[PATCH] CVE fix for xen-blkback allowing DISCARD operations on read-only disks.

[Konrad Rzeszutek Wilk]

Please put the following patch:
 [PATCH] xen/blkback: Check device permissions before allowing

in your tree for Linus. It fixes CVE-2013-2140.

The bug is that if a system admin provides a disk (which supports
the discard aka TRIM or SCSI UNMAP) to a guest as read-only - there
are no checks done. Which means that the OS can destroy the data.

The likehood of somebody using 'ro' disks I think is small - but
there is probably one person who does it and would be unhappy that
a guest OS can destroy the underlaying data.
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