tcp: Fix potential use-after-free due to double kfree()

Receiving ACK with a valid SYN cookie, cookie_v4_check() allocates struct
request_sock and then can allocate inet_rsk(req)->ireq_opt. After that,
tcp_v4_syn_recv_sock() allocates struct sock and copies ireq_opt to
inet_sk(sk)->inet_opt. Normally, tcp_v4_syn_recv_sock() inserts the full
socket into ehash and sets NULL to ireq_opt. Otherwise,
tcp_v4_syn_recv_sock() has to reset inet_opt by NULL and free the full
socket.

The commit 01770a16 ("tcp: fix race condition when creating child
sockets from syncookies") added a new path, in which more than one cores
create full sockets for the same SYN cookie. Currently, the core which
loses the race frees the full socket without resetting inet_opt, resulting
in that both sock_put() and reqsk_put() call kfree() for the same memory:

  sock_put
    sk_free
      __sk_free
        sk_destruct
          __sk_destruct
            sk->sk_destruct/inet_sock_destruct
              kfree(rcu_dereference_protected(inet->inet_opt, 1));

  reqsk_put
    reqsk_free
      __reqsk_free
        req->rsk_ops->destructor/tcp_v4_reqsk_destructor
          kfree(rcu_dereference_protected(inet_rsk(req)->ireq_opt, 1));

Calling kmalloc() between the double kfree() can lead to use-after-free, so
this patch fixes it by setting NULL to inet_opt before sock_put().

As a side note, this kind of issue does not happen for IPv6. This is
because tcp_v6_syn_recv_sock() clones both ipv6_opt and pktopts which
correspond to ireq_opt in IPv4.

Fixes: 01770a16 ("tcp: fix race condition when creating child sockets from syncookies")
CC: Ricardo Dias <rdias@singlestore.com>
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Reviewed-by: Benjamin Herrenschmidt <benh@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20210118055920.82516-1-kuniyu@amazon.co.jpSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/tcp_ipv4.c

@@ -1595,6 +1595,8 @@ struct sock *tcp_v4_syn_recv_sock(const struct sock *sk, struct sk_buff *skb,
 		tcp_move_syn(newtp, req);
 		ireq->ireq_opt = NULL;
 	} else {
+		newinet->inet_opt = NULL;
+
 		if (!req_unhash && found_dup_sk) {
 			/* This code path should only be executed in the
 			 * syncookie case only
@@ -1602,8 +1604,6 @@ struct sock *tcp_v4_syn_recv_sock(const struct sock *sk, struct sk_buff *skb,
 			bh_unlock_sock(newsk);
 			sock_put(newsk);
 			newsk = NULL;
-		} else {
-			newinet->inet_opt = NULL;
 		}
 	}
 	return newsk;