-24 17:58, Maxime Bélair wrote:
> > > aa_unix_file_perm lazily refreshes the AppArmor context cached on a unix
> > > socket. Two of the helpers it uses assume ctx->peer has already been set:
> > > 
> > > 	update_peer_ctx ->  l = aa_label_merge(old, label, GFP_ATOMIC);
> > > 	update_sk_ctx   ->  } else if (aa_label_is_subset(plabel, old)) {
> > > 
> > > where @old is ctx->peer. Neither aa_label_merge nor aa_label_is_subset
> > > allows NULL. So both fault on the aa_label->size load.
> > > 
> > >   BUG: kernel NULL pointer dereference, address: 000000000000004c
> > >   RIP: 0010:__aa_label_next_not_in_set+0xb/0xd0
> > >   Call Trace:
> > >    aa_label_is_subset+0x3f/0x70
> > >    aa_unix_file_perm+0x5e8/0x9d0
> > >    aa_file_perm+0x45a/0x550
> > >    apparmor_file_permission+0x44/0xb0
> > >    security_file_permission+0x40/0x100
> > >    rw_verify_area+0x56/0x180
> > >    vfs_write+0x7c/0x480
> > >    ksys_write+0xbf/0xf0
> > > 
> > > ctx->peer is only recorded for stream connections and socket pairs.
> > > unix_dgram_connect sets unix_peer(sk) without going through that path,
> > > so a connected AF_UNIX datagram socket has unix_peer(sk) set while
> > > ctx->peer is still NULL, and the first write that needs revalidation
> > > reaches the helpers above.
> > > 
> > > Both derefs date back to the Fixes: commit, but the update_sk_ctx() one
> > > was dormant until commit 4483efe4f215 ("apparmor: fix shadowing of plabel
> > > that prevents cache from being updated") stopped @plabel being shadowed,
> > > which is why bisecting the oops lands there.
> > > 
> > > A NULL @old just means no peer label has been recorded yet, so install
> > > the label directly instead of merging or comparing against it.
> > > 
> > > Fixes: 88fec3526e84 ("apparmor: make sure unix socket labeling is correctly updated.")
> > > Reported-by: Aurelien Jarno <aurelien@aurel32.net>
> > > Closes: https://bugs.debian.org/1145111
> > > Cc: stable@vger.kernel.org
> > > Signed-off-by: Maxime Bélair <maxime.belair@canonical.com>
> > > ---
> > >  security/apparmor/af_unix.c | 20 ++++++++++++--------
> > >  1 file changed, 12 insertions(+), 8 deletions(-)
> > 
> > Thanks a lot for the quick patch. I confirm it fixes the issue I 
> > reported.
> > 
> > Tested-by: Aurelien Jarno <aurelien@aurel32.net>
> 
> Any news about this patch? The kernel oops can be triggered as a simple 
> user, so it would be nice to get it fixed relatively soon.

Any news here? As Aurelien has pointed out this can be relatively easy
triggered, so having a fix landing would be great.

Regards,
Salvatore