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linux/drivers/block/xen-blkback
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Paul Durrant d75e7f63b7 xen-blkback: fix compatibility bug with single page rings 
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>
History 2021-04-23 09:34:07 +02:00
..
blkback.c xen: branch for v5.12-rc6 2021-03-30 08:55:47 -07:00
common.h xen-blkback: fix compatibility bug with single page rings 2021-04-23 09:34:07 +02:00
Makefile treewide: Add SPDX license identifier - Makefile/Kconfig 2019-05-21 10:50:46 +02:00
xenbus.c xen-blkback: fix compatibility bug with single page rings 2021-04-23 09:34:07 +02:00