sctp: hold transport instead of assoc when lookup assoc in rx path

Prior to this patch, in rx path, before calling lock_sock, it needed to
hold assoc when got it by __sctp_lookup_association, in case other place
would free/put assoc.

But in __sctp_lookup_association, it lookup and hold transport, then got
assoc by transport->assoc, then hold assoc and put transport. It means
it didn't hold transport, yet it was returned and later on directly
assigned to chunk->transport.

Without the protection of sock lock, the transport may be freed/put by
other places, which would cause a use-after-free issue.

This patch is to fix this issue by holding transport instead of assoc.
As holding transport can make sure to access assoc is also safe, and
actually it looks up assoc by searching transport rhashtable, to hold
transport here makes more sense.

Note that the function will be renamed later on on another patch.

Signed-off-by: Xin Long <[e-mail]>
Acked-by: Marcelo Ricardo Leitner <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/sctp/sctp.h

···
 struct sock *sctp_err_lookup(struct net *net, int family, struct sk_buff *,
 			     struct sctphdr *, struct sctp_association **,
 			     struct sctp_transport **);
-void sctp_err_finish(struct sock *, struct sctp_association *);
+void sctp_err_finish(struct sock *, struct sctp_transport *);
 void sctp_icmp_frag_needed(struct sock *, struct sctp_association *,
 			   struct sctp_transport *t, __u32 pmtu);
 void sctp_icmp_redirect(struct sock *, struct sctp_transport *,

net/sctp/input.c

···
 	 * bound to another interface, via SO_BINDTODEVICE, treat it as OOTB
 	 */
 	if (sk->sk_bound_dev_if && (sk->sk_bound_dev_if != af->skb_iif(skb))) {
-		if (asoc) {
-			sctp_association_put(asoc);
+		if (transport) {
+			sctp_transport_put(transport);
 			asoc = NULL;
+			transport = NULL;
 		} else {
 			sctp_endpoint_put(ep);
 			ep = NULL;
···
 	bh_unlock_sock(sk);
 
 	/* Release the asoc/ep ref we took in the lookup calls. */
-	if (asoc)
-		sctp_association_put(asoc);
+	if (transport)
+		sctp_transport_put(transport);
 	else
 		sctp_endpoint_put(ep);
 
···
 
 discard_release:
 	/* Release the asoc/ep ref we took in the lookup calls. */
-	if (asoc)
-		sctp_association_put(asoc);
+	if (transport)
+		sctp_transport_put(transport);
 	else
 		sctp_endpoint_put(ep);
 
···
 {
 	struct sctp_chunk *chunk = SCTP_INPUT_CB(skb)->chunk;
 	struct sctp_inq *inqueue = &chunk->rcvr->inqueue;
+	struct sctp_transport *t = chunk->transport;
 	struct sctp_ep_common *rcvr = NULL;
 	int backloged = 0;
 
···
 done:
 	/* Release the refs we took in sctp_add_backlog */
 	if (SCTP_EP_TYPE_ASSOCIATION == rcvr->type)
-		sctp_association_put(sctp_assoc(rcvr));
+		sctp_transport_put(t);
 	else if (SCTP_EP_TYPE_SOCKET == rcvr->type)
 		sctp_endpoint_put(sctp_ep(rcvr));
 	else
···
 static int sctp_add_backlog(struct sock *sk, struct sk_buff *skb)
 {
 	struct sctp_chunk *chunk = SCTP_INPUT_CB(skb)->chunk;
+	struct sctp_transport *t = chunk->transport;
 	struct sctp_ep_common *rcvr = chunk->rcvr;
 	int ret;
 
···
 		 * from us
 		 */
 		if (SCTP_EP_TYPE_ASSOCIATION == rcvr->type)
-			sctp_association_hold(sctp_assoc(rcvr));
+			sctp_transport_hold(t);
 		else if (SCTP_EP_TYPE_SOCKET == rcvr->type)
 			sctp_endpoint_hold(sctp_ep(rcvr));
 		else
···
 	return sk;
 
 out:
-	sctp_association_put(asoc);
+	sctp_transport_put(transport);
 	return NULL;
 }
 
 /* Common cleanup code for icmp/icmpv6 error handler. */
-void sctp_err_finish(struct sock *sk, struct sctp_association *asoc)
+void sctp_err_finish(struct sock *sk, struct sctp_transport *t)
 {
 	bh_unlock_sock(sk);
-	sctp_association_put(asoc);
+	sctp_transport_put(t);
 }
 
 /*
···
 	}
 
 out_unlock:
-	sctp_err_finish(sk, asoc);
+	sctp_err_finish(sk, transport);
 }
 
 /*
···
 		goto out;
 
 	asoc = t->asoc;
-	sctp_association_hold(asoc);
 	*pt = t;
-
-	sctp_transport_put(t);
 
 out:
 	return asoc;
···
 	struct sctp_transport *transport;
 
 	if ((asoc = sctp_lookup_association(net, laddr, paddr, &transport))) {
-		sctp_association_put(asoc);
+		sctp_transport_put(transport);
 		return 1;
 	}
 

net/sctp/ipv6.c

···
 	}
 
 out_unlock:
-	sctp_err_finish(sk, asoc);
+	sctp_err_finish(sk, transport);
 out:
 	if (likely(idev != NULL))
 		in6_dev_put(idev);