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 <cpaasch@apple.com>
Suggested-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>

net/mptcp/protocol.c

@@ -2116,6 +2116,14 @@ 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).
  *
@@ -2160,6 +2168,9 @@ 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,
@@ -2529,12 +2540,6 @@ 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().
  */
@@ -2559,6 +2564,7 @@ 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);
 }