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

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Date:   Tue, 28 Nov 2017 22:16:59 +0100
From:   "Luis R. Rodriguez" <mcgrof@...nel.org>
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, Nov 28, 2017 at 12:11:34PM -0800, Kees Cook wrote:
> On Tue, Nov 28, 2017 at 11:14 AM, Luis R. Rodriguez <mcgrof@...nel.org> wrote:
> > kmod is just a helper to poke userpsace to load a module, that's it.
> >
> > The old init_module() and newer finit_module() do the real handy work or
> > module loading, and both currently only use may_init_module():
> >
> > static int may_init_module(void)
> > {
> >         if (!capable(CAP_SYS_MODULE) || modules_disabled)
> >                 return -EPERM;
> >
> >         return 0;
> > }
> >
> > This begs the question:
> >
> >   o If userspace just tries to just use raw finit_module() do we want similar
> >     checks?
> >
> > Otherwise, correct me if I'm wrong this all seems pointless.
> 
> Hm? That's direct-loading, not auto-loading. This series is only about
> auto-loading.

And *all* auto-loading uses aliases? What's the difference between auto-loading
and direct-loading?

> We already have a global sysctl for blocking direct-loading (modules_disabled).

My point was that even if you have a CAP_NET_ADMIN check on request_module(),
finit_module() will not check for it, so a crafty userspace could still try
to just finit_module() directly, and completely then bypass the CAP_NET_ADMIN
check.

So unless I'm missing something, I see no point in adding extra checks for
request_module() but nothing for the respective load_module().

  Luis