Re: [dpdk-dev] [PATCH] net/mlx5: Fix possible NULL deref in RX path

[Sagi Grimberg <sagi@grimberg.me>]

On 02/08/16 12:58, Adrien Mazarguil wrote:
> On Tue, Aug 02, 2016 at 12:31:35PM +0300, Sagi Grimberg wrote:
>>
>>
>> On 01/08/16 19:43, Adrien Mazarguil wrote:
>>> Hi Sagi,
>>>
>>> On Mon, Aug 01, 2016 at 11:44:21AM +0300, Sagi Grimberg wrote:
>>>> The user is allowed to call ->rx_pkt_burst() even without free
>>>> mbufs in the pool. In this scenario we'll fail allocating a rep mbuf
>>>> on the first iteration (where pkt is still NULL). This would cause us
>>>> to deref a NULL pkt (reset refcount and free).
>>>>
>>>> Fix this by checking the pkt before freeing it.
>>>
>>> Just to be sure, did you get an actual NULL deref crash here or is that an
>>> assumed possibility?
>>>
>>> I'm asking because this problem was supposed to be addressed by:
>>>
>>> a1bdb71a32da ("net/mlx5: fix crash in Rx")
>>
>> I actually got the NULL deref. This happens when the application doesn't
>> restore mbufs to the pool correctly. In the case rte_mbuf_raw_alloc
>> will fail on the first iteration (pkt wasn't assigned) unlike the
>> condition handled in a1bdb71a32da.
>>
>> With this applied, I didn't see the crash.
>
> Thanks for confirming this,

Hey Adrien, I just noticed that I missed the rest of
your response in the previous message (pre-coffee mail
browsing...)

You analysis was on spot.

> now what about the different approach I
> suggested in my previous message to avoid the extra check in the inner loop:
>
>  if (!pkt)
>      pkt = seg;
>  while (pkt != seg) {
>       ...
>  }

We can go this way, but it looks kinda confusing to set pkt = seg and
then iterate on pkt != seg.

How about a more explicit approach:
--
diff --git a/drivers/net/mlx5/mlx5_rxtx.c b/drivers/net/mlx5/mlx5_rxtx.c
index fce3381ae87a..37573668e43e 100644
--- a/drivers/net/mlx5/mlx5_rxtx.c
+++ b/drivers/net/mlx5/mlx5_rxtx.c
@@ -1572,6 +1572,14 @@ mlx5_rx_burst(void *dpdk_rxq, struct rte_mbuf 
**pkts, uint16_t pkts_n)
                 rte_prefetch0(wqe);
                 rep = rte_mbuf_raw_alloc(rxq->mp);
                 if (unlikely(rep == NULL)) {
+                       ++rxq->stats.rx_nombuf;
+                       if (!pkt) {
+                               /*
+                                * no buffers before we even started,
+                                * bail out silently.
+                                */
+                               break;
+                       }
                         while (pkt != seg) {
                                 assert(pkt != (*rxq->elts)[idx]);
                                 seg = NEXT(pkt);
@@ -1579,7 +1587,6 @@ mlx5_rx_burst(void *dpdk_rxq, struct rte_mbuf 
**pkts, uint16_t pkts_n)
                                 __rte_mbuf_raw_free(pkt);
                                 pkt = seg;
                         }
-                       ++rxq->stats.rx_nombuf;
                         break;
                 }
                 if (!pkt) {
--


> Also the fixes line in your commit message?

I'll add it in v2. Thanks.