commit 18779149 upstream.

Since indirect descriptors are in memory shared with the frontend, the
frontend could alter the first_sect and last_sect values after they have
been validated but before they are recorded in the request.  This may
result in I/O requests that overflow the foreign page, possibly
overwriting local pages when the I/O request is executed.

When parsing indirect descriptors, only read first_sect and last_sect
once.

This is part of XSA155.
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
[ luis: backported to 3.16:
  - Use ACCESS_ONCE instead of READ_ONCE
  - Use PAGE_SIZE instead of XEN_PAGE_SIZE ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>