Re: NVMe induced NULL deref in bt_iter()

Max Gurtovoy
2017-07-02T14:37:05Z

On 7/2/2017 2:56 PM, Sagi Grimberg wrote:
>
>
> 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?)

Yes, this fixes the null deref issue.
I run some additional login/logout tests that passed too.
This fix is important also for stable kernel (with needed backports to
blk_mq_quiesce_queue/blk_mq_unquiesce_queue functions).
You can add my:
Tested-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>

Let me know if you want me to push this fix to the mailing list to save
time (can we make it to 4.12 ?)

>
> [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);
> --