linux-kernel - Re: [PATCH] mm: Check writable zero page in page table check

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Message-ID: <CA+CK2bDjoWNw_0WC_qAhzw9BRVPgbBAcs0=sUb3qcqhyVyvaDQ@mail.gmail.com>
Date:   Mon, 5 Sep 2022 20:38:37 -0400
From:   Pasha Tatashin <pasha.tatashin@...een.com>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: Check writable zero page in page table check

Hi Rock,

Good idea to add a check for write access to zero page. Can you please
also update Documentation/mm/page_table_check.rst ?

Thank you,
Pasha

On Fri, Sep 2, 2022 at 7:31 PM Rick Edgecombe
<rick.p.edgecombe@...el.com> wrote:
>
> The zero page should remain all zero, so that it can be mapped as
> read-only for read faults of memory that should be zeroed. If it is ever
> mapped writable to userspace, it could become non-zero and so other apps
> would unexpectedly get non-zero data. So the zero page should never be
> mapped writable to userspace. Check for this condition in
> page_table_check_set().
>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
>
> ---
>
> Hi,
>
> CONFIG_PAGE_TABLE_CHECK is pretty explicit about what it checks (and
> doesn't mention the zero page), but this condition seems to fit with the
> general category of "pages mapped wrongly to userspace". I added it
> locally to help me debug something. Maybe it's more widely useful.
>
>  mm/page_table_check.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/mm/page_table_check.c b/mm/page_table_check.c
> index e2062748791a..665ece0d55d4 100644
> --- a/mm/page_table_check.c
> +++ b/mm/page_table_check.c
> @@ -102,6 +102,8 @@ static void page_table_check_set(struct mm_struct *mm, unsigned long addr,
>         if (!pfn_valid(pfn))
>                 return;
>
> +       BUG_ON(is_zero_pfn(pfn) && rw);
> +
>         page = pfn_to_page(pfn);
>         page_ext = lookup_page_ext(page);
>         anon = PageAnon(page);
>
> base-commit: b90cb1053190353cc30f0fef0ef1f378ccc063c5
> --
> 2.17.1
>