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

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Date:	Wed, 23 Nov 2011 17:50:20 +0300
From:	Dan Carpenter <dan.carpenter@...cle.com>
To:	Xi Wang <xi.wang@...il.com>
Cc:	linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org,
	Mori Hess <fmhess@...rs.sourceforge.net>, security@...nel.org,
	Lucas De Marchi <lucas.demarchi@...fusion.mobi>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Ian Abbott <abbotti@....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 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?

regards,
dan carpenter


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