linux-kernel - Re: [PATCH] mm/page_table_check: fix accessing unmapped ptep

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Date:   Thu, 26 May 2022 09:17:14 -0400
From:   Pasha Tatashin <pasha.tatashin@...een.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/page_table_check: fix accessing unmapped ptep

On Thu, May 26, 2022 at 7:33 AM Miaohe Lin <linmiaohe@...wei.com> wrote:
>
> ptep is unmapped too early, so ptep will be accessed while it's unmapped.
> Fix it by deferring pte_unmap() until page table checking is done.

I would re-word this as a cleanup. While pte_unmap() is currently
unused, it is still better to call it after we are done with *ptep in
case of future changes in other architectures.

>
> Fixes: 80110bbfbba6 ("mm/page_table_check: check entries at pmd levels")

This is more a clean-up, there is no existing bug, so no need to
backport to stable. Please remove the above.

> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>

With the above changes:
Acked-by: Pasha Tatashin <pasha.tatashin@...een.com>


> ---
>  mm/page_table_check.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/page_table_check.c b/mm/page_table_check.c
> index 3692bea2ea2c..971c3129b0e3 100644
> --- a/mm/page_table_check.c
> +++ b/mm/page_table_check.c
> @@ -234,11 +234,11 @@ void __page_table_check_pte_clear_range(struct mm_struct *mm,
>                 pte_t *ptep = pte_offset_map(&pmd, addr);
>                 unsigned long i;
>
> -               pte_unmap(ptep);
>                 for (i = 0; i < PTRS_PER_PTE; i++) {
>                         __page_table_check_pte_clear(mm, addr, *ptep);
>                         addr += PAGE_SIZE;
>                         ptep++;
>                 }
> +               pte_unmap(ptep);
>         }
>  }
> --
> 2.23.0
>