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

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Message-ID: <56640B00.7040100@m4x.org>
Date:	Sun, 6 Dec 2015 11:16:32 +0100
From:	Nicolas Iooss <nicolas.iooss_linux@....org>
To:	Boris Brezillon <boris.brezillon@...e-electrons.com>,
	David Airlie <airlied@...ux.ie>,
	Jianwei Wang <jianwei.wang.chn@...il.com>,
	Alison Wang <alison.wang@...escale.com>,
	Thierry Reding <thierry.reding@...il.com>,
	Terje Bergström <tbergstrom@...dia.com>,
	dri-devel@...ts.freedesktop.org, linux-tegra@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm: do not use device name as a format string

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.

Regards,
Nicolas

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