[PATCH 6/6] xen-blkfront: prepare request locally, only then put it on the shared ring

["Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>]

Do not reuse data which theoretically might be already modified by the
backend. This is mostly about private copy of the request
(info->shadow[id].req) - make sure the request saved there is really the
one just filled.

This is complementary to XSA155.

CC: stable@vger.kernel.org
Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
---
 drivers/block/xen-blkfront.c | 76 +++++++++++++++++++++----------------
 1 file changed, 44 insertions(+), 32 deletions(-)

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 3926811..b100b55 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -525,19 +525,16 @@ static int blkif_ioctl(struct block_device *bdev, fmode_t mode,
 
 static unsigned long blkif_ring_get_request(struct blkfront_ring_info *rinfo,
 					    struct request *req,
-					    struct blkif_request **ring_req)
+					    struct blkif_request *ring_req)
 {
 	unsigned long id;
 
-	*ring_req = RING_GET_REQUEST(&rinfo->ring, rinfo->ring.req_prod_pvt);
-	rinfo->ring.req_prod_pvt++;
-
 	id = get_id_from_freelist(rinfo);
 	rinfo->shadow[id].request = req;
 	rinfo->shadow[id].status = REQ_WAITING;
 	rinfo->shadow[id].associated_id = NO_ASSOCIATED_ID;
 
-	(*ring_req)->u.rw.id = id;
+	ring_req->u.rw.id = id;
 
 	return id;
 }
@@ -545,23 +542,28 @@ 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 = { 0 };
 	unsigned long id;
 
 	/* Fill out a communications ring structure. */
 	id = blkif_ring_get_request(rinfo, req, &ring_req);
 
-	ring_req->operation = BLKIF_OP_DISCARD;
-	ring_req->u.discard.nr_sectors = blk_rq_sectors(req);
-	ring_req->u.discard.id = id;
-	ring_req->u.discard.sector_number = (blkif_sector_t)blk_rq_pos(req);
+	ring_req.operation = BLKIF_OP_DISCARD;
+	ring_req.u.discard.nr_sectors = blk_rq_sectors(req);
+	ring_req.u.discard.id = id;
+	ring_req.u.discard.sector_number = (blkif_sector_t)blk_rq_pos(req);
 	if (req_op(req) == REQ_OP_SECURE_ERASE && info->feature_secdiscard)
-		ring_req->u.discard.flag = BLKIF_DISCARD_SECURE;
+		ring_req.u.discard.flag = BLKIF_DISCARD_SECURE;
 	else
-		ring_req->u.discard.flag = 0;
+		ring_req.u.discard.flag = 0;
+
+	/* make the request available to the backend */
+	*RING_GET_REQUEST(&rinfo->ring, rinfo->ring.req_prod_pvt) = ring_req;
+	wmb();
+	rinfo->ring.req_prod_pvt++;
 
 	/* Keep a private copy so we can reissue requests when recovering. */
-	rinfo->shadow[id].req = *ring_req;
+	rinfo->shadow[id].req = ring_req;
 
