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

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Date:   Wed, 29 Nov 2017 13:46:12 +0000
From:   Alan Cox <gnomes@...rguk.ukuu.org.uk>
To:     Kees Cook <keescook@...omium.org>
Cc:     "Luis R. Rodriguez" <mcgrof@...nel.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, 28 Nov 2017 13:39:58 -0800
Kees Cook <keescook@...omium.org> 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.

If you have CAP_SYS_DAC you can rename any module to ppp.ko and ask the
network manager (which has the right permissions) to init a ppp
connection. Capabilities alone are simply not enough to do any kind of
useful protection on a current system and the Linux capability model is
broken architecturally and not fixable because fixing it would break lots
of real systems.

I really don't care what the module loading rules end up with and whether
we add CAP_SYS_YET_ANOTHER_MEANINGLESS_FLAG but what is actually needed
is to properly incorporate it into securiy ruiles for whatever LSM you
are using.

Alan