netdev - Re: [PATCH net] net/smc: fix NULL sndbuf_desc in smc_cdc_tx

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Date:   Mon, 06 Mar 2023 08:38:52 -0800
From:   Alexander H Duyck <alexander.duyck@...il.com>
To:     "D. Wythe" <alibuda@...ux.alibaba.com>, kgraul@...ux.ibm.com,
        wenjia@...ux.ibm.com, jaka@...ux.ibm.com
Cc:     kuba@...nel.org, davem@...emloft.net, netdev@...r.kernel.org,
        linux-s390@...r.kernel.org, linux-rdma@...r.kernel.org
Subject: Re: [PATCH net] net/smc: fix NULL sndbuf_desc in
 smc_cdc_tx_handler()

On Mon, 2023-03-06 at 11:36 +0800, D. Wythe wrote:
> From: "D. Wythe" <alibuda@...ux.alibaba.com>
> 
> When performing a stress test on SMC-R by rmmod mlx5_ib driver
> during the wrk/nginx test, we found that there is a probability
> of triggering a panic while terminating all link groups.
> 
> This issue dues to the race between smc_smcr_terminate_all()
> and smc_buf_create().
> 
> 			smc_smcr_terminate_all
> 
> smc_buf_create
> /* init */
> conn->sndbuf_desc = NULL;
> ...
> 
> 			__smc_lgr_terminate
> 				smc_conn_kill
> 					smc_close_abort
> 						smc_cdc_get_slot_and_msg_send
> 
> 			__softirqentry_text_start
> 				smc_wr_tx_process_cqe
> 					smc_cdc_tx_handler
> 						READ(conn->sndbuf_desc->len);
> 						/* panic dues to NULL sndbuf_desc */
> 
> conn->sndbuf_desc = xxx;
> 
> This patch tries to fix the issue by always to check the sndbuf_desc
> before send any cdc msg, to make sure that no null pointer is
> seen during cqe processing.
> 
> Fixes: 0b29ec643613 ("net/smc: immediate termination for SMCR link groups")
> Signed-off-by: D. Wythe <alibuda@...ux.alibaba.com>

Looking at the code for __smc_buf_create it seems like you might have
more issues hiding in the code. From what I can tell smc_buf_get_slot
can only return a pointer or NULL but it is getting checked for being
being a PTR_ERR or IS_ERR in several spots that are likely all dead
code.

> ---
>  net/smc/smc_cdc.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/net/smc/smc_cdc.c b/net/smc/smc_cdc.c
> index 53f63bf..2f0e2ee 100644
> --- a/net/smc/smc_cdc.c
> +++ b/net/smc/smc_cdc.c
> @@ -114,6 +114,9 @@ int smc_cdc_msg_send(struct smc_connection *conn,
>  	union smc_host_cursor cfed;
>  	int rc;
>  
> +	if (unlikely(!READ_ONCE(conn->sndbuf_desc)))
> +		return -EINVAL;
> +

This return value doesn't seem right to me. Rather than en EINVAL
should this be something like a ENOBUFS just to make it easier to debug
when this issue is encountered?

>  	smc_cdc_add_pending_send(conn, pend);
>  
>  	conn->tx_cdc_seq++;