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

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Message-ID: <20130911192250.GF25896@mwanda>
Date:	Wed, 11 Sep 2013 22:22:50 +0300
From:	Dan Carpenter <dan.carpenter@...cle.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	devel@...verdev.osuosl.org, Lidza Louina <lidza.louina@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Joe Perches <joe@...ches.com>,
	Tushar Behera <tushar.behera@...aro.org>
Subject: Re: [PATCH] staging: dgnc: fix potential format string flaw

On Wed, Sep 11, 2013 at 11:19:11AM -0700, Kees Cook wrote:
> 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
> 

Hm...  That's unfortunate.  The comments were shifted around so it says
it's ignored but it's not.

regards,
dan carpenter
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