linux-kernel - Re: [kernel-hardening] Re: [PATCH v5 next 5/5] net: modules: use request_module

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Date:   Tue, 28 Nov 2017 18:23:20 -0500
From:   Theodore Ts'o <tytso@....edu>
To:     Kees Cook <keescook@...omium.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Djalal Harouni <tixxdz@...il.com>,
        Jonathan Corbet <corbet@....net>,
        James Morris <james.l.morris@...cle.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        Geo Kozey <geokozey@...lfence.com>
Subject: Re: [kernel-hardening] Re: [PATCH v5 next 5/5] net: modules: use
 request_module_cap() to load 'netdev-%s' modules

On Tue, Nov 28, 2017 at 01:33:40PM -0800, Kees Cook wrote:
> As I've said before, this isn't a theoretical attack surface. This
> year alone there have been three known-exploitable flaws exposed by
> autoloading:
> 
> The exploit for CVE-2017-2636 uses int n_hdlc = N_HDLC; ioctl(fd,
> TIOCSETD, &n_hdlc) [1]. This is using the existing "tty-ldisc-"
> prefix, and is intentionally unprivileged.
> 
> The exploit for CVE-2017-6074 uses socket(PF_INET6, SOCK_DCCP,
> IPPROTO_IP) [2]. This is using the existing proto prefix, and is
> intentionally unprivileged.

So in these two cases, if the kernel was built w/o modules, and HDLC
and DCCP was built-in, you'd be screwed, then?

Is the goal here to protect people using distro kernels which build
the world as modules, including dodgy pieces of kernel code that are
bug-ridden?

If so, then presumably 90% of the problem you've cited can be done by
creating a script which takes a look of the modules that are normally
in use once the machine is in production, and then deleting everything
else?  Correct?

And yes, this will potentially break some users, but the security
folks who are advocating for the more aggressive version of this
change seem to be OK with breaking users, so they can do this without
making kernel changes.  Good luck getting Red Hat and SuSE to accept
such a change, though....

						- Ted