af_unix: fix holding spinlock in oob handling

syzkaller found that OOB code was holding spinlock
while calling a function in which it could sleep.

Reported-by: syzbot+8760ca6c1ee783ac4abd@syzkaller.appspotmail.com
Fixes: 314001f ("af_unix: Add OOB support")
Signed-off-by: Rao Shoaib <rao.shoaib@oracle.com>
Link: https://lore.kernel.org/r/20210811220652.567434-1-Rao.Shoaib@oracle.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/unix/af_unix.c

@@ -2426,19 +2426,37 @@ static int unix_stream_recv_urg(struct unix_stream_read_state *state)
 	struct sock *sk = sock->sk;
 	struct unix_sock *u = unix_sk(sk);
 	int chunk = 1;
+	struct sk_buff *oob_skb;
 
-	if (sock_flag(sk, SOCK_URGINLINE) || !u->oob_skb)
+	mutex_lock(&u->iolock);
+	unix_state_lock(sk);
+
+	if (sock_flag(sk, SOCK_URGINLINE) || !u->oob_skb) {
+		unix_state_unlock(sk);
+		mutex_unlock(&u->iolock);
 		return -EINVAL;
+	}
 
-	chunk = state->recv_actor(u->oob_skb, 0, chunk, state);
-	if (chunk < 0)
-		return -EFAULT;
+	oob_skb = u->oob_skb;
 
 	if (!(state->flags & MSG_PEEK)) {
-		UNIXCB(u->oob_skb).consumed += 1;
-		kfree_skb(u->oob_skb);
 		u->oob_skb = NULL;
 	}
+
+	unix_state_unlock(sk);
+
+	chunk = state->recv_actor(oob_skb, 0, chunk, state);
+
+	if (!(state->flags & MSG_PEEK)) {
+		UNIXCB(oob_skb).consumed += 1;
+		kfree_skb(oob_skb);
+	}
+
+	mutex_unlock(&u->iolock);
+
+	if (chunk < 0)
+		return -EFAULT;
+
 	state->msg->msg_flags |= MSG_OOB;
 	return 1;
 }
@@ -2498,13 +2516,7 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state,
 	if (unlikely(flags & MSG_OOB)) {
 		err = -EOPNOTSUPP;
 #if IS_ENABLED(CONFIG_AF_UNIX_OOB)
-		mutex_lock(&u->iolock);
-		unix_state_lock(sk);
-
 		err = unix_stream_recv_urg(state);
-
-		unix_state_unlock(sk);
-		mutex_unlock(&u->iolock);
 #endif
 		goto out;
 	}