[Xen-devel] [PATCH v5 2/2] 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>
---
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)

Changed since v4:
  * use BUG_ON() when (nr_grefs == 0) to reminder the developer
  * set err = -EINVAL before xenbus_dev_fatal()

 drivers/block/xen-blkback/xenbus.c | 69 ++++++++++++++++++++++----------------
 1 file changed, 40 insertions(+), 29 deletions(-)

diff --git a/drivers/block/xen-blkback/xenbus.c 
b/drivers/block/xen-blkback/xenbus.c
index a4aadac..f6146cd 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 +936,39 @@ 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;
+
+       BUG_ON(!nr_grefs);
+
+       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 (ring_page_order > xen_blkif_max_ring_order) {
+       if (err != 1) {
+               WARN_ON(nr_grefs != 1);
+
+               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 +1027,7 @@ 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 +1073,20 @@ 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


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