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From:   Yang Shi <shy828301@gmail.com>
Date:   Wed, 22 Jul 2020 10:54:41 -0700
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Subject: Re: [PATCH] khugepaged: Fix null-pointer dereference due to race
To:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        syzbot+ed318e8b790ca72c5ad0@syzkaller.appspotmail.com
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On Wed, Jul 22, 2020 at 5:14 AM Kirill A. Shutemov
<kirill.shutemov@linux.intel.com> 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@linux.intel.com>
> Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
> Reported-by: syzbot+ed318e8b790ca72c5ad0@syzkaller.appspotmail.com

Acked-by: Yang Shi <yang.shi@linux.alibaba.com>

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
>
>