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

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Message-ID: <CAGXu5jKqMeBUB4iArqw9_iyKqm3NjpCrU_byy90kZrAHDb0c9w@mail.gmail.com>
Date:   Tue, 28 Nov 2017 14:18:18 -0800
From:   Kees Cook <keescook@...omium.org>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:     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 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().

> 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.

-Kees

-- 
Kees Cook
Pixel Security