linux-kernel - Re: [RFC] Expose request

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Message-Id: <2ebf1a9d-77d5-472b-a99a-b141654725da@www.fastmail.com>
Date:   Sat, 18 Sep 2021 11:47:07 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Christian Brauner" <christian.brauner@...ntu.com>
Cc:     Thomas Weißschuh <thomas@...ch.de>,
        "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 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?

> 
> 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
>