[PATCH v4] nvme: fix corruption for passthrough meta/data

[Kanchan Joshi]

On 10/13/2023 7:24 PM, Keith Busch wrote:
> On Fri, Oct 13, 2023 at 03:44:38PM +0530, Kanchan Joshi wrote:
>> On 10/13/2023 10:56 AM, Christoph Hellwig wrote:
>>> On Fri, Oct 13, 2023 at 10:44:58AM +0530, Kanchan Joshi wrote:
>>>> Changes since v3:
>>>> - Block only unprivileged user
>>>
>>> That's not really what at least I had in mind.  I'd much rather
>>> completely disable unprivileged passthrough for now as an easy
>>> backportable patch.  And then only re-enable it later in a way
>>> where it does require using SGLs for all data transfers.
>>>
>>
>> I did not get how forcing SGLs can solve the issue at hand.
>> The problem happened because (i) user specified short buffer/len, and
>> (ii) kernel allocated buffer. Whether the buffer is fed to device using
>> PRP or SGL does not seem to solve the large DMA problem.
> 
> The problem is a disconnect between the buffer size provided and the
> implied size of the command. The idea with SGL is that it requires an
> explicit buffer size, so the device will know the buffer is short and
> return an appropriate error.

Thanks for clearing this up.
It seems we will have two limitations with this approach - (i) sgl for 
the external metadata buffer, and (ii) using sgl for data-transfer will 
reduce the speed of passthrough io, perhaps more than what can happen 
using the checks. And if we make the sgl opt-in, that means leaving the 
hole for the case when this was not chosen.