linux-kernel - Re: [PATCH v2] nvme-tcp: Check if request has started before processing it

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Date:   Wed, 31 Mar 2021 15:37:22 -0700
From:   Sagi Grimberg <sagi@...mberg.me>
To:     Keith Busch <kbusch@...nel.org>
Cc:     "Ewan D. Milne" <emilne@...hat.com>,
        Daniel Wagner <dwagner@...e.de>,
        linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Jens Axboe <axboe@...com>, Hannes Reinecke <hare@...e.de>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v2] nvme-tcp: Check if request has started before
 processing it


>>>> 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?