linux-kernel - Re: [PATCH] staging: dgnc: fix potential format string flaw

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Message-ID: <CAGXu5j+H_RLwj9c6CKxMpaYeR9kS-qDnK+e1mOHHt9zh6vAm=w@mail.gmail.com>
Date:	Wed, 11 Sep 2013 11:19:11 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Dan Carpenter <dan.carpenter@...cle.com>
Cc:	Joe Perches <joe@...ches.com>, devel@...verdev.osuosl.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Tushar Behera <tushar.behera@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Lidza Louina <lidza.louina@...il.com>
Subject: Re: [PATCH] staging: dgnc: fix potential format string flaw

On Wed, Sep 11, 2013 at 2:31 AM, Dan Carpenter <dan.carpenter@...cle.com> wrote:
> On Tue, Sep 10, 2013 at 10:19:17PM -0700, Kees Cook wrote:
>> In the former case, format characters will get processed by the
>> sprintf logic. In the latter, they are printed as-is. In this specific
>> case, if there was a way to inject strings like "ohai %n" into the
>> msgbuf string, the former would actually attempt to resolve the %n. In
>> the simple case, this could lead to Oopses, and in the unlucky case,
>> it could allow arbitrary memory writing and execution control.
>>
>> http://en.wikipedia.org/wiki/Uncontrolled_format_string
>
> The kernel ignores %n so hopefully it can't actually write to memory.

I wish! This is not the case, though. See FORMAT_TYPE_NRCHARS in
lib/vsprintf.c's vsnprintf().

$ git grep '%n' | wc -l
111

-Kees

-- 
Kees Cook
Chrome OS Security
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