net: mctp: hold key reference when looking up a general key

Currently, we have a race where we look up a sock through a "general"
(ie, not directly associated with the (src,dest,tag) tuple) key, then
drop the key reference while still holding the key's sock.

This change expands the key reference until we've finished using the
sock, and hence the sock reference too.

Commit message changes from Jeremy Kerr <jk@codeconstruct.com.au>.
Reported-by: NNoam Rathaus <noamr@ssd-disclosure.com>
Fixes: 73c61845 ("mctp: locking, lifetime and validity changes for sk_keys")
Signed-off-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NJeremy Kerr <jk@codeconstruct.com.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/mctp/route.c

@@ -317,8 +317,8 @@ static int mctp_frag_queue(struct mctp_sk_key *key, struct sk_buff *skb)
 
 static int mctp_route_input(struct mctp_route *route, struct sk_buff *skb)
 {
+	struct mctp_sk_key *key, *any_key = NULL;
 	struct net *net = dev_net(skb->dev);
-	struct mctp_sk_key *key;
 	struct mctp_sock *msk;
 	struct mctp_hdr *mh;
 	unsigned long f;
@@ -363,13 +363,11 @@ static int mctp_route_input(struct mctp_route *route, struct sk_buff *skb)
 			 * key for reassembly - we'll create a more specific
 			 * one for future packets if required (ie, !EOM).
 			 */
-			key = mctp_lookup_key(net, skb, MCTP_ADDR_ANY, &f);
-			if (key) {
-				msk = container_of(key->sk,
+			any_key = mctp_lookup_key(net, skb, MCTP_ADDR_ANY, &f);
+			if (any_key) {
+				msk = container_of(any_key->sk,
 						   struct mctp_sock, sk);
-				spin_unlock_irqrestore(&key->lock, f);
-				mctp_key_unref(key);
-				key = NULL;
+				spin_unlock_irqrestore(&any_key->lock, f);
 			}
 		}
 
@@ -475,6 +473,8 @@ static int mctp_route_input(struct mctp_route *route, struct sk_buff *skb)
 		spin_unlock_irqrestore(&key->lock, f);
 		mctp_key_unref(key);
 	}
+	if (any_key)
+		mctp_key_unref(any_key);
 out:
 	if (rc)
 		kfree_skb(skb);