[PATCH v4 2/2] xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront

[Dongli Zhang <dongli.zhang@oracle.com>]

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@oracle.com>
---
Changed since v1:
  * change the order of xenstore read in read_per_ring_refs
  * use xenbus_read_unsigned() in connect_ring()

Changed since v2:
  * simplify the condition check as "(err != 1 && nr_grefs > 1)"
  * avoid setting err as -EINVAL to remove extra one line of code

Changed since v3:
  * exit at the beginning if !nr_grefs
  * change the if statements to avoid test (err != 1) twice
  * initialize a 'blkif' stack variable (refer to PATCH 1/2)

 drivers/block/xen-blkback/xenbus.c | 76 +++++++++++++++++++++-----------------
 1 file changed, 43 insertions(+), 33 deletions(-)

diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index a4aadac..a2acbc9 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -926,7 +926,7 @@ static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir)
 	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);
@@ -936,43 +936,38 @@ 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) {
-		err = xenbus_scanf(XBT_NIL, dir, "ring-ref", "%u", &ring_ref[0]);
+	nr_grefs = blkif->nr_ring_pages;
+
+	if (unlikely(!nr_grefs))
+		return -EINVAL;
+
+	for (i = 0; i < nr_grefs; i++) {
+		char ring_ref_name[RINGREF_NAME_LEN];
+
+		snprintf(ring_ref_name, RINGREF_NAME_LEN, "ring-ref%u", i);
+		err = xenbus_scanf(XBT_NIL, dir, ring_ref_name,
+				   "%u", &ring_ref[i]);
+
 		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;
+			if (nr_grefs == 1)
+				break;
+
+			xenbus_dev_fatal(dev, err, "reading %s/%s",
+					 dir, ring_ref_name);
+			return -EINVAL;
 		}
+	}
 
-		nr_grefs = 1 << ring_page_order;
-		for (i = 0; i < nr_grefs; i++) {
-			char ring_ref_name[RINGREF_NAME_LEN];
-
-			snprintf(ring_ref_name, RINGREF_NAME_LEN, "ring-ref%u", i);
-			err = xenbus_scanf(XBT_NIL, dir, ring_ref_name,
-					   "%u", &ring_ref[i]);
-			if (err != 1) {
-				err = -EINVAL;
-				xenbus_dev_fatal(dev, err, "reading %s/%s",
-						 dir, ring_ref_name);
-				return err;
-			}
+	if (err != 1) {
+		WARN_ON(nr_grefs != 1);
+
+		err = xenbus_scanf(XBT_NIL, dir, "ring-ref", "%u",
+				   &ring_ref[0]);
+		if (err != 1) {
+			xenbus_dev_fatal(dev, err, "reading %s/ring-ref", dir);
+			return -EINVAL;
 		}
 	}
-	blkif->nr_ring_pages = nr_grefs;
 
 	for (i = 0; i < nr_grefs * XEN_BLKIF_REQS_PER_PAGE; i++) {
 		req = kzalloc(sizeof(*req), GFP_KERNEL);
@@ -1031,6 +1026,7 @@ 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;
+	unsigned int ring_page_order;
 
 	pr_debug("%s %s\n", __func__, dev->otherend);
 
@@ -1076,6 +1072,20 @@ static int connect_ring(struct backend_info *be)
 		 blkif->nr_rings, blkif->blk_protocol, protocol,
 		 pers_grants ? "persistent grants" : "");
 
+	ring_page_order = xenbus_read_unsigned(dev->otherend,
+					       "ring-page-order", 0);
+
+	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;
+	}
+
+	blkif->nr_ring_pages = 1 << ring_page_order;
+
 	if (blkif->nr_rings == 1)
 		return read_per_ring_refs(&blkif->rings[0], dev->otherend);
 	else {
-- 
2.7.4