[PATCH v2] xen-blkback: fix compatibility bug with single page rings

[Paul Durrant <paul@xxxxxxx>]

From: Paul Durrant <pdurrant@xxxxxxxxxx>

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@xxxxxxxxxx>
---
Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: "Roger Pau Monné" <roger.pau@xxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Dongli Zhang <dongli.zhang@xxxxxxxxxx>

v2:
 - Remove now-spurious error path special-case when nr_grefs == 1
---
 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..524a79f10de6 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 {
-- 
2.17.1