Re: [RFC] Expose request_module via syscall

[Andy Lutomirski]

On Sun, Sep 19, 2021 at 12:56 AM Thomas Weißschuh <thomas@xxxxxxxx> 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@xxxxxxxx> 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.

> > > 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
> > >