sctp: add the missing sock_owned_by_user check in sctp_icmp_redirect

Now sctp processes icmp redirect packet in sctp_icmp_redirect where
it calls sctp_transport_dst_check in which tp->dst can be released.

The problem is before calling sctp_transport_dst_check, it doesn't
check sock_owned_by_user, which means tp->dst could be freed while
a process is accessing it with owning the socket.

An use-after-free issue could be triggered by this.

This patch is to fix it by checking sock_owned_by_user before calling
sctp_transport_dst_check in sctp_icmp_redirect, so that it would not
release tp->dst if users still hold sock lock.

Besides, the same issue fixed in commit 45caeaa5ac0b ("dccp/tcp: fix
routing redirect race") on sctp also needs this check.

Fixes: 55be7a9c6074 ("ipv4: Add redirect support to all protocol icmp error handlers")
Reported-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/input.c

@@ -421,7 +421,7 @@ void sctp_icmp_redirect(struct sock *sk, struct sctp_transport *t,
 {
 	struct dst_entry *dst;
 
-	if (!t)
+	if (sock_owned_by_user(sk) || !t)
 		return;
 	dst = sctp_transport_dst_check(t);
 	if (dst)