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

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Date:	Wed, 27 Apr 2016 07:42:04 +0200 (CEST)
From:	Julia Lawall <julia.lawall@...6.fr>
To:	Kees Cook <keescook@...gle.com>
cc:	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 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?

thanks,
julia

> virtual report
> virtual org
> 
> @cfu_twice@
> position p;
> identifier src;
> expression dest1, dest2, size1, size2, offset;
> @@
> 
> *copy_from_user(dest1, src, size1)
>  ... when != src = offset
>      when != src += offset
> *copy_from_user@p(dest2, src, size2)
> 
> @script:python depends on org@
> p << cfu_twice.p;
> @@
> 
> cocci.print_main("potentially dangerous second copy_from_user()",p)
> 
> @script:python depends on report@
> p << cfu_twice.p;
> @@
> 
> coccilib.report.print_report(p[0],"potentially dangerous second
> copy_from_user()")
> 
> 
> It would be great to have some one go through all the reports to see
> which are legit. I'll send separate emails with the patch for
> coccicheck and the output.
> 
> -Kees
> 
> >
> > In function ioctl_send_fib(), the driver fetches user space data by pointer
> > arg via copy_from_user(), and this happens twice at line 81 and line 116
> > respectively. The first fetched value (stored in kfib) is used to get the
> > header and calculate the size at line 90 so as to copy the whole message
> > later at line 116, which means the copy size of the whole message is based
> > on the old value that came from the first fetch. Besides, the whole message
> > copied in the  second fetch also contains the header.
> >
> > However, when the function processes the message after the second fetch at
> > line 130, it uses kfib->header.Size that came from the second fetch, which
> > might be different from the one came from the first fetch as well as
> > calculated the size to copy the message from user space to driver.
> >
> > If the kfib->header.Size is modified by a user thread under race condition
> > between the fetch operations, for example changing to a very large value,
> > this will lead to over-boundary access or other serious consequences in
> > function aac_fib_send().
> >
> > I also reported this to bugzilla,
> > https://bugzilla.kernel.org/show_bug.cgi?id=116751
> >
> > I am expecting a reply to confirm this, thank you!
> >
> >
> >
> >
> >
> > Kind regards
> > Pengfei
> >
> 
> 
> 
> -- 
> Kees Cook
> Chrome OS & Brillo Security
>