net: mctp: add an explicit reference from a mctp_sk_key to sock

Currently, we correlate the mctp_sk_key lifetime to the sock lifetime
through the sock hash/unhash operations, but this is pretty tenuous, and
there are cases where we may have a temporary reference to an unhashed
sk.

This change makes the reference more explicit, by adding a hold on the
sock when it's associated with a mctp_sk_key, released on final key
unref.

Fixes: 73c61845 ("mctp: locking, lifetime and validity changes for sk_keys")
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mctp/route.c

@@ -147,6 +147,7 @@ static struct mctp_sk_key *mctp_key_alloc(struct mctp_sock *msk,
 	key->valid = true;
 	spin_lock_init(&key->lock);
 	refcount_set(&key->refs, 1);
+	sock_hold(key->sk);
 
 	return key;
 }
@@ -165,6 +166,7 @@ void mctp_key_unref(struct mctp_sk_key *key)
 	mctp_dev_release_key(key->dev, key);
 	spin_unlock_irqrestore(&key->lock, flags);
 
+	sock_put(key->sk);
 	kfree(key);
 }
 
@@ -419,14 +421,14 @@ static int mctp_route_input(struct mctp_route *route, struct sk_buff *skb)
 			 * this function.
 			 */
 			rc = mctp_key_add(key, msk);
-			if (rc) {
-				kfree(key);
-			} else {
+			if (!rc)
 				trace_mctp_key_acquire(key);
 
-				/* we don't need to release key->lock on exit */
+			/* we don't need to release key->lock on exit, so
+			 * clean up here and suppress the unlock via
+			 * setting to NULL
+			 */
 			mctp_key_unref(key);
-			}
 			key = NULL;
 
 		} else {