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Subject[PATCH 1/1] 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>
---
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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