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

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Date:	Thu, 14 Jan 2016 11:12:00 +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 Mon, 11 Jan, at 06:16:14PM, Luck, Tony wrote:
> >> 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?
> 
> Looks like overkill to me.  We only use this function in two places:
> 
>         if (cper_dimm_err_location(cmem, rcd_decode_str))
>                 trace_seq_printf(p, "%s", rcd_decode_str);
> 
>         if (cper_dimm_err_location(&cmem, rcd_decode_str))
>                 printk("%s%s\n", pfx, rcd_decode_str);
> 
> Neither would care if there were garbage after the NUL and before the
> end of the rcd_decode_str[] array.
> 
> This buffer isn't visible to user space, so we aren't leaking data by having
> garbage bytes after the NUL.

What about *before* the NUL? That was the point I was trying to make.
If the string you print into the buffer isn't 'len' bytes in size the
buffer will look like,

  "DIMM location: Foo bar"<garbage.....>\0

Doing the memset() before the call to snprintf() guarantees this won't
happen and means you don't have to try and calculate where the NUL
needs to be placed.