[AF_UNIX]: Fix datagram connect race causing an OOPS.

Based upon an excellent bug report and initial patch by
Frederik Deweerdt.

The UNIX datagram connect code blindly dereferences other->sk_socket
via the call down to the security_unix_may_send() function.

Without locking 'other' that pointer can go NULL via unix_release_sock()
which does sock_orphan() which also marks the socket SOCK_DEAD.

So we have to lock both 'sk' and 'other' yet avoid all kinds of
potential deadlocks (connect to self is OK for datagram sockets and it
is possible for two datagram sockets to perform a simultaneous connect
to each other).  So what we do is have a "double lock" function similar
to how we handle this situation in other areas of the kernel.  We take
the lock of the socket pointer with the smallest address first in
order to avoid ABBA style deadlocks.

Once we have them both locked, we check to see if SOCK_DEAD is set
for 'other' and if so, drop everything and retry the lookup.
Signed-off-by: David S. Miller <davem@davemloft.net>

net/unix/af_unix.c

@@ -858,6 +858,31 @@ static int unix_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
 	goto out_up;
 }
 
+static void unix_state_double_lock(struct sock *sk1, struct sock *sk2)
+{
+	if (unlikely(sk1 == sk2) || !sk2) {
+		unix_state_lock(sk1);
+		return;
+	}
+	if (sk1 < sk2) {
+		unix_state_lock(sk1);
+		unix_state_lock_nested(sk2);
+	} else {
+		unix_state_lock(sk2);
+		unix_state_lock_nested(sk1);
+	}
+}
+
+static void unix_state_double_unlock(struct sock *sk1, struct sock *sk2)
+{
+	if (unlikely(sk1 == sk2) || !sk2) {
+		unix_state_unlock(sk1);
+		return;
+	}
+	unix_state_unlock(sk1);
+	unix_state_unlock(sk2);
+}
+
 static int unix_dgram_connect(struct socket *sock, struct sockaddr *addr,
 			      int alen, int flags)
 {
@@ -877,11 +902,19 @@ static int unix_dgram_connect(struct socket *sock, struct sockaddr *addr,
 		    !unix_sk(sk)->addr && (err = unix_autobind(sock)) != 0)
 			goto out;
 
+restart:
 		other=unix_find_other(sunaddr, alen, sock->type, hash, &err);
 		if (!other)
 			goto out;
 
-		unix_state_lock(sk);
+		unix_state_double_lock(sk, other);
+
+		/* Apparently VFS overslept socket death. Retry. */
+		if (sock_flag(other, SOCK_DEAD)) {
+			unix_state_double_unlock(sk, other);
+			sock_put(other);
+			goto restart;
+		}
 
 		err = -EPERM;
 		if (!unix_may_send(sk, other))
@@ -896,7 +929,7 @@ static int unix_dgram_connect(struct socket *sock, struct sockaddr *addr,
 		 *	1003.1g breaking connected state with AF_UNSPEC
 		 */
 		other = NULL;
-		unix_state_lock(sk);
+		unix_state_double_lock(sk, other);
 	}
 
 	/*
@@ -905,19 +938,19 @@ static int unix_dgram_connect(struct socket *sock, struct sockaddr *addr,
 	if (unix_peer(sk)) {
 		struct sock *old_peer = unix_peer(sk);
 		unix_peer(sk)=other;
-		unix_state_unlock(sk);
+		unix_state_double_unlock(sk, other);
 
 		if (other != old_peer)
 			unix_dgram_disconnected(sk, old_peer);
 		sock_put(old_peer);
 	} else {
 		unix_peer(sk)=other;
-		unix_state_unlock(sk);
+		unix_state_double_unlock(sk, other);
 	}
 	return 0;
 
 out_unlock:
-	unix_state_unlock(sk);
+	unix_state_double_unlock(sk, other);
 	sock_put(other);
 out:
 	return err;