linux-kernel - Re: [PATCH v5 next 1/5] modules:capabilities: add request_module

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Message-ID: <20171129074956.7xg4gcon3zxd4xck@unicorn.suse.cz>
Date:   Wed, 29 Nov 2017 08:49:56 +0100
From:   Michal Kubecek <mkubecek@...e.cz>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Djalal Harouni <tixxdz@...il.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        James Morris <james.l.morris@...cle.com>,
        Ben Hutchings <ben.hutchings@...ethink.co.uk>,
        Solar Designer <solar@...nwall.com>,
        Serge Hallyn <serge@...lyn.com>, Jessica Yu <jeyu@...nel.org>,
        Rusty Russell <rusty@...tcorp.com.au>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        kernel-hardening@...ts.openwall.com,
        Jonathan Corbet <corbet@....net>,
        Ingo Molnar <mingo@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v5 next 1/5] modules:capabilities: add
 request_module_cap()

On Tue, Nov 28, 2017 at 11:48:49PM +0100, Luis R. Rodriguez wrote:
> On Tue, Nov 28, 2017 at 02:18:18PM -0800, Kees Cook wrote:
> > On Tue, Nov 28, 2017 at 2:12 PM, Luis R. Rodriguez <mcgrof@...nel.org> wrote:
> > > On Tue, Nov 28, 2017 at 01:39:58PM -0800, Kees Cook wrote:
> > >> On Tue, Nov 28, 2017 at 1:16 PM, Luis R. Rodriguez <mcgrof@...nel.org> wrote:
> > >> > And *all* auto-loading uses aliases? What's the difference
> > >> > between auto-loading and direct-loading?
> > >>
> > >> The difference is the process privileges. Unprivilged autoloading
> > >> (e.g. int n_hdlc = N_HDLC; ioctl(fd,
> > >> TIOCSETD, &n_hdlc)), triggers a privileged call to finit_module()
> > >> under CAP_SYS_MODULE.
> > >
> > > Ah, so system call implicated request_module() calls.
> > 
> > Yup. Unprivileged user does something that ultimately hits a
> > request_module() in the kernel. Then the kernel calls out with the
> > usermode helper (which has CAP_SYS_MODULE) and calls finit_module().
> 
> Thanks, using this terminology is much better to understand than
> auto-loading, given it does make it clear an unprivileged call was one
> that initiated the request_module() call, there are many uses of
> request_module() which *are* privileged.
> 
> > > OK and since CAP_SYS_MODULE is much more restrictive one could
> > > argue, what's the point here?
> > 
> > The goal is to block an unprivileged user from being able to trigger a
> > module load without blocking root from loading modules directly.
> 
> I see now. Do we have an audit of all system calls which implicate a
> request_module() call? Networking is a good example for sure to start
> off with but I was curious if we have a grasp of how wide spread this
> could be.

I'm not sure it makes sense to classify this by syscalls. In networking,
request_module() can be triggered e.g. by a netlink message (genetlink
family lookup is an example not needing any privileges) so that one of
the syscalls would be sendmsg().

Michal Kubecek