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

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

[Maxime Bélair]

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(-)

diff --git a/security/apparmor/af_unix.c b/security/apparmor/af_unix.c
index b908e744818c..d04d9cd268aa 100644
--- a/security/apparmor/af_unix.c
+++ b/security/apparmor/af_unix.c
@@ -682,7 +682,7 @@ static void update_sk_ctx(struct sock *sk, struct aa_label *label,
 		if (old == plabel) {
 			rcu_assign_pointer(ctx->peer_lastupdate,
 					   aa_get_label(plabel));
-		} else if (aa_label_is_subset(plabel, old)) {
+		} else if (!old || aa_label_is_subset(plabel, old)) {
 			rcu_assign_pointer(ctx->peer_lastupdate,
 					   aa_get_label(plabel));
 			rcu_assign_pointer(ctx->peer, aa_get_label(plabel));
@@ -700,13 +700,17 @@ static void update_peer_ctx(struct sock *sk, struct aa_sk_ctx *ctx,
 	spin_lock(&unix_sk(sk)->lock);
 	old = rcu_dereference_protected(ctx->peer,
 					lockdep_is_held(&unix_sk(sk)->lock));
-	l = aa_label_merge(old, label, GFP_ATOMIC);
-	if (l) {
-		if (l != old) {
-			rcu_assign_pointer(ctx->peer, l);
-			aa_put_label(old);
-		} else
-			aa_put_label(l);
+	if (!old)
+		rcu_assign_pointer(ctx->peer, aa_get_label(label));
+	else {
+		l = aa_label_merge(old, label, GFP_ATOMIC);
+		if (l) {
+			if (l != old) {
+				rcu_assign_pointer(ctx->peer, l);
+				aa_put_label(old);
+			} else
+				aa_put_label(l);
+		}
 	}
 	spin_unlock(&unix_sk(sk)->lock);
 }
-- 
2.51.0

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

[Aurelien Jarno]

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>

Regards
Aurelien

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