mptcp: dispose initial struct socket when its subflow is closed

Christoph Paasch reported following crash:
dst_release underflow
WARNING: CPU: 0 PID: 1319 at net/core/dst.c:175 dst_release+0xc1/0xd0 net/core/dst.c:175
CPU: 0 PID: 1319 Comm: syz-executor217 Not tainted 5.11.0-rc6af8e85128b4d0d24083c5cac646e891227052e0c #70
Call Trace:
 rt_cache_route+0x12e/0x140 net/ipv4/route.c:1503
 rt_set_nexthop.constprop.0+0x1fc/0x590 net/ipv4/route.c:1612
 __mkroute_output net/ipv4/route.c:2484 [inline]
...

The worker leaves msk->subflow alone even when it
happened to close the subflow ssk associated with it.

Fixes: 866f26f ("mptcp: always graft subflow socket to parent")
Closes: #157
Reported-by: Christoph Paasch <[email protected]>
Suggested-by: Paolo Abeni <[email protected]>
Acked-by: Paolo Abeni <[email protected]>
Signed-off-by: Florian Westphal <[email protected]>

net/mptcp/protocol.c

@@ -2115,6 +2115,14 @@ static struct sock *mptcp_subflow_get_retrans(const struct mptcp_sock *msk)
 	return backup;
 }
 
+static void mptcp_dispose_initial_subflow(struct mptcp_sock *msk)
+{
+	if (msk->subflow) {
+		iput(SOCK_INODE(msk->subflow));
+		msk->subflow = NULL;
+	}
+}
+
 /* subflow sockets can be either outgoing (connect) or incoming
  * (accept).
  *
@@ -2159,6 +2167,9 @@ static void __mptcp_close_ssk(struct sock *sk, struct sock *ssk,
 
 	if (ssk == msk->last_snd)
 		msk->last_snd = NULL;
+
+	if (msk->subflow && ssk == msk->subflow->sk)
+		mptcp_dispose_initial_subflow(msk);
 }
 
 void mptcp_close_ssk(struct sock *sk, struct sock *ssk,
@@ -2528,12 +2539,6 @@ static void __mptcp_destroy_sock(struct sock *sk)
 
 	might_sleep();
 
-	/* dispose the ancillatory tcp socket, if any */
-	if (msk->subflow) {
-		iput(SOCK_INODE(msk->subflow));
-		msk->subflow = NULL;
-	}
-
 	/* be sure to always acquire the join list lock, to sync vs
 	 * mptcp_finish_join().
 	 */
@@ -2558,6 +2563,7 @@ static void __mptcp_destroy_sock(struct sock *sk)
 	sk_stream_kill_queues(sk);
 	xfrm_sk_free_policy(sk);
 	sk_refcnt_debug_release(sk);
+	mptcp_dispose_initial_subflow(msk);
 	sock_put(sk);
 }