[PATCH 5.7 164/179] khugepaged: fix null-pointer dereference due to race

From: Greg Kroah-Hartman
Date: Mon Jul 27 2020 - 10:25:45 EST

From: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

commit 594cced14ad3903166c8b091ff96adac7552f0b3 upstream.

khugepaged has to drop mmap lock several times while collapsing a page.
The situation can change while the lock is dropped and we need to
re-validate that the VMA is still in place and the PMD is still subject
for collapse.

But we miss one corner case: while collapsing an anonymous pages the VMA
could be replaced with file VMA. If the file VMA doesn't have any
private pages we get NULL pointer dereference:

general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
anon_vma_lock_write include/linux/rmap.h:120 [inline]
collapse_huge_page mm/khugepaged.c:1110 [inline]
khugepaged_scan_pmd mm/khugepaged.c:1349 [inline]
khugepaged_scan_mm_slot mm/khugepaged.c:2110 [inline]
khugepaged_do_scan mm/khugepaged.c:2193 [inline]
khugepaged+0x3bba/0x5a10 mm/khugepaged.c:2238

The fix is to make sure that the VMA is anonymous in
hugepage_vma_revalidate(). The helper is only used for collapsing
anonymous pages.

Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
Reported-by: syzbot+ed318e8b790ca72c5ad0@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
Acked-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/20200722121439.44328-1-kirill.shutemov@xxxxxxxxxxxxxxx
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

mm/khugepaged.c | 3 +++
1 file changed, 3 insertions(+)

--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -873,6 +873,9 @@ static int hugepage_vma_revalidate(struc
if (!hugepage_vma_check(vma, vma->vm_flags))
+ /* Anon VMA expected */
+ if (!vma->anon_vma || vma->vm_ops)
+ return SCAN_VMA_CHECK;
return 0;