linux-kernel - Re: [PATCH] devtmpfs: mount with noexec and nosuid

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Date:	Mon, 19 Nov 2012 10:14:46 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Kay Sievers <kay@...y.org>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, ellyjones@...omium.org
Subject: Re: [PATCH] devtmpfs: mount with noexec and nosuid

On Fri, Nov 16, 2012 at 4:39 PM, Kay Sievers <kay@...y.org> wrote:
> On Sat, Nov 17, 2012 at 1:27 AM, Greg Kroah-Hartman
> <gregkh@...uxfoundation.org> wrote:
>> On Fri, Nov 16, 2012 at 04:20:16PM -0800, Kees Cook wrote:
>>> Since devtmpfs is writable, make the default noexec nosuid as well. This
>>> protects from the case of a privileged process having an arbitrary file
>>> write flaw and an argumentless arbitrary execution (i.e. it would lack
>>> the ability to run "mount -o remount,exec,suid /dev"), with a system
>>> that already has nosuid,noexec on all other writable mounts.
>>>
>>> Cc: ellyjones@...omium.org
>>> Signed-off-by: Kees Cook <keescook@...omium.org>
>>> ---
>>>  drivers/base/devtmpfs.c |    6 ++++--
>>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> Have you tested this to verify that it doesn't break anything?
>>
>> Kay, could this cause any problems that you could think of?
>
> It breaks all sorts of old, possibly outdated, stuff, that does things
> like mapping /dev/mem executable. It for sure used to break X drivers,
> that fiddle with the BIOS of cards.

Ah, yeah, you're totally right. Attempting an mmap with PROT_EXEC on
/dev/mem would be denied.

Is this something we could put behind a CONFIG?

-Kees

-- 
Kees Cook
Chrome OS Security
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