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

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 <redacted>
---
 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