af_unix: fix struct pid memory leak

commit fa0dc04d upstream.

Dmitry reported a struct pid leak detected by a syzkaller program.

Bug happens in unix_stream_recvmsg() when we break the loop when a
signal is pending, without properly releasing scm.

Fixes: b3ca9b02 ("net: fix multithreaded signal handling in unix recv routines")
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Rainer Weikusat <rweikusat@mobileactivedefense.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2: cookie is *sicob->scm, not scm]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

net/unix/af_unix.c

@@ -2100,6 +2100,7 @@ static int unix_stream_recvmsg(struct kiocb *iocb, struct socket *sock,
 
 			if (signal_pending(current)) {
 				err = sock_intr_errno(timeo);
+				scm_destroy(siocb->scm);
 				goto out;
 			}