[PATCH 1/1] xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront

[Dongli Zhang]

The xenstore 'ring-page-order' is used globally for each blkback queue and
therefore should be read from xenstore only once. However, it is obtained
in read_per_ring_refs() which might be called multiple times during the
initialization of each blkback queue.

If the blkfront is malicious and the 'ring-page-order' is set in different
value by blkfront every time before blkback reads it, this may end up at
the "WARN_ON(i != (XEN_BLKIF_REQS_PER_PAGE * blkif->nr_ring_pages));" in
xen_blkif_disconnect() when frontend is destroyed.

This patch reworks connect_ring() to read xenstore 'ring-page-order' only
once.

Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
---
 drivers/block/xen-blkback/xenbus.c | 49 ++++++++++++++++++++++++--------------
 1 file changed, 31 insertions(+), 18 deletions(-)

diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index a4bc74e..4a8ce20 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -919,14 +919,15 @@ static void connect(struct backend_info *be)
 /*
  * Each ring may have multi pages, depends on "ring-page-order".
  */
-static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir)
+static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir,
+			      bool use_ring_page_order)
 {
 	unsigned int ring_ref[XENBUS_MAX_RING_GRANTS];
 	struct pending_req *req, *n;
 	int err, i, j;
 	struct xen_blkif *blkif = ring->blkif;
 	struct xenbus_device *dev = blkif->be->dev;
-	unsigned int ring_page_order, nr_grefs, evtchn;
+	unsigned int nr_grefs, evtchn;
 
 	err = xenbus_scanf(XBT_NIL, dir, "event-channel", "%u",
 			  &evtchn);
@@ -936,28 +937,18 @@ static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir)
 		return err;
 	}
 
-	err = xenbus_scanf(XBT_NIL, dev->otherend, "ring-page-order", "%u",
-			  &ring_page_order);
-	if (err != 1) {
+	nr_grefs = blkif->nr_ring_pages;
+
+	if (!use_ring_page_order) {
 		err = xenbus_scanf(XBT_NIL, dir, "ring-ref", "%u", &ring_ref[0]);
 		if (err != 1) {
 			err = -EINVAL;
 			xenbus_dev_fatal(dev, err, "reading %s/ring-ref", dir);
 			return err;
 		}
-		nr_grefs = 1;
 	} else {
 		unsigned int i;
 
-		if (ring_page_order > xen_blkif_max_ring_order) {
-			err = -EINVAL;
-			xenbus_dev_fatal(dev, err, "%s/request %d ring page order exceed max:%d",
-					 dir, ring_page_order,
-					 xen_blkif_max_ring_order);
-			return err;
-		}
-
-		nr_grefs = 1 << ring_page_order;
 		for (i = 0; i < nr_grefs; i++) {
 			char ring_ref_name[RINGREF_NAME_LEN];
 
@@ -972,7 +963,6 @@ static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir)
 			}
 		}
 	}
-	blkif->nr_ring_pages = nr_grefs;
 
 	for (i = 0; i < nr_grefs * XEN_BLKIF_REQS_PER_PAGE; i++) {
 		req = kzalloc(sizeof(*req), GFP_KERNEL);
@@ -1030,6 +1020,8 @@ static int connect_ring(struct backend_info *be)
 	size_t xspathsize;
 	const size_t xenstore_path_ext_size = 11; /* sufficient for "/queue-NNN" */
 	unsigned int requested_num_queues = 0;
+	bool use_ring_page_order = false;
+	unsigned int ring_page_order;
 
 	pr_debug("%s %s\n", __func__, dev->otherend);
 
@@ -1075,8 +1067,28 @@ static int connect_ring(struct backend_info *be)
 		 be->blkif->nr_rings, be->blkif->blk_protocol, protocol,
 		 pers_grants ? "persistent grants" : "");
 
+	err = xenbus_scanf(XBT_NIL, dev->otherend, "ring-page-order", "%u",
+			   &ring_page_order);
+
+	if (err != 1) {
+		be->blkif->nr_ring_pages = 1;
+	} else {
+		if (ring_page_order > xen_blkif_max_ring_order) {
+			err = -EINVAL;
+			xenbus_dev_fatal(dev, err,
+					 "requested ring page order %d exceed max:%d",
+					 ring_page_order,
+					 xen_blkif_max_ring_order);
+			return err;
+		}
+
+		use_ring_page_order = true;
+		be->blkif->nr_ring_pages = 1 << ring_page_order;
+	}
+
 	if (be->blkif->nr_rings == 1)
-		return read_per_ring_refs(&be->blkif->rings[0], dev->otherend);
+		return read_per_ring_refs(&be->blkif->rings[0], dev->otherend,
+					  use_ring_page_order);
 	else {
 		xspathsize = strlen(dev->otherend) + xenstore_path_ext_size;
 		xspath = kmalloc(xspathsize, GFP_KERNEL);
@@ -1088,7 +1100,8 @@ static int connect_ring(struct backend_info *be)
 		for (i = 0; i < be->blkif->nr_rings; i++) {
 			memset(xspath, 0, xspathsize);
 			snprintf(xspath, xspathsize, "%s/queue-%u", dev->otherend, i);
-			err = read_per_ring_refs(&be->blkif->rings[i], xspath);
+			err = read_per_ring_refs(&be->blkif->rings[i], xspath,
+						 use_ring_page_order);
 			if (err) {
 				kfree(xspath);
 				return err;
-- 
2.7.4