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

[Aurelien Jarno <aurelien@aurel32.net>]

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.

Thanks
Aurelien

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurelien@aurel32.net                     http://aurel32.net