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From:   Andy Lutomirski <luto@kernel.org>
Date:   Sun, 19 Sep 2021 07:37:16 -0700
Message-ID: <CALCETrWA1TBvbknH1Jzt=newTd4sHzNFm0RPuRxazjuRQRsR7w@mail.gmail.com>
Subject: Re: [RFC] Expose request_module via syscall
To:     =?UTF-8?Q?Thomas_Wei=C3=9Fschuh?= <thomas@t-8ch.de>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Linux API <linux-api@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Jessica Yu <jeyu@kernel.org>
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On Sun, Sep 19, 2021 at 12:56 AM Thomas Wei=C3=9Fschuh <thomas@t-8ch.de> wr=
ote:
>
> 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 Wei=C3=9Fschuh <thomas@t-8ch=
.de> wrote:
> > > > >
> > > > > Hi,
> > > > >
> > > > > I would like to propose a new syscall that exposes the functional=
ity 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 kern=
el modules.
> > > > > Currently we have to always pass around the contents of /lib/modu=
les from the
> > > > > root namespace which contains the modules.
> > > > > (Also the containers need to have userspace components for module=
loading
> > > > > installed)
> > > > >
> > > > > The syscall would remove the need for this bookkeeping work.
> > > >
> > > > I feel like I'm missing something, and I don't understand the purpo=
se
> > > > of this syscall.  Wouldn't the right solution be for the container =
to
> > > > have a stub module loader (maybe doable with a special /sbin/modpro=
be
> > > > 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 reques=
t
> > > > 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=E2=80=99m a bit confused.  What exactly is the container =
doing that causes the container=E2=80=99s 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 lo=
ad 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 h=
ow
> > > we currently handle mount.
> > >
> > > Christian
> > >