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

[Dust Li <dust.li@linux.alibaba.com>]

On Wed, Sep 20, 2023 at 08:08:34PM +0800, D. Wythe wrote:
>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;

Is it possible smc != NULL here
> 
> 	if (READ_ONCE(sk->sk_ack_backlog) + atomic_read(&smc->queued_smc_hs) >
But later turns to NULL in 'atomic_read(&smc->queue_smc_hs)'
> 				sk->sk_max_ack_backlog)

Seems there is still a race ?

>-- 
>1.8.3.1