 	return 0;
 }
@@ -693,7 +695,7 @@ static void blkif_setup_extra_req(struct blkif_request *first,
 static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *rinfo)
 {
 	struct blkfront_info *info = rinfo->dev_info;
-	struct blkif_request *ring_req, *extra_ring_req = NULL;
+	struct blkif_request ring_req = { 0 }, extra_ring_req = { 0 };
 	unsigned long id, extra_id = NO_ASSOCIATED_ID;
 	bool require_extra_req = false;
 	int i;
@@ -758,16 +760,16 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri
 		 * BLKIF_OP_WRITE
 		 */
 		BUG_ON(req_op(req) == REQ_OP_FLUSH || req->cmd_flags & REQ_FUA);
-		ring_req->operation = BLKIF_OP_INDIRECT;
-		ring_req->u.indirect.indirect_op = rq_data_dir(req) ?
+		ring_req.operation = BLKIF_OP_INDIRECT;
+		ring_req.u.indirect.indirect_op = rq_data_dir(req) ?
 			BLKIF_OP_WRITE : BLKIF_OP_READ;
-		ring_req->u.indirect.sector_number = (blkif_sector_t)blk_rq_pos(req);
-		ring_req->u.indirect.handle = info->handle;
-		ring_req->u.indirect.nr_segments = num_grant;
+		ring_req.u.indirect.sector_number = (blkif_sector_t)blk_rq_pos(req);
+		ring_req.u.indirect.handle = info->handle;
+		ring_req.u.indirect.nr_segments = num_grant;
 	} else {
-		ring_req->u.rw.sector_number = (blkif_sector_t)blk_rq_pos(req);
-		ring_req->u.rw.handle = info->handle;
-		ring_req->operation = rq_data_dir(req) ?
+		ring_req.u.rw.sector_number = (blkif_sector_t)blk_rq_pos(req);
+		ring_req.u.rw.handle = info->handle;
+		ring_req.operation = rq_data_dir(req) ?
 			BLKIF_OP_WRITE : BLKIF_OP_READ;
 		if (req_op(req) == REQ_OP_FLUSH || req->cmd_flags & REQ_FUA) {
 			/*
@@ -778,15 +780,15 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri
 			 * since it is guaranteed ordered WRT previous writes.)
 			 */
 			if (info->feature_flush && info->feature_fua)
-				ring_req->operation =
+				ring_req.operation =
 					BLKIF_OP_WRITE_BARRIER;
 			else if (info->feature_flush)
-				ring_req->operation =
+				ring_req.operation =
 					BLKIF_OP_FLUSH_DISKCACHE;
 			else
-				ring_req->operation = 0;
+				ring_req.operation = 0;
 		}
-		ring_req->u.rw.nr_segments = num_grant;
+		ring_req.u.rw.nr_segments = num_grant;
 		if (unlikely(require_extra_req)) {
 			extra_id = blkif_ring_get_request(rinfo, req,
 							  &extra_ring_req);
@@ -796,7 +798,7 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri
 			 */
 			rinfo->shadow[extra_id].num_sg = 0;
 
-			blkif_setup_extra_req(ring_req, extra_ring_req);
+			blkif_setup_extra_req(&ring_req, &extra_ring_req);
 
 			/* Link the 2 requests together */
 			rinfo->shadow[extra_id].associated_id = id;
@@ -804,12 +806,12 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri
 		}
 	}
 
-	setup.ring_req = ring_req;
+	setup.ring_req = &ring_req;
 	setup.id = id;
 
 	setup.require_extra_req = require_extra_req;
 	if (unlikely(require_extra_req))
-		setup.extra_ring_req = extra_ring_req;
+		setup.extra_ring_req = &extra_ring_req;
 
 	for_each_sg(rinfo->shadow[id].sg, sg, num_sg, i) {
 		BUG_ON(sg->offset + sg->length > PAGE_SIZE);
@@ -831,10 +833,20 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri
 	if (setup.segments)
 		kunmap_atomic(setup.segments);
 
+	/* make the request available to the backend */
+	*RING_GET_REQUEST(&rinfo->ring, rinfo->ring.req_prod_pvt) = ring_req;
+	wmb();
+	rinfo->ring.req_prod_pvt++;
 	/* Keep a private copy so we can reissue requests when recovering. */
-	rinfo->shadow[id].req = *ring_req;
-	if (unlikely(require_extra_req))
-		rinfo->shadow[extra_id].req = *extra_ring_req;
+	rinfo->shadow[id].req = ring_req;
+
+	if (unlikely(require_extra_req)) {
+		*RING_GET_REQUEST(&rinfo->ring, rinfo->ring.req_prod_pvt) = extra_ring_req;
+		wmb();
+		rinfo->ring.req_prod_pvt++;
+		/* Keep a private copy so we can reissue requests when recovering. */
+		rinfo->shadow[extra_id].req = extra_ring_req;
+	}
 
 	if (new_persistent_gnts)
 		gnttab_free_grant_references(setup.gref_head);
-- 
git-series 0.9.1