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

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Date:	Wed, 27 Apr 2016 10:07:05 +0200 (CEST)
From:	Julia Lawall <julia.lawall@...6.fr>
To:	Dan Carpenter <dan.carpenter@...cle.com>
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, 27 Apr 2016, Dan Carpenter wrote:

> 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.

OK, so the problem is when data that was checked on the first copy is used
after the second copy?  It would probably be possible to get rid of a lot
of false positives with that.

thanks,
julia