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

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@xxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- 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)
- /* 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.


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