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

From: Qi Zheng
Date: Thu May 26 2022 - 09:23:34 EST




On 2022/5/26 9:11 PM, Pasha Tatashin wrote:
On Thu, May 26, 2022 at 9:04 AM Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx> wrote:



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.

pte_unmap() is needed only with CONFIG_HIGHPTE. I do not see this
config for riskv-32?

My bad, but it's better to call pte_unmap() after the check is done.


Pasha


So:

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


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

--
Thanks,
Qi