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 03:32 PM, Jan Beulich wrote:
>>>> On 30.01.15 at 15:22, <JBeulich@suse.com> 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.
>
> I should have clarified that I didn't mean we don't need to change
> anything here: We should still move down the point where the
> ring slot gets accounted as consumed.

My solution is more robust, I think. You don't have to be careful not
to introduce another double read somewhere.

Juergen