x86/mm: PV superpage handling lacks sanity checks

x86/mm: PV superpage handling lacks sanity checks

MMUEXT_{,UN}MARK_SUPER fail to check the input MFN for validity before
dereferencing pointers into the superpage frame table.

Reported-by: Qinghao Tang <luodalongde@gmail.com>
get_superpage() has a similar issue.

This is CVE-2016-1570 / XSA-167.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
master commit: 47abf29a9255b2e7b94e56d66b455d0a584b68b8
master date: 2016-01-20 13:49:23 +0100