Re: [PATCH v3 2/3] xen/blkfront: don't take local copy of a request from the ring page

Juergen Gross
2021-09-10T10:36:50Z

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On 10.09.21 12:14, Marek Marczykowski-Górecki wrote:
> On Fri, Jul 30, 2021 at 12:38:53PM +0200, Juergen Gross wrote:
>> In order to avoid a malicious backend being able to influence the local
>> copy of a request build the request locally first and then copy it to
>> the ring page instead of doing it the other way round as today.
>>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> Reviewed-by: Jan Beulich <jbeulich@suse.com>
>> Acked-by: Roger Pau Monné <roger.pau@citrix.com>
>> ---
>> V2:
>> - init variable to avoid potential compiler warning (Jan Beulich)
>> ---
>> drivers/block/xen-blkfront.c | 25 +++++++++++++++----------
>> 1 file changed, 15 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
>> index 15e840287734..b7301006fb28 100644
>
> (...)
>
>> @@ -827,10 +832,10 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri
>> if (setup.segments)
>> kunmap_atomic(setup.segments);
>>
>> - /* Keep a private copy so we can reissue requests when recovering. */
>> - rinfo->shadow[id].req = *ring_req;
>> + /* Copy request(s) to the ring page. */
>> + *final_ring_req = *ring_req;
>
> Is this guaranteed to not be optimized by the compiler in an unsafe way
> (like, do the operation the other way around)?

I don't think the C standard allows that. AFAIK reordering writes is
allowed only between sequence points. And each external function call is
a sequence point, making such an optimization in our case illegal.

Juergen

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