BACKPORT: pagemap: do not leak physical addresses to non-privileged userspace

(cherry pick from commit ab676b7d6fbf4b294bf198fb27ade5b0e865c7ce)

As pointed by recent post[1] on exploiting DRAM physical imperfection,
/proc/PID/pagemap exposes sensitive information which can be used to do
attacks.

This disallows anybody without CAP_SYS_ADMIN to read the pagemap.

[1] http://googleprojectzero.blogspot.com/2015/03/exploiting-dram-rowhammer-bug-to-gain.html



[ Eventually we might want to do anything more finegrained, but for now
  this is the simple model.   - Linus ]

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Acked-by: Andy Lutomirski <luto@amacapital.net>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Mark Seaborn <mseaborn@chromium.org>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 25739721
Git-commit: 19dc2c8b1ccfe130a8d2fc4093acacdf2ed1395f
Git-repo: https://android.googlesource.com/kernel/common


Change-Id: I5582c993dbeb0b7287e6d41b57e494d123d33083
Signed-off-by: Nirmal Abraham <nabrah@codeaurora.org>