linux-kernel - Re: [RFC] Expose request

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Message-Id: <59e230b3-0e85-42ff-84a8-6b30ad0719d8@www.fastmail.com>
Date:   Wed, 22 Sep 2021 08:34:23 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Christian Brauner" <christian.brauner@...ntu.com>
Cc:     "Luis Chamberlain" <mcgrof@...nel.org>,
        Thomas Weißschuh <thomas@...ch.de>,
        "Linux API" <linux-api@...r.kernel.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        "Jessica Yu" <jeyu@...nel.org>
Subject: Re: [RFC] Expose request_module via syscall

On Wed, Sep 22, 2021, at 5:25 AM, Christian Brauner wrote:
> On Mon, Sep 20, 2021 at 11:36:47AM -0700, Andy Lutomirski wrote:
>> On Mon, Sep 20, 2021 at 11:16 AM Luis Chamberlain <mcgrof@...nel.org> wrote:
>> >
>> > On Mon, Sep 20, 2021 at 04:51:19PM +0200, Thomas Weißschuh wrote:
>> 
>> > > > 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.
>> >
>> > Would documentation guiding you in that way have helped? If so
>> > I welcome a patch that does just that.
>> 
>> If someone wants to make this classy, we should probably have the
>> container counterpart of a standardized paravirt interface.  There
>> should be a way for a container to, in a runtime-agnostic way, issue
>> requests to its manager, and requesting a module by (name, Linux
>> kernel version for which that name makes sense) seems like an
>> excellent use of such an interface.
>
> I always thought of this in two ways we currently do this:
>
> 1. Caller transparent container manager requests.
>    This is the seccomp notifier where we transparently handle syscalls
>    including intercepting init_module() where we parse out the module to
>    be loaded from the syscall args of the container and if it is
>    allow-listed load it for the container otherwise continue the syscall
>    letting it fail or failing directly through seccomp return value.

Specific problems here include aliases and dependencies.  My modules.alias file, for example, has:

alias net-pf-16-proto-16-family-wireguard wireguard

If I do modprobe net-pf-16-proto-16-family-wireguard, modprobe parses some files in /lib/modules/`uname -r` and issues init_module() asking for 'wireguard'.  So hooking init_module() is at the wrong layer -- for that to work, the container's /sbin/modprobe needs to already have figured out that the desired module is wireguard and have a .ko for it.

>
> 2. A process in the container explicitly calling out to the container
>    manager.
>    One example how this happens is systemd-nspawn via dbus messages
>    between systemd in the container and systemd outside the container to
>    e.g. allocate a new terminal in the container (kinda insecure but
>    that's another issue) or other stuff.
>
> So what was your idea: would it be like a device file that could be
> exposed to the container where it writes requestes to the container
> manager? What would be the advantage to just standardizing a socket
> protocol which is what we do for example (it doesn't do module loading
> of course as we handle that differently):

My idea is standardizing *something*.  I think it would be nice if, for example, distros could ship a /sbin/modprobe that would do the right thing inside any compliant container runtime as well as when running outside a container.

I suppose container managers could also bind-mount over /sbin/modprobe, but that's more intrusive.