linux-kernel - Re: [PATCH] x86: mm: Check if PUD is large when validating a kernel address

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Message-ID: <51194960.5080909@redhat.com>
Date:	Mon, 11 Feb 2013 14:41:20 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Mel Gorman <mgorman@...e.de>
CC:	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] x86: mm: Check if PUD is large when validating a kernel
 address

On 02/11/2013 09:52 AM, Mel Gorman wrote:
> A user reported the following oops when a backup process read
> /proc/kcore.
>
>   BUG: unable to handle kernel paging request at ffffbb00ff33b000

> Investigation determined that the bug triggered when reading system RAM
> at the 4G mark. On this system, that was the first address using 1G pages
> for the virt->phys direct mapping so the PUD is pointing to a physical
> address, not a PMD page.  The problem is that the page table walker in
> kern_addr_valid() is not checking pud_large() and treats the physical
> address as if it was a PMD.  If it happens to look like pmd_none then it'll
> silently fail, probably returning zeros instead of real data. If the data
> happens to look like a present PMD though, it will be walked resulting in
> the oops above. This patch adds the necessary pud_large() check.
>
> Unfortunately the problem was not readily reproducible and now they are
> running the backup program without accessing /proc/kcore so the patch has
> not been validated but I think it makes sense. If reviewers agree then it
> should also be included in -stable back as far as 3.0-stable.
>
> Cc: stable@...r.kernel.org
> Signed-off-by: Mel Gorman <mgorman@...e.de>

Reviewed-by: Rik van Riel <riel@...hat.coM>

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