unix: fix a race condition in unix_release()

As reported by Jan, and others over the past few years, there is a
race condition caused by unix_release setting the sock->sk pointer
to NULL before properly marking the socket as dead/orphaned.  This
can cause a problem with the LSM hook security_unix_may_send() if
there is another socket attempting to write to this partially
released socket in between when sock->sk is set to NULL and it is
marked as dead/orphaned.  This patch fixes this by only setting
sock->sk to NULL after the socket has been marked as dead; I also
take the opportunity to make unix_release_sock() a void function
as it only ever returned 0/success.

Dave, I think this one should go on the -stable pile.

Special thanks to Jan for coming up with a reproducer for this
problem.
Reported-by: Jan Stancek <jan.stancek@gmail.com>
Signed-off-by: Paul Moore <pmoore@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/unix/af_unix.c

@@ -382,7 +382,7 @@ static void unix_sock_destructor(struct sock *sk)
 #endif
 }
 
-static int unix_release_sock(struct sock *sk, int embrion)
+static void unix_release_sock(struct sock *sk, int embrion)
 {
 	struct unix_sock *u = unix_sk(sk);
 	struct path path;
@@ -451,8 +451,6 @@ static int unix_release_sock(struct sock *sk, int embrion)
 
 	if (unix_tot_inflight)
 		unix_gc();		/* Garbage collect fds */
-
-	return 0;
 }
 
 static void init_peercred(struct sock *sk)
@@ -699,9 +697,10 @@ static int unix_release(struct socket *sock)
 	if (!sk)
 		return 0;
 
+	unix_release_sock(sk, 0);
 	sock->sk = NULL;
 
-	return unix_release_sock(sk, 0);
+	return 0;
 }
 
 static int unix_autobind(struct socket *sock)