linux-kernel - Re: [PATCH 1/3] mm/page_table_check: Check writable zero page in page table check

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Date:   Mon, 12 Sep 2022 15:58:14 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "corbet@....net" <corbet@....net>,
        "pasha.tatashin@...een.com" <pasha.tatashin@...een.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH 1/3] mm/page_table_check: Check writable zero page in page
 table check

On Sun, 2022-09-11 at 09:59 +0000, Pasha Tatashin wrote:
> From: Rick Edgecombe <rick.p.edgecombe@...el.com>
> 
> 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>
> Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>
> ---
>  mm/page_table_check.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
Thanks. Should we put this at the end, in order to not add any more
BUG_ON()'s to the kernel? Or I can just send a follow up and add the
docs you asked for.