linux-kernel - Re: [PATCH] vsprintf: drop comment claiming %n is ignored

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Date:	Thu, 12 Sep 2013 00:31:12 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Jan Beulich <JBeulich@...e.com>
Cc:	Joe Perches <joe@...ches.com>, David Miller <davem@...emloft.net>,
	Eldad Zack <eldad@...refinery.com>,
	George Spelvin <linux@...izon.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dan Carpenter <dan.carpenter@...cle.com>,
	Jiri Kosina <jkosina@...e.cz>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] vsprintf: drop comment claiming %n is ignored

On Thu, Sep 12, 2013 at 12:03 AM, Jan Beulich <JBeulich@...e.com> wrote:
>>>> On 11.09.13 at 22:18, Kees Cook <keescook@...omium.org> wrote:
>> On Wed, Sep 11, 2013 at 1:06 PM, Joe Perches <joe@...ches.com> wrote:
>>> On Wed, 2013-09-11 at 12:30 -0700, Kees Cook wrote:
>>>> The %n format is not ignored, so remove the incorrect comment about it.
>>>
>>> I think it may be better to reimplement the ignoring.
>>
>> Yeah, just had a quick look, and scanf doesn't use this code at all.
>> I'd much rather remove %n again instead.
>
> Why would you want to artificially make the function diverge
> from the spec? People shouldn't be caught by surprises if at all
> possible, and one can certainly not expect people to go look at
> the comment before the function implementation to find out
> what basic (standard) features _do not_ work (one can expect
> so when trying to find out about _extensions_).

The spec, in this case, is considered harmful. Without %n, format
string flaws are at worst "just" information leaks. Being able to
downgrade potential arbitrary memory writing vulnerabilities into info
leak vulnerabilities would be a very nice improvement. As another
point of reference, even Android's libc replacement, bionic, doesn't
implement %n for the very same reasons.

Since there are so few users of %n in the kernel, it seems that this
goal is not totally outside the realm of possibility in the
short-term. The inclusion of the WARN_ON() was suggested for the very
reason of helping a potential user of %n to notice that it's not
supported.

-Kees

-- 
Kees Cook
Chrome OS Security
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