linux-kernel - [PATCH 4.8 07/35] thp: fix corner case of munlock() of PTE-mapped THPs

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Date:   Wed,  7 Dec 2016 08:08:23 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Konstantin Khlebnikov <koct9i@...il.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        syzkaller <syzkaller@...glegroups.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 4.8 07/35] thp: fix corner case of munlock() of PTE-mapped THPs

4.8-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

commit 655548bf6271b212cd1e4c259da9dbe616348d38 upstream.

The following program triggers BUG() in munlock_vma_pages_range():

	// autogenerated by syzkaller (http://github.com/google/syzkaller)
	#include <sys/mman.h>

	int main()
	{
	  mmap((void*)0x20105000ul, 0xc00000ul, 0x2ul, 0x2172ul, -1, 0);
	  mremap((void*)0x201fd000ul, 0x4000ul, 0xc00000ul, 0x3ul, 0x203f0000ul);
	  return 0;
	}

The test-case constructs the situation when munlock_vma_pages_range()
finds PTE-mapped THP-head in the middle of page table and, by mistake,
skips HPAGE_PMD_NR pages after that.

As result, on the next iteration it hits the middle of PMD-mapped THP
and gets upset seeing mlocked tail page.

The solution is only skip HPAGE_PMD_NR pages if the THP was mlocked
during munlock_vma_page().  It would guarantee that the page is
PMD-mapped as we never mlock PTE-mapeed THPs.

Fixes: e90309c9f772 ("thp: allow mlocked THP again")
Link: http://lkml.kernel.org/r/20161115132703.7s7rrgmwttegcdh4@black.fi.intel.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
Cc: Konstantin Khlebnikov <koct9i@...il.com>
Cc: Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc: syzkaller <syzkaller@...glegroups.com>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 mm/mlock.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -190,10 +190,13 @@ unsigned int munlock_vma_page(struct pag
 	 */
 	spin_lock_irq(zone_lru_lock(zone));

-	nr_pages = hpage_nr_pages(page);
-	if (!TestClearPageMlocked(page))
+	if (!TestClearPageMlocked(page)) {
+		/* Potentially, PTE-mapped THP: do not skip the rest PTEs */
+		nr_pages = 1;
 		goto unlock_out;
+	}

+	nr_pages = hpage_nr_pages(page);
 	__mod_zone_page_state(zone, NR_MLOCK, -nr_pages);

 	if (__munlock_isolate_lru_page(page, true)) {