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

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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>
This commit is contained in:
Dongli Zhang 2019-02-24 10:17:27 -05:00 committed by Konrad Rzeszutek Wilk
parent 5b8e432dbb
commit 4a8c31a1c6
1 changed files with 45 additions and 31 deletions
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76
drivers/block/xen-blkback/xenbus.c
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@ -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);
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/%s",
dir, ring_ref_name);
return err;
}
}
if (err != 1) {
err = xenbus_scanf(XBT_NIL, dir, "ring-ref", "%u", &ring_ref[0]);
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, "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];
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 {