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xen-blkback: fix compatibility bug with single page rings 

[ Upstream commit d75e7f63b7c95c527cde42efb5d410d7f961498f ]

Prior to commit 4a8c31a1c6 ("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: 4a8c31a1c6 ("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>
This commit is contained in:
Paul Durrant 2021-02-02 17:56:59 +00:00 committed by Greg Kroah-Hartman
parent aafb5e38e6
commit f276d195ce
2 changed files with 17 additions and 22 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
drivers/block/xen-blkback/common.h
View File

@ -316,6 +316,7 @@ 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;

38
drivers/block/xen-blkback/xenbus.c
View File

@ -949,14 +949,17 @@ 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);
@ -964,18 +967,6 @@ 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);
@ -1079,10 +1070,15 @@ 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 = 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",
@ -1091,8 +1087,6 @@ 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 {