sctp: Fix another socket race during accept/peeloff

There is a race between sctp_rcv() and sctp_accept() where we
have moved the association from the listening socket to the
accepted socket, but sctp_rcv() processing cached the old
socket and continues to use it.

The easy solution is to check for the socket mismatch once we've
grabed the socket lock.  If we hit a mis-match, that means
that were are currently holding the lock on the listening socket,
but the association is refrencing a newly accepted socket.  We need
to drop the lock on the old socket and grab the lock on the new one.

A more proper solution might be to create accepted sockets when
the new association is established, similar to TCP.  That would
eliminate the race for 1-to-1 style sockets, but it would still
existing for 1-to-many sockets where a user wished to peeloff an
association.  For now, we'll live with this easy solution as
it addresses the problem.

Reported-by: Michal Hocko <mhocko@suse.cz>
Reported-by: Karsten Keil <kkeil@suse.de>
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 13 insertions, 0 deletions

diff --git a/net/sctp/input.c b/net/sctp/input.c
index bf612d954d41..2e4a8646dbc3 100644
--- a/net/sctp/input.c
+++ b/net/sctp/input.c
@@ -249,6 +249,19 @@ int sctp_rcv(struct sk_buff *skb)
          */
         sctp_bh_lock_sock(sk);
 
+        if (sk != rcvr->sk) {
+                /* Our cached sk is different from the rcvr->sk.  This is
+                 * because migrate()/accept() may have moved the association
+                 * to a new socket and released all the sockets.  So now we
+                 * are holding a lock on the old socket while the user may
+                 * be doing something with the new socket.  Switch our veiw
+                 * of the current sk.
+                 */
+                sctp_bh_unlock_sock(sk);
+                sk = rcvr->sk;
+                sctp_bh_lock_sock(sk);
+        }
+
         if (sock_owned_by_user(sk)) {
                 SCTP_INC_STATS_BH(SCTP_MIB_IN_PKT_BACKLOG);
                 sctp_add_backlog(sk, skb);