Date:	Mon, 27 Jan 2014 16:19:59 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Ryan Mallon <rmallon@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.cz>, Joe Perches <joe@...ches.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Olof Johansson <olof@...om.net>,
	Stepan Moskovchenko <stepanm@...eaurora.org>,
	Daniel Borkmann <dborkman@...hat.com>
Subject: Re: [PATCH] vsprintf: BUG on %n

On Mon, Jan 27, 2014 at 4:11 PM, Ryan Mallon <rmallon@...il.com> wrote:
> On 28/01/14 10:56, Kees Cook wrote:
>> On Mon, Jan 27, 2014 at 3:11 PM, Ryan Mallon <rmallon@...il.com> wrote:
>>> On 28/01/14 10:03, Kees Cook wrote:
>>>> Now that there has been a full release of the kernel, and all users
>>>> of %n have been dropped, switch to %n use triggering a BUG. Ignoring
>>>> arguments could be used to assist in information leaks if an arbitrary
>>>> format string was under the control of an attacker.
>>>
>>> Not sure I follow the reasoning. %n no longer does anything in the
>>> kernel, so there is no risk if it does manage to find its way into a
>>> printed string. BUG() is for unrecoverable errors, which this clearly isn't.
>>>
>>> Information leaks via injectable strings are still possible if an
>>> attacker can insert %x, %d, etc. %n is more problematic since it allows
>>> for code injection, which is why it got removed. %n is not however,
>>> required to get an infoleak via a format string, so I think the summary
>>> is also a bit misleading.
>>
>> Yeah, I'm a bit uncomfortable with the BUG() too. The issue with %n is
>> that it would produce no output at all to skip arguments. With other
>> things, you have to take up output space, which may be limited. How
>> about just not skipping the argument? Leave the warn_on, etc?
>
> If you are trying to catch in kernel users of %n, then the warning is
> probably fine. I don't think the presense of a %n in a format string,
> without any injection vulnerability is going to cause a problem.
>
> If you are trying to catch %n being injected by a malicious user into a
> vulnerable string then a warning is fine as long as the string doesn't
> allow code injection through some other means. I don't think you can
> easily prevent infoleaks at runtime, since any vulnerable can have %x,
> %s, or whatever injected to leak information on the stack. There was
> some work on detecting potentially vulnerable strings at compile time I
> think?

Yeah, that's in my tree. Since gcc loves reporting false positives, I
haven't wanted to try to push it into mainline. But with Fengguang
Wu's robots doing the builds, I've been trying to keep real flaws out
of the tree.

> The reason to get rid of %n is to remove the ability to escalate an
> infoleak on a vulnerable format string into code execution. Vulnerable
> strings and infoleaks via them are really a separate issue, and
> detecting %n does nothing to solve them.

Right, I totally agree.

> %n should probably just be treated the same as any other %FOO which is
> not a valid format string directive. Keeping the warning might be useful
> for kernel developers who don't know that they shouldn't be using it.
> Then again, sparse, checkpatch or code review might be the better place
> to do that.

In that regard, then, I think we should just not skip the argument.
(Imagine, as an attacker trying to find some deep value on the stack,
but using %x would fill up the buffer that is being written to. %n
doing arg-skipping would help in that case.)

I'll send a different patch.

-Kees

-- 
Kees Cook
Chrome OS Security
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