Re: [Xen-devel] [PATCH] xen: Add RING_COPY_RESPONSE()

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 02.05.18 at 07:29, <andr2000@xxxxxxxxx> wrote:
> On 05/01/2018 12:54 AM, Marek Marczykowski-Górecki wrote:
>> Using RING_GET_RESPONSE() on a shared ring is easy to use incorrectly
>> (i.e., by not considering that the other end may alter the data in the
>> shared ring while it is being inspected).  Safe usage of a response
>> generally requires taking a local copy.
> I do not agree with that. Copying still doesn't make all the above safe
> as nothing prevents the backend from overwriting the response while
> frontend makes its local copy.

But that's not the point here: What the frontend wants is a single, consistent
(i.e. not further changing) view of the response, i.e. avoid the multiple reads
issue addressed in XSA-155 for backends. Once it has that, it could still apply
(sanity) checks to that local copy (just like backends to for requests).

Jan


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