linux-kernel - Re: [RFC] Expose request

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Message-ID: <20210916092719.v4pkhhugdiq7ytcp@wittgenstein>
Date:   Thu, 16 Sep 2021 11:27:19 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Thomas Weißschuh <thomas@...ch.de>,
        Linux API <linux-api@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Jessica Yu <jeyu@...nel.org>
Subject: Re: [RFC] Expose request_module via syscall

On Wed, Sep 15, 2021 at 09:47:25AM -0700, Andy Lutomirski wrote:
> On Wed, Sep 15, 2021 at 8:50 AM Thomas Weißschuh <thomas@...ch.de> wrote:
> >
> > Hi,
> >
> > I would like to propose a new syscall that exposes the functionality of
> > request_module() to userspace.
> >
> > Propsed signature: request_module(char *module_name, char **args, int flags);
> > Where args and flags have to be NULL and 0 for the time being.
> >
> > Rationale:
> >
> > We are using nested, privileged containers which are loading kernel modules.
> > Currently we have to always pass around the contents of /lib/modules from the
> > root namespace which contains the modules.
> > (Also the containers need to have userspace components for moduleloading
> > installed)
> >
> > The syscall would remove the need for this bookkeeping work.
> 
> I feel like I'm missing something, and I don't understand the purpose
> of this syscall.  Wouldn't the right solution be for the container to
> have a stub module loader (maybe doable with a special /sbin/modprobe
> or maybe a kernel patch would be needed, depending on the exact use
> case) and have the stub call out to the container manager to request
> the module?  The container manager would check its security policy and
> load the module or not load it as appropriate.

I don't see the need for a syscall like this yet either.

This should be the job of the container manager. modprobe just calls the
init_module() syscall, right?

If so the seccomp notifier can be used to intercept this system call for
the container and verify the module against an allowlist similar to how
we currently handle mount.

Christian