NVMe induced NULL deref in bt_iter()

[Sagi Grimberg]

On 02/07/17 13:45, Max Gurtovoy wrote:
> 
> 
> On 6/30/2017 8:26 PM, Jens Axboe wrote:
>> Hi Max,
> 
> Hi Jens,
> 
>>
>> I remembered you reporting this. I think this is a regression introduced
>> with the scheduling, since ->rqs[] isn't static anymore. ->static_rqs[]
>> is, but that's not indexable by the tag we find. So I think we need to
>> guard those with a NULL check. The actual requests themselves are
>> static, so we know the memory itself isn't going away. But if we race
>> with completion, we could find a NULL there, validly.
>>
>> Since you could reproduce it, can you try the below?
> 
> I still can repro the null deref with this patch applied.
> 
>>
>> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
>> index d0be72ccb091..b856b2827157 100644
>> --- a/block/blk-mq-tag.c
>> +++ b/block/blk-mq-tag.c
>> @@ -214,7 +214,7 @@ static bool bt_iter(struct sbitmap *bitmap, 
>> unsigned int bitnr, void *data)
>>          bitnr += tags->nr_reserved_tags;
>>      rq = tags->rqs[bitnr];
>>
>> -    if (rq->q == hctx->queue)
>> +    if (rq && rq->q == hctx->queue)
>>          iter_data->fn(hctx, rq, iter_data->data, reserved);
>>      return true;
>>  }
>> @@ -249,8 +249,8 @@ static bool bt_tags_iter(struct sbitmap *bitmap, 
>> unsigned int bitnr, void *data)
>>      if (!reserved)
>>          bitnr += tags->nr_reserved_tags;
>>      rq = tags->rqs[bitnr];
>> -
>> -    iter_data->fn(rq, iter_data->data, reserved);
>> +    if (rq)
>> +        iter_data->fn(rq, iter_data->data, reserved);
>>      return true;
>>  }
> 
> see the attached file for dmesg output.
> 
> output of gdb:
> 
> (gdb) list *(blk_mq_flush_busy_ctxs+0x48)
> 0xffffffff8127b108 is in blk_mq_flush_busy_ctxs 
> (./include/linux/sbitmap.h:234).
> 229
> 230             for (i = 0; i < sb->map_nr; i++) {
> 231                     struct sbitmap_word *word = &sb->map[i];
> 232                     unsigned int off, nr;
> 233
> 234                     if (!word->word)
> 235                             continue;
> 236
> 237                     nr = 0;
> 238                     off = i << sb->shift;
> 
> 
> when I change the "if (!word->word)" to  "if (word && !word->word)"
> I can get null deref at "nr = find_next_bit(&word->word, word->depth, 
> nr);". Seems like somehow word becomes NULL.
> 
> Adding the linux-nvme guys too.
> Sagi has mentioned that this can be null only if we remove the tagset 
> while I/O is trying to get a tag and when killing the target we get into
> error recovery and periodic reconnects, which does _NOT_ include freeing
> the tagset, so this is probably the admin tagset.
> 
> Sagi,
> you've mention a patch for centrelizing the treatment of the admin 
> tagset to the nvme core. I think I missed this patch, so can you please 
> send a pointer to it and I'll check if it helps ?

Hmm,

In the above flow we should not be freeing the tag_set, not on admin as
well. The target keep removing namespaces and finally removes the
subsystem which generates a error recovery flow. What we at least try
to do is:

1. mark rdma queues as not live
2. stop all the sw queues (admin and io)
3. fail inflight I/Os
4. restart all sw queues (to fast fail until we recover)

We shouldn't be freeing the tagsets (although we might update them
when we recover and cpu map changed - which I don't think is happening).

However, I do see a difference between bt_tags_for_each
and blk_mq_flush_busy_ctxs (checks tags->rqs not being NULL).

Unrelated to this I think we should quiesce/unquiesce the admin_q
instead of stop/start because it respects the submission path rcu [1].

It might hide the issue, but given that we never free the tagset its
seems like it's not in nvme-rdma (max, can you see if this makes the
issue go away?)

[1]:
--
diff --git a/drivers/nvme/host/rdma.c b/drivers/nvme/host/rdma.c
index e3996db22738..094873a4ee38 100644
--- a/drivers/nvme/host/rdma.c
+++ b/drivers/nvme/host/rdma.c
@@ -785,7 +785,7 @@ static void nvme_rdma_error_recovery_work(struct 
work_struct *work)

         if (ctrl->ctrl.queue_count > 1)
                 nvme_stop_queues(&ctrl->ctrl);
-       blk_mq_stop_hw_queues(ctrl->ctrl.admin_q);
+       blk_mq_quiesce_queue(ctrl->ctrl.admin_q);

         /* We must take care of fastfail/requeue all our inflight 
requests */
         if (ctrl->ctrl.queue_count > 1)
@@ -798,7 +798,8 @@ static void nvme_rdma_error_recovery_work(struct 
work_struct *work)
          * queues are not a live anymore, so restart the queues to fail 
fast
          * new IO
          */
-       blk_mq_start_stopped_hw_queues(ctrl->ctrl.admin_q, true);
+       blk_mq_unquiesce_queue(ctrl->ctrl.admin_q);
+       blk_mq_kick_requeue_list(ctrl->ctrl.admin_q);
         nvme_start_queues(&ctrl->ctrl);

         nvme_rdma_reconnect_or_remove(ctrl);
@@ -1651,7 +1652,7 @@ static void nvme_rdma_shutdown_ctrl(struct 
nvme_rdma_ctrl *ctrl)
         if (test_bit(NVME_RDMA_Q_LIVE, &ctrl->queues[0].flags))
                 nvme_shutdown_ctrl(&ctrl->ctrl);

-       blk_mq_stop_hw_queues(ctrl->ctrl.admin_q);
+       blk_mq_quiesce_queue(ctrl->ctrl.admin_q);
         blk_mq_tagset_busy_iter(&ctrl->admin_tag_set,
                                 nvme_cancel_request, &ctrl->ctrl);
         nvme_rdma_destroy_admin_queue(ctrl);
--