linux-kernel - Re: [PATCH] drm: do not use device name as a format string

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Date:	Mon, 07 Dec 2015 11:53:01 +0200
From:	Jani Nikula <jani.nikula@...ux.intel.com>
To:	Daniel Vetter <daniel@...ll.ch>,
	Nicolas Iooss <nicolas.iooss_linux@....org>
Cc:	Terje Bergström <tbergstrom@...dia.com>,
	Alison Wang <alison.wang@...escale.com>,
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	linux-tegra@...r.kernel.org
Subject: Re: [PATCH] drm: do not use device name as a format string

On Mon, 07 Dec 2015, Daniel Vetter <daniel@...ll.ch> wrote:
> On Sun, Dec 06, 2015 at 11:16:32AM +0100, Nicolas Iooss wrote:
>> On 12/06/2015 10:35 AM, Daniel Vetter wrote:
>> >> On 11/18/2015 06:58 PM, Nicolas Iooss wrote:
>> >>> drm_dev_set_unique() formats its parameter using kvasprintf() but many
>> >>> of its callers directly pass dev_name(dev) as printf format string,
>> >>> without any format parameter.  This can cause some issues when the
>> >>> device name contains '%' characters.
>> >>>
>> >>> To avoid any potential issue, always use "%s" when using
>> >>> drm_dev_set_unique() with dev_name().
>> > 
>> > Not sure this is worth it really, normally people don't place % characters
>> > into their device names, ever. And if they do it'll blow up. There's also
>> > no security issue here since userspace can't set this name.
>> > 
>> > If the maintainers of the affected drivers don't want this I won't merge
>> > this patch.
>> 
>> Actually I had the same opinion before I began to add __printf
>> attributes and "%s" in several places in the kernel to make
>> -Wformat-security useful.  This led me to discover some funny issues
>> like the one fixed by commit 3958b79266b1 ("configfs: fix kernel
>> infoleak through user-controlled format string",
>> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=3958b79266b14729edd61daf9dfb84de45f4ec6d
>> ).  The patch I sent is in fact a very small step towards making
>> -Wformat-security useful again to detect "real" issues.
>> 
>> Of course, if you do not feel it is worth it and believe that dev_name
>> is fully controlled by trusted sources which will never introduce any %
>> character, I understand your will of not merging my patch.
>
> Ah, that's the context I was missing, that really should be in the commit
> message. If this is part of an overall effort to enable something useful
> it makes more sense to get it in.
>
> On the patch itself it feels rather funny to do a "%s", str); combo, maybe
> we should have a drm_dev_set_unique_static instead? Including kerneldoc
> explaining why there's too.

No caller of drm_dev_set_unique() actually uses the formatting for
anything... so you'd end up with drm_dev_set_unique_static() and an
orphaned drm_dev_set_unique()...

BR,
Jani.



-- 
Jani Nikula, Intel Open Source Technology Center
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