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To:     Sagi Grimberg <sagi@grimberg.me>, Keith Busch <kbusch@kernel.org>
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From:   Hannes Reinecke <hare@suse.de>
Organization: SUSE Linux GmbH
Subject: Re: [PATCH v2] nvme-tcp: Check if request has started before
 processing it
Message-ID: <492b8393-fc35-f58a-3768-94632a083c93@suse.de>
Date:   Thu, 6 May 2021 17:36:32 +0200
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On 4/1/21 12:37 AM, Sagi Grimberg wrote:
> 
>>>>> It is, but in this situation, the controller is sending a second
>>>>> completion that results in a use-after-free, which makes the
>>>>> transport irrelevant. Unless there is some other flow (which is
>>>>> unclear
>>>>> to me) that causes this which is a bug that needs to be fixed rather
>>>>> than hidden with a safeguard.
>>>>>
>>>>
>>>> The kernel should not crash regardless of any network traffic that is
>>>> sent to the system.  It should not be possible to either intentionally
>>>> of mistakenly contruct packets that will deny service in this way.
>>>
>>> This is not specific to nvme-tcp. I can build an rdma or pci controller
>>> that can trigger the same crash... I saw a similar patch from Hannes
>>> implemented in the scsi level, and not the individual scsi transports..
>>
>> If scsi wants this too, this could be made generic at the blk-mq level.
>> We just need to make something like blk_mq_tag_to_rq(), but return NULL
>> if the request isn't started.
> 
> Makes sense...
> 
>>> I would also mention, that a crash is not even the scariest issue that
>>> we can see here, because if the request happened to be reused we are
>>> in the silent data corruption realm...
>>
>> If this does happen, I think we have to come up with some way to
>> mitigate it. We're not utilizing the full 16 bits of the command_id, so
>> maybe we can append something like a generation sequence number that can
>> be checked for validity.
> 
> That's actually a great idea. scsi needs unique tags so it encodes the
> hwq in the upper 16 bits giving the actual tag the lower 16 bits which
> is more than enough for a single queue. We can do the same with
> a gencnt that will increment in both submission and completion and we
> can validate against it.
> 
> This will be useful for all transports, so maintaining it in
> nvme_req(rq)->genctr and introducing a helper like:
> rq = nvme_find_tag(tagset, cqe->command_id)
> That will filter genctr, locate the request.
> 
> Also:
> nvme_validate_request_gen(rq, cqe->command_id) that would
> compare against it.
> 
> 
> And then a helper to set the command_id like:
> cmd->common.command_id = nvme_request_command_id(rq)
> that will both increment the genctr and build a command_id
> from it.
> 
> Thoughts?
> 

Well, that would require a modification to the CQE specification, no?
fmds was not amused when I proposed that :-(

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		        Kernel Storage Architect
hare@suse.de			               +49 911 74053 688
SUSE Software Solutions Germany GmbH, 90409 Nürnberg
GF: F. Imendörffer, HRB 36809 (AG Nürnberg)