On Wed, Jul 22, 2020 at 5:14 AM Kirill A. Shutemov
<kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
>
> 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.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
> Reported-by: syzbot+ed318e8b790ca72c5ad0@xxxxxxxxxxxxxxxxxxxxxxxxx
Acked-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
I think this is worth backporting to stable as well.
> ---
> mm/khugepaged.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index b043c40a21d4..700f5160f3e4 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -958,6 +958,9 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
> return SCAN_ADDRESS_RANGE;
> if (!hugepage_vma_check(vma, vma->vm_flags))
> return SCAN_VMA_CHECK;
> + /* Anon VMA expected */
> + if (!vma->anon_vma || vma->vm_ops)
> + return SCAN_VMA_CHECK;
> return 0;
> }
>
> --
> 2.26.2
>
>
