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Subject[PATCH v6 2/2] xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront
Date
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)

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

Changed since v5:
* use WARN_ON() instead of BUG_ON()

drivers/block/xen-blkback/xenbus.c | 72 +++++++++++++++++++++++---------------
1 file changed, 43 insertions(+), 29 deletions(-)

diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index a4aadac..0878e55 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 {
--
2.7.4
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