linux-kernel - Re: [PATCH] vsprintf: ignore arguments to %n

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Message-ID: <CAGXu5jKKMim0M1cpvt4XP5YYzYvBqaSRR8k47zPDkWJCaj4FBw@mail.gmail.com>
Date:	Tue, 28 Jan 2014 13:16:59 -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: ignore arguments to %n

On Tue, Jan 28, 2014 at 10:54 AM, Ryan Mallon <rmallon@...il.com> wrote:
> On 29/01/14 09:51, Kees Cook wrote:
>
>> On Mon, Jan 27, 2014 at 5:02 PM, Ryan Mallon <rmallon@...il.com> wrote:
>>> On 28/01/14 11:39, Kees Cook wrote:
>>>> If arguments are consumed without output when encountering %n, it
>>>> could be used to benefit or improve information leak attacks that were
>>>> exposed via a limited size buffer. Since %n is not used by the kernel,
>>>> there is no reason to make an info leak attack any easier.
>>>
>>> I was thinking more like the following. Print the warning if %n is
>>> detected in format_decode(), but otherwise just remove the handling of
>>> %n outright and treat it like any other invalid format specifier.
>>> Something like this completely untested patch. Thoughts?
>>
>> I'd be totally fine with it. Minor typo in the comment before the
>> WARN_ONCE (should be "its" instead of "it"), but otherwise looks good.
>> Consider it:
>>
>> Acked-by: Kees Cook <keescook@...omium.org>
>>
>> It builds and boots fine for me, FWIW.
>>
>> -Kees
>>
>
>
> It looks like your second version already got added to Andrew's mm tree.
> I'm happy to repost mine with a fixed typo and proper signed-off by if
> you'd rather use that version.

I think yours is much cleaner: it entirely removes the %n processing logic.

-Kees

-- 
Kees Cook
Chrome OS Security
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