netdev - [PATCH net-next 1/1] ipv6: tcp: fix potential NULL deref in tcp_v6_send

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Message-Id: <20190607192348.189876-1-edumazet@google.com>
Date:   Fri,  7 Jun 2019 12:23:48 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     "David S . Miller" <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        syzbot <syzkaller@...glegroups.com>
Subject: [PATCH net-next 1/1] ipv6: tcp: fix potential NULL deref in tcp_v6_send_reset()

syzbot found a crash in tcp_v6_send_reset() caused by my latest
change.

Problem is that if an skb has been queued to socket prequeue,
skb_dst(skb)->dev can not anymore point to the device.

Fortunately in this case the socket pointer is not NULL.

A similar issue has been fixed in commit 0f85feae6b71 ("tcp: fix
more NULL deref after prequeue changes"), I should have known better.

Fixes: 323a53c41292 ("ipv6: tcp: enable flowlabel reflection in some RST packets")
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Reported-by: syzbot <syzkaller@...glegroups.com>
---
 net/ipv6/tcp_ipv6.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index d8d18386c99a82e112dd3a1aeee01e4c328ba5d7..c1da52c7f990f2fa3e020e3f3a33934149ad225e 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -934,7 +934,7 @@ static void tcp_v6_send_reset(const struct sock *sk, struct sk_buff *skb)
 	if (!sk && !ipv6_unicast_destination(skb))
 		return;
 
-	net = dev_net(skb_dst(skb)->dev);
+	net = sk ? sock_net(sk) : dev_net(skb_dst(skb)->dev);
 #ifdef CONFIG_TCP_MD5SIG
 	rcu_read_lock();
 	hash_location = tcp_parse_md5sig_option(th);
-- 
2.22.0.rc2.383.gf4fbbf30c2-goog