linux-kernel - Re: [RFC] Expose request

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Message-ID: <8cbf0703-5734-4e92-a6cc-12de69094f95@t-8ch.de>
Date:   Mon, 20 Sep 2021 16:51:19 +0200
From:   Thomas Weißschuh <thomas@...ch.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        Linux API <linux-api@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Jessica Yu <jeyu@...nel.org>
Subject: Re: [RFC] Expose request_module via syscall

On 2021-09-19T07:37-0700, Andy Lutomirski wrote:
> On Sun, Sep 19, 2021 at 12:56 AM Thomas Weißschuh <thomas@...ch.de> wrote:
> >
> > On 2021-09-18T11:47-0700, Andy Lutomirski wrote:
> > > On Thu, Sep 16, 2021, at 2:27 AM, Christian Brauner wrote:
> > > > 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?
> > >
> > > Not quite so simple. modprobe parses things in /lib/modules and maybe /etc to decide what init_module() calls to do.
> > >
> > > But I admit I’m a bit confused.  What exactly is the container doing that causes the container’s copy of modprobe to be called?
> >
> > The container is running an instance of the docker daemon in swarm mode.
> > That needs the "ip_vs" module (amongst others) and explicitly tries to load it
> > via modprobe.
> >
> 
> Do you mean it literally invokes /sbin/modprobe?  If so, hooking this
> at /sbin/modprobe and calling out to the container manager seems like
> a decent solution.

Yes it does. Thanks for the idea, I'll see how this works out.

> > > > 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
> > > >