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

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Date:   Tue, 28 Nov 2017 22:33:27 +0100
From:   Djalal Harouni <tixxdz@...il.com>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>,
        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 10:16 PM, Luis R. Rodriguez <mcgrof@...nel.org> wrote:
> 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?

Not all auto-loading uses aliases, auto-loading is when kernel code
calls request_module() to loads the feature that was not present, and
direct-loading in this thread is the direct syscalls like
finit_module().

>> 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.

The finit_module() uses CAP_SYS_MODULE which should allow all modules
and in this context it should be more privileged than CAP_NET_ADMIN
which is only for "netdev-%s" (to not load arbitrary modules with it).

finit_module() coming from request_module() always has the
CAP_NET_ADMIN, hence the check is done before.

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

I see, request_module() is called from kernel context which runs in
init namespace will full capabilities, the spawned userspace modprobe
will get CAP_SYS_MODULE and all other caps, then after comes modprobe
and load_module().

Btw as suggested by Linus I will update with request_module_cap() and
I can offer my help maintaining these bits too.


>
>   Luis



-- 
tixxdz