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

[Qi Zheng <zhengqi.arch@bytedance.com>]

On 2022/5/26 7:33 PM, 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.

In the beginning, page_table_check only supported x86_64, so there
is no problem. But then the commit 3fee229a8eb9 ("riscv/mm: enable
ARCH_SUPPORTS_PAGE_TABLE_CHECK") added support for riscv-32, it is 
indeed a problem in this case.

So:

Reviewed-by: Qi Zheng <zhengqi.arch@bytedance.com>

> 
> Fixes: 80110bbfbba6 ("mm/page_table_check: check entries at pmd levels")
> Signed-off-by: Miaohe Lin <linmiaohe@huawei.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);
>   	}
>   }

-- 
Thanks,
Qi