net: Unpublish sk from sk_reuseport_cb before call_rcu

The "reuse->sock[]" array is shared by multiple sockets.  The going away
sk must unpublish itself from "reuse->sock[]" before making call_rcu()
call.  However, this unpublish-action is currently done after a grace
period and it may cause use-after-free.

The fix is to move reuseport_detach_sock() to sk_destruct().
Due to the above reason, any socket with sk_reuseport_cb has
to go through the rcu grace period before freeing it.

It is a rather old bug (~3 yrs).  The Fixes tag is not necessary
the right commit but it is the one that introduced the SOCK_RCU_FREE
logic and this fix is depending on it.

Fixes: a4298e45 ("net: add SOCK_RCU_FREE socket flag")
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/sock.c

@@ -1700,8 +1700,6 @@ static void __sk_destruct(struct rcu_head *head)
 		sk_filter_uncharge(sk, filter);
 		RCU_INIT_POINTER(sk->sk_filter, NULL);
 	}
-	if (rcu_access_pointer(sk->sk_reuseport_cb))
-		reuseport_detach_sock(sk);
 
 	sock_disable_timestamp(sk, SK_FLAGS_TIMESTAMP);
 
@@ -1728,7 +1726,14 @@ static void __sk_destruct(struct rcu_head *head)
 
 void sk_destruct(struct sock *sk)
 {
-	if (sock_flag(sk, SOCK_RCU_FREE))
+	bool use_call_rcu = sock_flag(sk, SOCK_RCU_FREE);
+
+	if (rcu_access_pointer(sk->sk_reuseport_cb)) {
+		reuseport_detach_sock(sk);
+		use_call_rcu = true;
+	}
+
+	if (use_call_rcu)
 		call_rcu(&sk->sk_rcu, __sk_destruct);
 	else
 		__sk_destruct(&sk->sk_rcu);