linux-kernel - Re: Double-Fetch bug in Linux-4.5/drivers/scsi/aacraid/commctrl.c

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Date:	Wed, 27 Apr 2016 11:02:43 +0300
From:	Dan Carpenter <dan.carpenter@...cle.com>
To:	Julia Lawall <julia.lawall@...6.fr>
Cc:	Kees Cook <keescook@...gle.com>,
	Pengfei Wang <wpengfeinudt@...il.com>,
	"security@...nel.org" <security@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Double-Fetch bug in Linux-4.5/drivers/scsi/aacraid/commctrl.c

On Wed, Apr 27, 2016 at 07:42:04AM +0200, Julia Lawall wrote:
> 
> 
> On Tue, 26 Apr 2016, Kees Cook wrote:
> 
> > On Mon, Apr 25, 2016 at 7:50 AM, Pengfei Wang <wpengfeinudt@...il.com> wrote:
> > > Hello,
> > >
> > > I found this Double-Fetch bug in Linux-4.5/drivers/scsi/aacraid/commctrl.c
> > > when I was examining the source code.
> > 
> > Thanks for these reports! I wrote a coccinelle script to find these,
> > but it requires some manual checking. For what it's worth, it found
> > your report as well:
> > 
> > ./drivers/scsi/aacraid/commctrl.c:116:5-19: potentially dangerous
> > second copy_from_user()
> > 
> > So I should probably get this added to the coccicheck run... Maybe it
> > can get some clean up from Julia. :)
> 
> I looked a bit at the results, and didn't see anything obvious.  What is 
> the problem, exactly, and what would be a characteristic of a false 
> positive?
> 


	copy_from_user(dest, src, sizeof(dest));

	if (dest.extra > MAX_SIZE)
		return -EINVAL;

	copy_from_user(dest, src, sizeof(dest) + dest.extra);

	for (i = 0; i < dest.extra; i++) {
		dest.foo[i] = xxx;


We get dest.extra from the user, we verify the size, then we copy more
data from the user but that over writes dest.extra again.  We use
dest.extra a second time without checking that it's still <= MAX_SIZE.

regards,
dan carpenter