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

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Date:   Thu, 26 May 2022 14:34:25 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     akpm@...ux-foundation.org, pasha.tatashin@...een.com,
        rientjes@...gle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/page_table_check: fix accessing unmapped ptep

On Thu, May 26, 2022 at 07:33:50PM +0800, Miaohe Lin 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.
> 
> Fixes: 80110bbfbba6 ("mm/page_table_check: check entries at pmd levels")
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.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);

But ptep was mutated in the loop.  So surely this needs to be:

		pte_unmap(ptep - PTRS_PER_PTE);

or you'll be unmapping the wrong page.