xen/blkback: Check device permissions before allowing OP_DISCARD

[Konrad Rzeszutek Wilk <konrad.wilk () oracle com>]

Hey,

John Haxby and Dan Carpenter recommended I ask for an CVE number here.

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.

I have a patch (and a test-case) ready (see attached). I think
I just need an CVE number and need to send the mentioned patch
to Linus?

Attachment: 0001-xen-blkback-Check-device-permissions-before-allowing.patch
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Attachment: _bin
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