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

[Juergen Gross]

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>
> > ---
> > 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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