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Message-ID: <CAGXu5jLV+WZyj+xnxVFkFEgEthNt6eXdcSgHT-=85mJ1ECZ1Rw@mail.gmail.com> Date: Sat, 22 Apr 2017 12:29:37 -0700 From: Kees Cook <keescook@...omium.org> To: Andy Lutomirski <luto@...nel.org> Cc: Djalal Harouni <tixxdz@...il.com>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Andrew Morton <akpm@...ux-foundation.org>, "Serge E. Hallyn" <serge@...lyn.com>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, LSM List <linux-security-module@...r.kernel.org>, Linux API <linux-api@...r.kernel.org>, Dongsu Park <dpark@...teo.net>, Casey Schaufler <casey@...aufler-ca.com>, James Morris <james.l.morris@...cle.com>, Paul Moore <paul@...l-moore.com>, Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jonathan Corbet <corbet@....net>, Jessica Yu <jeyu@...hat.com>, Rusty Russell <rusty@...tcorp.com.au>, Arnaldo Carvalho de Melo <acme@...hat.com>, Mauro Carvalho Chehab <mchehab@...nel.org>, Ingo Molnar <mingo@...nel.org>, belakhdar abdeldjalil <zendyani@...il.com>, Peter Zijlstra <peterz@...radead.org> Subject: Re: [PATCH v3 2/2] modules:capabilities: add a per-task modules autoload restriction On Fri, Apr 21, 2017 at 11:51 PM, Andy Lutomirski <luto@...nel.org> wrote: > On Fri, Apr 21, 2017 at 5:12 PM, Djalal Harouni <tixxdz@...il.com> wrote: >> On Sat, Apr 22, 2017 at 1:51 AM, Andy Lutomirski <luto@...nel.org> wrote: >> [...] >>>>> I personally like my implicit_rights idea, and it might be interesting >>>>> to prototype it. >>>> >>>> I don't like blocking a needed feature behind a large super-feature >>>> that doesn't exist yet. We'd be able to refactor this code into using >>>> such a thing in the future, so I'd prefer to move ahead with this >>>> since it would stop actual exploits. >>> >>> I don't think the super-feature is so hard, and I think we should not >>> add the per-task thing the way it's done in this patch. Let's not add >>> per-task things where the best argument for their security is "not >>> sure how it would be exploited". >> >> Actually the XFRM framework CVE-2017-7184 [1] is one real example, of >> course there are others. The exploit was used on a generic distro >> during a security contest that distro is Ubuntu. That distro will >> never provide a module autoloading restriction by default to not harm >> it's users. Consumers or containers/sandboxes then can run their >> confined apps using such facilities. >> >> These bugs will stay in embedded devices that use these generic >> distros for ever. >> >>> Anyway, I think the sysctl is really the important bit. The per-task >>> setting is icing on the cake IMO. One upon a time autoload was more >>> important, but these days modaliases are supposed to do most of the >>> work. I bet that modern distros don't need unprivileged autoload at >>> all. >> >> Actually I think they do and we can't just change that. Users may >> depend on it, it is a well established facility. >> >> Now the other problem is CAP_NET_ADMIN which does lot of things, it is >> more like the CAP_SYS_ADMIN. >> >> This is a quick list that I got from only the past months, I'm pretty >> sure there are more: >> >> * DCCP use after free CVE-2017-6074 >> * n_hldc CVE-2017-2636 >> * XFRM framework CVE-2017-7184 >> * L2TPv3 CVE-2016-10200 >> >> Most of these need CAP_NET_ADMIN to be autoloaded, however we also >> need CAP_NET_ADMIN for other things... therefore it is better to have >> an extra facility that could coexist with CAP_NET_ADMIN and other >> sandbox features. >> > > I agree that the feature is important, but I think your implementation > is needlessly dangerous. I imagine that the main uses that you care > about involve containers. How about doing it in a safer way that > works for containers? I can think of a few. For example: > > 1. A sysctl that, if set, prevents autoloading outside the root > userns. This isn't very flexible at all, but it might work. > > 2. Your patch, but require privilege within the calling namespace to > set the prctl. How about CAP_SYS_ADMIN || no_new_privs? -Kees > > 3. A per-user-ns sysctl. -- Kees Cook Pixel Security
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