linux-kernel - Re: [PATCH] getsockopt() early argument sanity checking

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Date:	Mon, 21 Aug 2006 12:24:08 +0400
From:	Solar Designer <solar@...nwall.com>
To:	David Wagner <daw-usenet@...erner.cs.berkeley.edu>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] getsockopt() early argument sanity checking

(I realize that the patch has been rejected and this message is in no
way meant to affect that decision.)

On Mon, Aug 21, 2006 at 03:00:22AM +0000, David Wagner wrote:
> Solar Designer  wrote:
> >The patch makes getsockopt(2) sanity-check the value pointed to by
> >the optlen argument early on.  This is a security hardening measure
> >intended to prevent exploitation of certain potential vulnerabilities in
> >socket type specific getsockopt() code on UP systems.
> 
> This looks broken to me.  It has a TOCTTOU (time-of-check-to-time-of-use)

Yes it does, using Matt Bishop's classification.

> vulnerability (i.e., race condition): you read the length value twice,
> and assume that you will get the same value both times.  That assumption
> is not valid.

I don't assume that.  I realize that there's the race condition on SMP
and, as pointed out by Andi Kleen, in many cases also on UP systems.
That's why I had the XXX comment in there from the very beginning.

This added check is not supposed to be relied upon; rather, it is a
hardening measure in case we miss a bug in underlying *getsockopt()
functions.

> It looks like it will be easy to bypass this check.

It depends.  You might have missed this description of a special case
where it does not appear to be possible to bypass the check:

	http://lkml.org/lkml/2006/8/20/148

Yes, the patch is highly controversial and I mostly agree with its
opponents (I had much of the same thoughts myself, except for DaveM's
concern that *optlen might be uninitialized or negative on purpose),
yet I am going to keep it in -ow.

BTW, I had previously submitted a very similar check to do_sysctl(),
also with an XXX comment, which got in a few years back.

I won't be surprised if one of these checks saves a system from a
compromise one day.  This world is not perfect - neither the rest of the
Linux kernel code nor vulnerability exploit programs are perfect.

Alexander

P.S. Please CC me on your replies.
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