[PATCH net] net/smc: fix panic smc_tcp_syn_recv_sock() while closing listen socket

["D. Wythe" <alibuda@linux.alibaba.com>]

From: "D. Wythe" <alibuda@linux.alibaba.com>

Consider the following scenarios:

smc_release
	smc_close_active
		write_lock_bh(&smc->clcsock->sk->sk_callback_lock);
		smc->clcsock->sk->sk_user_data = NULL;
		write_unlock_bh(&smc->clcsock->sk->sk_callback_lock);

smc_tcp_syn_recv_sock
	smc = smc_clcsock_user_data(sk);
	/* now */
	/* smc == NULL */

Hence, we may read the a NULL value in smc_tcp_syn_recv_sock(). And
since we only unset sk_user_data during smc_release, it's safe to
drop the incoming tcp reqsock.

Fixes:  ("net/smc: net/smc: Limit backlog connections"
Signed-off-by: D. Wythe <alibuda@linux.alibaba.com>
---
 net/smc/af_smc.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
index bacdd97..b4acf47 100644
--- a/net/smc/af_smc.c
+++ b/net/smc/af_smc.c
@@ -125,6 +125,8 @@ static struct sock *smc_tcp_syn_recv_sock(const struct sock *sk,
 	struct sock *child;
 
 	smc = smc_clcsock_user_data(sk);
+	if (unlikely(!smc))
+		goto drop;
 
 	if (READ_ONCE(sk->sk_ack_backlog) + atomic_read(&smc->queued_smc_hs) >
 				sk->sk_max_ack_backlog)
-- 
1.8.3.1