Re: [PATCH v2 2/3] xen/blkfront: don't take local copy of a request from the ring page
From: Roger Pau Monné <roger.pau@citrix.com>
Date: 2021-07-09 09:02:51
Also in:
lkml, xen-devel
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 <redacted>
Acked-by: Roger Pau Monné <redacted> Thanks! One unrelated comment below.
quoted hunk ↗ jump to hunk
--- 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.