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

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Message-ID: <CAGXu5j+JaFCo+pZKvet==Kh7GQgNXi30P50SMF4cpOGoUw-8ug@mail.gmail.com>
Date:   Tue, 28 Nov 2017 13:39:58 -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 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.

>> 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.

You need CAP_SYS_MODULE to run finit_module().

-Kees

-- 
Kees Cook
Pixel Security