linux-kernel - Re: [PATCH] comedi: integer overflow in do_insnlist

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Date:	Wed, 23 Nov 2011 16:06:25 +0000
From:	Ian Abbott <abbotti@....co.uk>
To:	Dan Carpenter <dan.carpenter@...cle.com>
CC:	Xi Wang <xi.wang@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
	Mori Hess <fmhess@...rs.sourceforge.net>,
	"security@...nel.org" <security@...nel.org>,
	Lucas De Marchi <lucas.demarchi@...fusion.mobi>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Ian Abbott <ian.abbott@....co.uk>,
	Franky Lin <frankyl@...adcom.com>,
	Greg Dietsche <Gregory.Dietsche@....edu>,
	Mark Pearson <markpearson_de@...oo.de>
Subject: Re: [PATCH] comedi: integer overflow in do_insnlist_ioctl()

On 2011-11-23 14:50, Dan Carpenter wrote:
> On Wed, Nov 23, 2011 at 08:59:52AM -0500, Xi Wang wrote:
>> Thanks for the pointer.  However you cannot do the overflow check using
>>
>>    if (sizeof(struct comedi_insn) * insnlist.n_insns <  insnlist.n_insns)
>>
>> Let's assume 32-bit system, sizeof(struct comedi_insn) = 32, and
>> insnlist.n_insns = 0x7fffffff.
>>
>> Note that 32 * 0x7fffffff = 0xffffffe0 overflows but bypasses your check.
>>
>
> Argh...  You're right, my check is wrong.  What I like about my patch
> though is that it doesn't introduce an arbitrary limit.  Could you
> redo your check without the MAX_INSNS?

Could use something like:

	if (insnlist.n_insns <= ULONG_MAX / sizeof(struct comedi_insn))
		insns =
			kmalloc(sizeof(struct comedi_insn) * insnlist.n_insns,
				GFP_KERNEL);
	if (!insns)
		...

(note that insns is initialized to NULL).

-- 
-=( Ian Abbott @ MEV Ltd.    E-mail: <abbotti@....co.uk>        )=-
-=( Tel: +44 (0)161 477 1898   FAX: +44 (0)161 718 3587         )=-
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