tcp: do not release socket ownership in tcp_close()

syzkaller was able to hit the WARN_ON(sock_owned_by_user(sk));
in tcp_close()

While a socket is being closed, it is very possible other
threads find it in rtnetlink dump.

tcp_get_info() will acquire the socket lock for a short amount
of time (slow = lock_sock_fast(sk)/unlock_sock_fast(sk, slow);),
enough to trigger the warning.

Fixes: 67db3e4bfbc9 ("tcp: no longer hold ehash lock while calling tcp_get_info()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -1492,6 +1492,7 @@ static inline void lock_sock(struct sock *sk)
 	lock_sock_nested(sk, 0);
 }
 
+void __release_sock(struct sock *sk);
 void release_sock(struct sock *sk);
 
 /* BH context may only use the following locking interface. */

net/core/sock.c

@@ -2317,7 +2317,7 @@ static void __lock_sock(struct sock *sk)
 	finish_wait(&sk->sk_lock.wq, &wait);
 }
 
-static void __release_sock(struct sock *sk)
+void __release_sock(struct sock *sk)
 	__releases(&sk->sk_lock.slock)
 	__acquires(&sk->sk_lock.slock)
 {

net/ipv4/tcp.c

@@ -2416,16 +2416,10 @@ adjudge_to_death:
 	sock_hold(sk);
 	sock_orphan(sk);
 
-	/* It is the last release_sock in its life. It will remove backlog. */
-	release_sock(sk);
-
-
-	/* Now socket is owned by kernel and we acquire BH lock
-	 *  to finish close. No need to check for user refs.
-	 */
 	local_bh_disable();
 	bh_lock_sock(sk);
-	WARN_ON(sock_owned_by_user(sk));
+	/* remove backlog if any, without releasing ownership. */
+	__release_sock(sk);
 
 	percpu_counter_inc(sk->sk_prot->orphan_count);
 
@@ -2494,6 +2488,7 @@ adjudge_to_death:
 out:
 	bh_unlock_sock(sk);
 	local_bh_enable();
+	release_sock(sk);
 	sock_put(sk);
 }
 EXPORT_SYMBOL(tcp_close);