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

[Hannes Reinecke]

On 3/31/21 1:28 AM, Keith Busch wrote:
> On Tue, Mar 30, 2021 at 10:34:25AM -0700, 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.
>  
>> 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.
> 
... which will be near impossible.
We can protect against crashing on invalid frames.
We can _not_ protect against maliciously crafted packets referencing any
random _existing_ tag; that's what TLS is for.

What we can do, though, is checking the 'state' field in the tcp
request, and only allow completions for commands which are in a state
allowing for completions.

Let's see if I can whip up a patch.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		           Kernel Storage Architect
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