netdev - Re: [PATCH] net/rds: Accept peer connection reject messages due to incompatible version

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Date:   Wed, 6 Mar 2019 16:41:00 +0800
From:   Yanjun Zhu <yanjun.zhu@...cle.com>
To:     Gerd Rausch <gerd.rausch@...cle.com>,
        Santosh Shilimkar <santosh.shilimkar@...cle.com>,
        netdev@...r.kernel.org
Cc:     David Miller <davem@...emloft.net>
Subject: Re: [PATCH] net/rds: Accept peer connection reject messages due to
 incompatible version


On 2019/3/6 15:04, Gerd Rausch wrote:
> Prior to
> commit d021fabf525ff ("rds: rdma: add consumer reject")
>
> function "rds_rdma_cm_event_handler_cmn" would always honor a rejected
> connection attempt by issuing a "rds_conn_drop".
>
> The commit mentioned above added a "break", eliminating
> the "fallthrough" case and made the "rds_conn_drop" rather conditional:
>
> Now it only happens if a "consumer defined" reject (i.e. "rdma_reject")
> carries an integer-value of "1" inside "private_data":
>
>>                 if (!conn)
>> +                       break;
>> +               err = (int *)rdma_consumer_reject_data(cm_id, event, &len);
>> +               if (!err || (err && ((*err) == RDS_RDMA_REJ_INCOMPAT))) {
>> +                       pr_warn("RDS/RDMA: conn <%pI6c, %pI6c> rejected, dropping connection\n",
>> +                               &conn->c_laddr, &conn->c_faddr);
>> +                       conn->c_proposed_version = RDS_PROTOCOL_COMPAT_VERSION;
>> +                       rds_conn_drop(conn);
>> +               }
>>                  rdsdebug("Connection rejected: %s\n",
>>                           rdma_reject_msg(cm_id, event->status));
>> +               break;
>>                  /* FALLTHROUGH */
> A number of issues are worth mentioning here:
>    #1) Previous versions of the RDS code simply rejected a connection
>        by calling "rdma_reject(cm_id, NULL, 0);"
>        So the value of the payload in "private_data" will not be "1",
>        but "0".
>
>    #2) Now the code has become dependent on host byte order and sizing.
>        If one peer is big-endian, the other is little-endian,
>        or there's a difference in sizeof(int) (e.g. ILP64 vs LP64),
>        the *err check does not work as intended.
>
>    #3) There is no check for "len" to see if the data behind *err is even valid.
>        Luckily, it appears that the "rdma_reject(cm_id, NULL, 0)" will always
>        carry 148 bytes of zeroized payload.
>        But that should probably not be relied upon here.
>
>    #4) With the added "break;",
>        we might as well drop the misleading "/* FALLTHROUGH */" comment.
>
> This commit does _not_ address issue #2, as the sender would have to
> agree on a byte order as well.
>
> Here is the sequence of messages in this observed error-scenario:
>    Host-A is pre-QoS changes (excluding the commit mentioned above)
>    Host-B is post-QoS changes (including the commit mentioned above)
>
>    #1 Host-B
>       issues a connection request via function "rds_conn_path_transition"
>       connection state transitions to "RDS_CONN_CONNECTING"
>
>    #2 Host-A
>       rejects the incompatible connection request (from #1)
>       It does so by calling "rdma_reject(cm_id, NULL, 0);"
>
>    #3 Host-B
>       receives an "RDMA_CM_EVENT_REJECTED" event (from #2)
>       But since the code is changed in the way described above,
>       it won't drop the connection here, simply because "*err == 0".
>
>    #4 Host-A
>       issues a connection request
>
>    #5 Host-B
>       receives an "RDMA_CM_EVENT_CONNECT_REQUEST" event
>       and ends up calling "rds_ib_cm_handle_connect".
>       But since the state is already in "RDS_CONN_CONNECTING"
>       (as of #1) it will end up issuing a "rdma_reject" without
>       dropping the connection:
>          if (rds_conn_state(conn) == RDS_CONN_CONNECTING) {
>              /* Wait and see - our connect may still be succeeding */
>              rds_ib_stats_inc(s_ib_connect_raced);
>          }
>          goto out;
>
>    #6 Host-A
>       receives an "RDMA_CM_EVENT_REJECTED" event (from #5),
>       drops the connection and tries again (goto #4) until it gives up.
>
> Orabug: 29444532

This is the internal bug. It should be removed.

Zhu Yanjun

>
> Signed-off-by: Gerd Rausch <gerd.rausch@...cle.com>
> ---
>   net/rds/rdma_transport.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/net/rds/rdma_transport.c b/net/rds/rdma_transport.c
> index 46bce8389066..f628e7fda66d 100644
> --- a/net/rds/rdma_transport.c
> +++ b/net/rds/rdma_transport.c
> @@ -112,7 +112,7 @@ static int rds_rdma_cm_event_handler_cmn(struct rdma_cm_id *cm_id,
>   		if (!conn)
>   			break;
>   		err = (int *)rdma_consumer_reject_data(cm_id, event, &len);
> -		if (!err || (err && ((*err) == RDS_RDMA_REJ_INCOMPAT))) {
> +		if (!err || (err && len >= sizeof(*err) && ((*err) <= RDS_RDMA_REJ_INCOMPAT))) {
>   			pr_warn("RDS/RDMA: conn <%pI6c, %pI6c> rejected, dropping connection\n",
>   				&conn->c_laddr, &conn->c_faddr);
>   			conn->c_proposed_version = RDS_PROTOCOL_COMPAT_VERSION;
> @@ -122,7 +122,6 @@ static int rds_rdma_cm_event_handler_cmn(struct rdma_cm_id *cm_id,
>   		rdsdebug("Connection rejected: %s\n",
>   			 rdma_reject_msg(cm_id, event->status));
>   		break;
> -		/* FALLTHROUGH */
>   	case RDMA_CM_EVENT_ADDR_ERROR:
>   	case RDMA_CM_EVENT_ROUTE_ERROR:
>   	case RDMA_CM_EVENT_CONNECT_ERROR: