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References: <20220526113350.30806-1-linmiaohe@huawei.com> <09f38cf3-282a-61fa-9916-057b01b57d30@bytedance.com>
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From:   Pasha Tatashin <pasha.tatashin@soleen.com>
Date:   Thu, 26 May 2022 09:11:07 -0400
Message-ID: <CA+CK2bC0cUACMbSDFhxxPEv2J2pgEvVk=n=Pu00OgLadjNhEMA@mail.gmail.com>
Subject: Re: [PATCH] mm/page_table_check: fix accessing unmapped ptep
To:     Qi Zheng <zhengqi.arch@bytedance.com>
Cc:     Miaohe Lin <linmiaohe@huawei.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Rientjes <rientjes@google.com>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Thu, May 26, 2022 at 9:04 AM Qi Zheng <zhengqi.arch@bytedance.com> 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?

Pasha

>
> 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