rmap: fix NULL-pointer dereference on THP munlocking

The following test case triggers NULL-pointer derefernce in
try_to_unmap_one():

	#include <fcntl.h>
	#include <stdlib.h>
	#include <unistd.h>
	#include <sys/mman.h>

	int main(int argc, char *argv[])
	{
		int fd;

		system("mount -t tmpfs -o huge=always none /mnt");
		fd = open("/mnt/test", O_CREAT | O_RDWR);
		ftruncate(fd, 2UL << 20);
		mmap(NULL, 2UL << 20, PROT_READ | PROT_WRITE,
				MAP_SHARED | MAP_FIXED | MAP_LOCKED, fd, 0);
		mmap(NULL, 2UL << 20, PROT_READ | PROT_WRITE,
				MAP_SHARED | MAP_LOCKED, fd, 0);
		munlockall();
		return 0;
	}

Apparently, there's a case when we call try_to_unmap() on huge PMDs:
it's TTU_MUNLOCK.

Let's handle this case correctly.

Fixes: c7ab0d2fdc84 ("mm: convert try_to_unmap_one() to use page_vma_mapped_walk()")
Link: http://lkml.kernel.org/r/20170302151159.30592-1-kirill.shutemov@linux.intel.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/rmap.c b/mm/rmap.c
index 2da487d6cea83b4f51db93bcbd05feaad31b927c..250635dc47b77badb2ad3ea86f6135535b0b6418 100644 (file)
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1316,12 +1316,6 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
        }
 
        while (page_vma_mapped_walk(&pvmw)) {
-               subpage = page - page_to_pfn(page) + pte_pfn(*pvmw.pte);
-               address = pvmw.address;
-
-               /* Unexpected PMD-mapped THP? */
-               VM_BUG_ON_PAGE(!pvmw.pte, page);
-
                /*
                 * If the page is mlock()d, we cannot swap it out.
                 * If it's recently referenced (perhaps page_referenced
@@ -1345,6 +1339,13 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
                                continue;
                }
 
+               /* Unexpected PMD-mapped THP? */
+               VM_BUG_ON_PAGE(!pvmw.pte, page);
+
+               subpage = page - page_to_pfn(page) + pte_pfn(*pvmw.pte);
+               address = pvmw.address;
+
+
                if (!(flags & TTU_IGNORE_ACCESS)) {
                        if (ptep_clear_flush_young_notify(vma, address,
                                                pvmw.pte)) {