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

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>
static void smaps_account(struct mem_size_stats *mss, struct page *page,
		unsigned long size, bool young, bool dirty)
{
	int mapcount;

	if (PageAnon(page))
		mss->anonymous += size;

	mss->resident += size;
	/* Accumulate the size in pages that have been accessed. */
	if (young || PageReferenced(page))
		mss->referenced += size;
	mapcount = page_mapcount(page);
	if (mapcount >= 2) {
		u64 pss_delta;

		if (dirty || PageDirty(page))
			mss->shared_dirty += size;
		else
			mss->shared_clean += size;
		pss_delta = (u64)size << PSS_SHIFT;
		do_div(pss_delta, mapcount);
		mss->pss += pss_delta;
	} else {
		if (dirty || PageDirty(page))
			mss->private_dirty += size;
		else
			mss->private_clean += size;
		mss->pss += (u64)size << PSS_SHIFT;
	}
}

static void smaps_account(struct mem_size_stats *mss, struct page *page,
		unsigned long size, bool young, bool dirty)
{
	int mapcount;

	if (PageAnon(page))
		mss->anonymous += size;

	mss->resident += size;
	/* Accumulate the size in pages that have been accessed. */
	if (young || PageReferenced(page))
		mss->referenced += size;
	mapcount = page_mapcount(page);
	if (mapcount >= 2) {
		u64 pss_delta;

		if (dirty || PageDirty(page))
			mss->shared_dirty += size;
		else
			mss->shared_clean += size;
		pss_delta = (u64)size << PSS_SHIFT;
		do_div(pss_delta, mapcount);
		mss->pss += pss_delta;
	} else {
		if (dirty || PageDirty(page))
			mss->private_dirty += size;
		else
			mss->private_clean += size;
		mss->pss += (u64)size << PSS_SHIFT;
	}
}
