[PATCH net] unix: Fix race in SOCK_SEQPACKET's unix_dgram_sendmsg()

[Kirill Tkhai <tkhai@ya.ru>]

There is a race resulting in alive SOCK_SEQPACKET socket
may change its state from TCP_ESTABLISHED to TCP_CLOSE:

unix_release_sock(peer)                  unix_dgram_sendmsg(sk)
  sock_orphan(peer)
    sock_set_flag(peer, SOCK_DEAD)
                                           sock_alloc_send_pskb()
                                             if !(sk->sk_shutdown & SEND_SHUTDOWN)
                                               OK
                                           if sock_flag(peer, SOCK_DEAD)
                                             sk->sk_state = TCP_CLOSE
  sk->sk_shutdown = SHUTDOWN_MASK


After that socket sk remains almost normal: it is able to connect, listen, accept
and recvmsg, while it can't sendmsg.

Since this is the only possibility for alive SOCK_SEQPACKET to change
the state in such way, we should better fix this strange and potentially
danger corner case.

Also, move TCP_CLOSE assignment for SOCK_DGRAM sockets under state lock.

Signed-off-by: Kirill Tkhai <tkhai@ya.ru>
---
 net/unix/af_unix.c |   11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index b3545fc68097..6fd745cfc492 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1999,13 +1999,20 @@ static int unix_dgram_sendmsg(struct socket *sock, struct msghdr *msg,
 			unix_state_lock(sk);
 
 		err = 0;
-		if (unix_peer(sk) == other) {
+		if (sk->sk_type == SOCK_SEQPACKET) {
+			/* We are here only when racing with unix_release_sock()
+			 * is clearing @other. Never change state to TCP_CLOSE
+			 * unlike SOCK_DGRAM wants.
+			 */
+			unix_state_unlock(sk);
+			err = -EPIPE;
+		} else if (unix_peer(sk) == other) {
 			unix_peer(sk) = NULL;
 			unix_dgram_peer_wake_disconnect_wakeup(sk, other);
 
+			sk->sk_state = TCP_CLOSE;
 			unix_state_unlock(sk);
 
-			sk->sk_state = TCP_CLOSE;
 			unix_dgram_disconnected(sk, other);
 			sock_put(other);
 			err = -ECONNREFUSED;