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

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Message-ID: <CA+CK2bBBLjr3kGqc=zA6M5773G6gj83LB_kwwOCNuFX8YoUp+w@mail.gmail.com>
Date:   Thu, 26 May 2022 09:37:43 -0400
From:   Pasha Tatashin <pasha.tatashin@...een.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Miaohe Lin <linmiaohe@...wei.com>,
        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 9:34 AM Matthew Wilcox <willy@...radead.org> 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@...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.

Right, thank you Matthew.

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

Thanks,
Pasha