[PATCH 5.11 305/601] xen-blkback: fix compatibility bug with single page rings

Greg Kroah-Hartman
2021-05-12T17:30:59Z

From: Paul Durrant <pdurrant@amazon.com>

[ Upstream commit d75e7f63b7c95c527cde42efb5d410d7f961498f ]

Prior to commit 4a8c31a1c6f5 ("xen/blkback: rework connect_ring() to avoid
inconsistent xenstore 'ring-page-order' set by malicious blkfront"), the
behaviour of xen-blkback when connecting to a frontend was:

- read 'ring-page-order'
- if not present then expect a single page ring specified by 'ring-ref'
- else expect a ring specified by 'ring-refX' where X is between 0 and
1 << ring-page-order

This was correct behaviour, but was broken by the afforementioned commit to
become:

- read 'ring-page-order'
- if not present then expect a single page ring (i.e. ring-page-order = 0)
- expect a ring specified by 'ring-refX' where X is between 0 and
1 << ring-page-order
- if that didn't work then see if there's a single page ring specified by
'ring-ref'

This incorrect behaviour works most of the time but fails when a frontend
that sets 'ring-page-order' is unloaded and replaced by one that does not
because, instead of reading 'ring-ref', xen-blkback will read the stale
'ring-ref0' left around by the previous frontend will try to map the wrong
grant reference.

This patch restores the original behaviour.

Fixes: 4a8c31a1c6f5 ("xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-order' set by malicious blkfront")
Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: Dongli Zhang <dongli.zhang@oracle.com>
Reviewed-by: "Roger Pau Monné" <roger.pau@citrix.com>
Link: https://lore.kernel.org/r/20210202175659.18452-1-paul@xen.org
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/block/xen-blkback/common.h | 1 +
drivers/block/xen-blkback/xenbus.c | 38 +++++++++++++-----------------
2 files changed, 17 insertions(+), 22 deletions(-)

diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
index b0c71d3a81a0..bda5c815e441 100644
--- a/drivers/block/xen-blkback/common.h
+++ b/drivers/block/xen-blkback/common.h
@@ -313,6 +313,7 @@ struct xen_blkif {

struct work_struct free_work;
unsigned int nr_ring_pages;
+ bool multi_ref;
/* All rings for this device. */
struct xen_blkif_ring *rings;
unsigned int nr_rings;
diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index 9860d4842f36..6c5e9373e91c 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -998,14 +998,17 @@ static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir)
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);
+ if (blkif->multi_ref)
+ snprintf(ring_ref_name, RINGREF_NAME_LEN, "ring-ref%u", i);
+ else {
+ WARN_ON(i != 0);
+ snprintf(ring_ref_name, RINGREF_NAME_LEN, "ring-ref");
+ }
+
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);
@@ -1013,18 +1016,6 @@ static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir)
}
}

- 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, "reading %s/ring-ref", dir);
- return err;
- }
- }
-
err = -ENOMEM;
for (i = 0; i < nr_grefs * XEN_BLKIF_REQS_PER_PAGE; i++) {
req = kzalloc(sizeof(*req), GFP_KERNEL);
@@ -1129,10 +1120,15 @@ static int connect_ring(struct backend_info *be)
blkif->nr_rings, blkif->blk_protocol, protocol,
blkif->vbd.feature_gnt_persistent ? "persistent grants" : "");

- ring_page_order = xenbus_read_unsigned(dev->otherend,
- "ring-page-order", 0);
-
- if (ring_page_order > xen_blkif_max_ring_order) {
+ err = xenbus_scanf(XBT_NIL, dev->otherend, "ring-page-order", "%u",
+ &ring_page_order);
+ if (err != 1) {
+ blkif->nr_ring_pages = 1;
+ blkif->multi_ref = false;
+ } else if (ring_page_order <= xen_blkif_max_ring_order) {
+ blkif->nr_ring_pages = 1 << ring_page_order;
+ blkif->multi_ref = true;
+ } else {
err = -EINVAL;
xenbus_dev_fatal(dev, err,
"requested ring page order %d exceed max:%d",
@@ -1141,8 +1137,6 @@ static int connect_ring(struct backend_info *be)
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 {
--
2.30.2