Re: [RFC] Expose request_module via syscall

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:

quoted

On Wed, Sep 15, 2021 at 8:50 AM Thomas Weißschuh [off-list ref] wrote:

quoted

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?

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