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

[Dongli Zhang <dongli.zhang@xxxxxxxxxx>]

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


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