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

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Date:	Mon, 11 Jan 2016 18:16:14 +0000
From:	"Luck, Tony" <tony.luck@...el.com>
To:	Matt Fleming <matt@...eblueprint.co.uk>
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

>> 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.

-Tony