| Message ID | 20210708124345.10173-3-jgross@suse.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | xen: harden blkfront against malicious backends | expand |
On Thu, Jul 08, 2021 at 02:43:44PM +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@citrx.com> Thanks! One unrelated comment below. > --- > 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 86356014d35e..80701860870a 100644 > --- a/drivers/block/xen-blkfront.c > +++ b/drivers/block/xen-blkfront.c > @@ -546,7 +546,7 @@ static unsigned long blkif_ring_get_request(struct blkfront_ring_info *rinfo, > rinfo->shadow[id].status = REQ_WAITING; > rinfo->shadow[id].associated_id = NO_ASSOCIATED_ID; > > - (*ring_req)->u.rw.id = id; > + rinfo->shadow[id].req.u.rw.id = id; > > return id; > } > @@ -554,11 +554,12 @@ static unsigned long blkif_ring_get_request(struct blkfront_ring_info *rinfo, > static int blkif_queue_discard_req(struct request *req, struct blkfront_ring_info *rinfo) > { > struct blkfront_info *info = rinfo->dev_info; > - struct blkif_request *ring_req; > + struct blkif_request *ring_req, *final_ring_req; > unsigned long id; > > /* Fill out a communications ring structure. */ > - id = blkif_ring_get_request(rinfo, req, &ring_req); > + id = blkif_ring_get_request(rinfo, req, &final_ring_req); > + ring_req = &rinfo->shadow[id].req; > > ring_req->operation = BLKIF_OP_DISCARD; > ring_req->u.discard.nr_sectors = blk_rq_sectors(req); > @@ -569,8 +570,8 @@ static int blkif_queue_discard_req(struct request *req, struct blkfront_ring_inf > else > ring_req->u.discard.flag = 0; > > - /* Keep a private copy so we can reissue requests when recovering. */ > - rinfo->shadow[id].req = *ring_req; > + /* Copy the request to the ring page. */ > + *final_ring_req = *ring_req; > > return 0; > } > @@ -703,6 +704,7 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri > { > struct blkfront_info *info = rinfo->dev_info; > struct blkif_request *ring_req, *extra_ring_req = NULL; > + struct blkif_request *final_ring_req, *final_extra_ring_req = NULL; > unsigned long id, extra_id = NO_ASSOCIATED_ID; > bool require_extra_req = false; > int i; > @@ -747,7 +749,8 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri > } > > /* Fill out a communications ring structure. */ > - id = blkif_ring_get_request(rinfo, req, &ring_req); > + id = blkif_ring_get_request(rinfo, req, &final_ring_req); > + ring_req = &rinfo->shadow[id].req; > > num_sg = blk_rq_map_sg(req->q, req, rinfo->shadow[id].sg); > num_grant = 0; > @@ -798,7 +801,9 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri > ring_req->u.rw.nr_segments = num_grant; > if (unlikely(require_extra_req)) { > extra_id = blkif_ring_get_request(rinfo, req, > - &extra_ring_req); > + &final_extra_ring_req); > + extra_ring_req = &rinfo->shadow[extra_id].req; I'm slightly confused about this extra request stuff because I cannot find any check that asserts we have two empty slots on the ring before getting here (I only see a RING_FULL check in blkif_queue_rq). This is AFAIK only used on Arm when guest page size > 4KB. Roger.
On 09.07.21 10:55, Roger Pau Monné wrote: > On Thu, Jul 08, 2021 at 02:43:44PM +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@citrx.com> > > Thanks! > > One unrelated comment below. > ... >> @@ -798,7 +801,9 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri >> ring_req->u.rw.nr_segments = num_grant; >> if (unlikely(require_extra_req)) { >> extra_id = blkif_ring_get_request(rinfo, req, >> - &extra_ring_req); >> + &final_extra_ring_req); >> + extra_ring_req = &rinfo->shadow[extra_id].req; > > I'm slightly confused about this extra request stuff because I cannot > find any check that asserts we have two empty slots on the ring before > getting here (I only see a RING_FULL check in blkif_queue_rq). > > This is AFAIK only used on Arm when guest page size > 4KB. I agree that this is a bug and should be fixed. Juergen
diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c index 86356014d35e..80701860870a 100644 --- a/drivers/block/xen-blkfront.c +++ b/drivers/block/xen-blkfront.c @@ -546,7 +546,7 @@ static unsigned long blkif_ring_get_request(struct blkfront_ring_info *rinfo, rinfo->shadow[id].status = REQ_WAITING; rinfo->shadow[id].associated_id = NO_ASSOCIATED_ID; - (*ring_req)->u.rw.id = id; + rinfo->shadow[id].req.u.rw.id = id; return id; } @@ -554,11 +554,12 @@ static unsigned long blkif_ring_get_request(struct blkfront_ring_info *rinfo, static int blkif_queue_discard_req(struct request *req, struct blkfront_ring_info *rinfo) { struct blkfront_info *info = rinfo->dev_info; - struct blkif_request *ring_req; + struct blkif_request *ring_req, *final_ring_req; unsigned long id; /* Fill out a communications ring structure. */ - id = blkif_ring_get_request(rinfo, req, &ring_req); + id = blkif_ring_get_request(rinfo, req, &final_ring_req); + ring_req = &rinfo->shadow[id].req; ring_req->operation = BLKIF_OP_DISCARD; ring_req->u.discard.nr_sectors = blk_rq_sectors(req); @@ -569,8 +570,8 @@ static int blkif_queue_discard_req(struct request *req, struct blkfront_ring_inf else ring_req->u.discard.flag = 0; - /* Keep a private copy so we can reissue requests when recovering. */ - rinfo->shadow[id].req = *ring_req; + /* Copy the request to the ring page. */ + *final_ring_req = *ring_req; return 0; } @@ -703,6 +704,7 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri { struct blkfront_info *info = rinfo->dev_info; struct blkif_request *ring_req, *extra_ring_req = NULL; + struct blkif_request *final_ring_req, *final_extra_ring_req = NULL; unsigned long id, extra_id = NO_ASSOCIATED_ID; bool require_extra_req = false; int i; @@ -747,7 +749,8 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri } /* Fill out a communications ring structure. */ - id = blkif_ring_get_request(rinfo, req, &ring_req); + id = blkif_ring_get_request(rinfo, req, &final_ring_req); + ring_req = &rinfo->shadow[id].req; num_sg = blk_rq_map_sg(req->q, req, rinfo->shadow[id].sg); num_grant = 0; @@ -798,7 +801,9 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri ring_req->u.rw.nr_segments = num_grant; if (unlikely(require_extra_req)) { extra_id = blkif_ring_get_request(rinfo, req, - &extra_ring_req); + &final_extra_ring_req); + extra_ring_req = &rinfo->shadow[extra_id].req; + /* * Only the first request contains the scatter-gather * list. @@ -840,10 +845,10 @@ 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; if (unlikely(require_extra_req)) - rinfo->shadow[extra_id].req = *extra_ring_req; + *final_extra_ring_req = *extra_ring_req; if (new_persistent_gnts) gnttab_free_grant_references(setup.gref_head);