x86: guard against undue super page PTE creation

x86: guard against undue super page PTE creation

When optional super page support got added (commit bd1cd81d64 "x86: PV
support for hugepages"), two adjustments were missed: mod_l2_entry()
needs to consider the PSE and RW bits when deciding whether to use the
fast path, and the PSE bit must not be removed from L2_DISALLOW_MASK
unconditionally.

This is CVE-2015-7835 / XSA-148.

Reported-by: "栾尚聪(好风)" <shangcong.lsc@alibaba-inc.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>
master commit: fe360c90ea13f309ef78810f1a2b92f2ae3b30b8
master date: 2015-10-29 13:35:07 +0100

(cherry picked from commit 2d094bd87072e26ac29b07917d31fcbf13892288)

Patch-Name: CVE-2015-7835.diff

Gbp-Pq: Name CVE-2015-7835.diff