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

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>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

1 files changed, 43 insertions, 29 deletions

diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index a4aadac772e5..24896ffb04ed 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 +936,42 @@ 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)) {
+                WARN_ON(true);
+                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) {
+                        if (nr_grefs == 1)
+                                break;
+
                         err = -EINVAL;
-                        xenbus_dev_fatal(dev, err, "reading %s/ring-ref", dir);
+                        xenbus_dev_fatal(dev, err, "reading %s/%s",
+                                         dir, ring_ref_name);
                         return err;
                 }
-                nr_grefs = 1;
-        } else {
-                unsigned int i;
+        }
+
+        if (err != 1) {
+                WARN_ON(nr_grefs != 1);
 
-                if (ring_page_order > xen_blkif_max_ring_order) {
+                err = xenbus_scanf(XBT_NIL, dir, "ring-ref", "%u",
+                                   &ring_ref[0]);
+                if (err != 1) {
                         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);
+                        xenbus_dev_fatal(dev, err, "reading %s/ring-ref", dir);
                         return err;
                 }
-
-                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;
-                        }
-                }
         }
-        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 +1030,7 @@ 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 +1076,20 @@ 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 {