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

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On 9/22/23 7:59 AM, Wenjia Zhang wrote:

On 20.09.23 14:08, D. Wythe wrote:
From: "D. Wythe" <alibuda@xxxxxxxxxxxxxxxxx>

Consider the following scenarios:

        smc->clcsock->sk->sk_user_data = NULL;

    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@xxxxxxxxxxxxxxxxx>
  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) >

Hi Wenjia,

this is unfortunately not sufficient for this fix. You have to make sure that is not a life-time problem. Even so, READ_ONCE() is also needed in this case.

Life-time problem? If you means the smc will still be NULL in the future,  I don't really think so, smc is a local variable assigned by smc_clcsock_user_data.
it's either NULL or a valid and unchanged value.

And READ_ONCE() is needed indeed, considering not make too much change, maybe we can protected following

smc = smc_clcsock_user_data(sk);

with sk_callback_lock, which solves the same problem. What do you think?

Best Wishes
D. Wythe

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