[PATCH 5/6] xen-blkfront: make local copy of response before using it

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

Data on the shared page can be changed at any time by the backend. Make
a local copy, which is no longer controlled by the backend. And only
then access it.

This is complementary to XSA155.

CC: stable@vger.kernel.org
Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
---
 drivers/block/xen-blkfront.c | 34 +++++++++++++++++-----------------
 1 file changed, 17 insertions(+), 17 deletions(-)

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 2a8e781..3926811 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -1549,7 +1549,7 @@ static bool blkif_completion(unsigned long *id,
 static irqreturn_t blkif_interrupt(int irq, void *dev_id)
 {
 	struct request *req;
-	struct blkif_response *bret;
+	struct blkif_response bret;
 	RING_IDX i, rp;
 	unsigned long flags;
 	struct blkfront_ring_info *rinfo = (struct blkfront_ring_info *)dev_id;
@@ -1566,8 +1566,8 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id)
 	for (i = rinfo->ring.rsp_cons; i != rp; i++) {
 		unsigned long id;
 
-		bret = RING_GET_RESPONSE(&rinfo->ring, i);
-		id   = bret->id;
+		RING_COPY_RESPONSE(&rinfo->ring, i, &bret);
+		id   = bret.id;
 		/*
 		 * The backend has messed up and given us an id that we would
 		 * never have given to it (we stamp it up to BLK_RING_SIZE -
@@ -1575,39 +1575,39 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id)
 		 */
 		if (id >= BLK_RING_SIZE(info)) {
 			WARN(1, "%s: response to %s has incorrect id (%ld)\n",
-			     info->gd->disk_name, op_name(bret->operation), id);
+			     info->gd->disk_name, op_name(bret.operation), id);
 			/* We can't safely get the 'struct request' as
 			 * the id is busted. */
 			continue;
 		}
 		req  = rinfo->shadow[id].request;
 
-		if (bret->operation != BLKIF_OP_DISCARD) {
+		if (bret.operation != BLKIF_OP_DISCARD) {
 			/*
 			 * We may need to wait for an extra response if the
 			 * I/O request is split in 2
 			 */
-			if (!blkif_completion(&id, rinfo, bret))
+			if (!blkif_completion(&id, rinfo, &bret))
 				continue;
 		}
 
 		if (add_id_to_freelist(rinfo, id)) {
 			WARN(1, "%s: response to %s (id %ld) couldn't be recycled!\n",
-			     info->gd->disk_name, op_name(bret->operation), id);
+			     info->gd->disk_name, op_name(bret.operation), id);
 			continue;
 		}
 
-		if (bret->status == BLKIF_RSP_OKAY)
+		if (bret.status == BLKIF_RSP_OKAY)
 			blkif_req(req)->error = BLK_STS_OK;
 		else
 			blkif_req(req)->error = BLK_STS_IOERR;
 
-		switch (bret->operation) {
+		switch (bret.operation) {
 		case BLKIF_OP_DISCARD:
-			if (unlikely(bret->status == BLKIF_RSP_EOPNOTSUPP)) {
+			if (unlikely(bret.status == BLKIF_RSP_EOPNOTSUPP)) {
 				struct request_queue *rq = info->rq;
 				printk(KERN_WARNING "blkfront: %s: %s op failed\n",
-					   info->gd->disk_name, op_name(bret->operation));
+					   info->gd->disk_name, op_name(bret.operation));
 				blkif_req(req)->error = BLK_STS_NOTSUPP;
 				info->feature_discard = 0;
 				info->feature_secdiscard = 0;
@@ -1617,15 +1617,15 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id)
 			break;
 		case BLKIF_OP_FLUSH_DISKCACHE:
 		case BLKIF_OP_WRITE_BARRIER:
-			if (unlikely(bret->status == BLKIF_RSP_EOPNOTSUPP)) {
+			if (unlikely(bret.status == BLKIF_RSP_EOPNOTSUPP)) {
 				printk(KERN_WARNING "blkfront: %s: %s op failed\n",
-				       info->gd->disk_name, op_name(bret->operation));
+				       info->gd->disk_name, op_name(bret.operation));
 				blkif_req(req)->error = BLK_STS_NOTSUPP;
 			}
-			if (unlikely(bret->status == BLKIF_RSP_ERROR &&
+			if (unlikely(bret.status == BLKIF_RSP_ERROR &&
 				     rinfo->shadow[id].req.u.rw.nr_segments == 0)) {
 				printk(KERN_WARNING "blkfront: %s: empty %s op failed\n",
-				       info->gd->disk_name, op_name(bret->operation));
+				       info->gd->disk_name, op_name(bret.operation));
 				blkif_req(req)->error = BLK_STS_NOTSUPP;
 			}
 			if (unlikely(blkif_req(req)->error)) {
@@ -1638,9 +1638,9 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id)
 			/* fall through */
 		case BLKIF_OP_READ:
 		case BLKIF_OP_WRITE:
-			if (unlikely(bret->status != BLKIF_RSP_OKAY))
+			if (unlikely(bret.status != BLKIF_RSP_OKAY))
 				dev_dbg(&info->xbdev->dev, "Bad return from blkdev data "
-					"request: %x\n", bret->status);
+					"request: %x\n", bret.status);
 
 			break;
 		default:
-- 
git-series 0.9.1