Subject: Re: [PATCH net] net/smc: fix application data exception

From: Wenjia Zhang <[email-masked]>
Date: Wed, 15 Feb 2023 10:27:55 +0100

On 26.11.22 09:22, D.Wythe wrote:
> From: "D. Wythe" <[email-masked]>
> 
> There is a certain probability that following
> exceptions will occur in the wrk benchmark test:
> 
> Running 10s test @ http://11.213.45.6:80
>    8 threads and 64 connections
>    Thread Stats   Avg      Stdev     Max   +/- Stdev
>      Latency     3.72ms   13.94ms 245.33ms   94.17%
>      Req/Sec     1.96k   713.67     5.41k    75.16%
>    155262 requests in 10.10s, 23.10MB read
> Non-2xx or 3xx responses: 3
> 
> We will find that the error is HTTP 400 error, which is a serious
> exception in our test, which means the application data was
> corrupted.
> 
> Consider the following scenarios:
> 
> CPU0                            CPU1
> 
> buf_desc->used = 0;
>                                  cmpxchg(buf_desc->used, 0, 1)
>                                  deal_with(buf_desc)
> 
> memset(buf_desc->cpu_addr,0);
> 
> This will cause the data received by a victim connection to be cleared,
> thus triggering an HTTP 400 error in the server.
> 
> This patch exchange the order between clear used and memset, add
> barrier to ensure memory consistency.
> 
> Fixes: 1c5526968e27 ("net/smc: Clear memory when release and reuse buffer")
> Signed-off-by: D. Wythe <[email-masked]>
> ---
>   net/smc/smc_core.c | 17 ++++++++---------
>   1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/net/smc/smc_core.c b/net/smc/smc_core.c
> index c305d8d..c19d4b7 100644
> --- a/net/smc/smc_core.c
> +++ b/net/smc/smc_core.c
> @@ -1120,8 +1120,9 @@ static void smcr_buf_unuse(struct smc_buf_desc *buf_desc, bool is_rmb,
>   
>   		smc_buf_free(lgr, is_rmb, buf_desc);
>   	} else {
> -		buf_desc->used = 0;
> -		memset(buf_desc->cpu_addr, 0, buf_desc->len);
> +		/* memzero_explicit provides potential memory barrier semantics */
> +		memzero_explicit(buf_desc->cpu_addr, buf_desc->len);
> +		WRITE_ONCE(buf_desc->used, 0);
>   	}
>   }
>   
> @@ -1132,19 +1133,17 @@ static void smc_buf_unuse(struct smc_connection *conn,
>   		if (!lgr->is_smcd && conn->sndbuf_desc->is_vm) {
>   			smcr_buf_unuse(conn->sndbuf_desc, false, lgr);
>   		} else {
> -			conn->sndbuf_desc->used = 0;
> -			memset(conn->sndbuf_desc->cpu_addr, 0,
> -			       conn->sndbuf_desc->len);
> +			memzero_explicit(conn->sndbuf_desc->cpu_addr, conn->sndbuf_desc->len);
> +			WRITE_ONCE(conn->sndbuf_desc->used, 0);
>   		}
>   	}
>   	if (conn->rmb_desc) {
>   		if (!lgr->is_smcd) {
>   			smcr_buf_unuse(conn->rmb_desc, true, lgr);
>   		} else {
> -			conn->rmb_desc->used = 0;
> -			memset(conn->rmb_desc->cpu_addr, 0,
> -			       conn->rmb_desc->len +
> -			       sizeof(struct smcd_cdc_msg));
> +			memzero_explicit(conn->rmb_desc->cpu_addr,
> +					 conn->rmb_desc->len + sizeof(struct smcd_cdc_msg));
> +			WRITE_ONCE(conn->rmb_desc->used, 0);
>   		}
>   	}
>   }

Hi David,

Thank you for remembering me again about this patch. I did forget to 
answer you, sorry!

My consideration was if memzero_explicit() is necessary in this case. 
But sure, it makes sense, especiall when the dereferencing is in 
somewhere else.

Thank you for the fix!

Reviewed-by: Wenjia Zhang <[email-masked]>