Messages in this thread | | | From | Juergen Gross <> | Subject | Re: [PATCH v3 2/3] xen/blkfront: don't take local copy of a request from the ring page | Date | Fri, 10 Sep 2021 12:36:46 +0200 |
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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 [unhandled content-type:application/pgp-keys][unhandled content-type:application/pgp-signature] | |