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

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Message-ID: <353188b6-1fa6-0d53-b699-869a0588ee39@redhat.com>
Date:   Mon, 26 Sep 2022 10:26:52 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Pasha Tatashin <pasha.tatashin@...een.com>,
        akpm@...ux-foundation.org, corbet@....net, linux-mm@...ck.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        rick.p.edgecombe@...el.com
Subject: Re: [PATCH 1/3] mm/page_table_check: Check writable zero page in page
 table check

On 11.09.22 11:59, 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(+)
> 
> 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);

We most probably don't want that:

https://lkml.kernel.org/r/20220923113426.52871-2-david@redhat.com

-- 
Thanks,

David / dhildenb