linux-kernel - Re: [PATCH] efi: fix out-of-bounds null overwrite vulnerability

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Message-ID: <20160111141605.GC2644@codeblueprint.co.uk>
Date:	Mon, 11 Jan 2016 14:16:05 +0000
From:	Matt Fleming <matt@...eblueprint.co.uk>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	Insu Yun <wuninsu@...il.com>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"taesoo@...ech.edu" <taesoo@...ech.edu>,
	"yeongjin.jang@...ech.edu" <yeongjin.jang@...ech.edu>,
	"insu@...ech.edu" <insu@...ech.edu>,
	"changwoo@...ech.edu" <changwoo@...ech.edu>
Subject: Re: [PATCH] efi: fix out-of-bounds null overwrite vulnerability

On Fri, 08 Jan, at 04:47:17PM, Luck, Tony wrote:
> > But this function doesn't use snprintf(), it uses scnprintf() which
> > returns the number of characters written into buf and, because
> > scnprintf() largely follows vnsprintf(), it will never write more than
> > 'size' bytes into the buffer.
> 
>         if (bank && device)
>                 n = snprintf(msg, len, "DIMM location: %s %s ", bank, device);
> 
> That looks like "snprintf", not "scnprintf" to me :-)
 
Oops! Can you believe I looked at the wrong function?

> What about using:
> 
> 	msg[len] = '\0';
> 
> to guarantee NUL termination?

But that may leave garbage bytes in 'rcd_decode_str' in the case where
the string isn't as long as 'len'.

How about memset()'ing the buffer to zero and deleting the NUL
termination line?