Re: [PATCH linux-2.6.18] xen: mark pvscsi frontend request consumed only after last read

[Juergen Gross <jgross@suse.com>]

On 01/30/2015 03:22 PM, Jan Beulich wrote:
>>>> On 30.01.15 at 14:51, <"jgross@suse.com".non-mime.internet> wrote:
>> A request in the ring buffer mustn't be read after it has been marked
>> as consumed. Otherwise it might already have been reused by the
>> frontend without violating the ring protocol.
>>
>> To avoid inconsistencies in the backend only work on a private copy
>> of the request. This will ensure a malicious guest not being able to
>> bypass consistency checks of the backend by modifying an active
>> request.
>
> I'm not convinced we need this in this version of the driver: c/s
> 590:c4134d1a3e3f took care of reading each ring_req field just
> once.

This might be true. But the consumer index is incremented before the
last item of the request is read. This is a violation of the ring
interface: the frontend is free to put another request in this slot
while the backend is still using it.


Juergen