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

[Pasha Tatashin]

On Thu, May 26, 2022 at 9:34 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> 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@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);
>
> 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.

Right, thank you Matthew.

Miaohe, please store the ptep, or maybe drop this patch entirely.

Thanks,
Pasha