linux-kernel - Re: [RFC 1/3] seccomp: add a return code to trap to userspace

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Date:   Wed, 14 Feb 2018 10:23:00 -0700
From:   Tycho Andersen <tycho@...ho.ws>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Kees Cook <keescook@...omium.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Tyler Hicks <tyhicks@...onical.com>,
        Akihiro Suda <suda.akihiro@....ntt.co.jp>,
        Tom Hromatka <tom.hromatka@...cle.com>,
        Sargun Dhillon <sargun@...gun.me>,
        Paul Moore <pmoore@...hat.com>
Subject: Re: [RFC 1/3] seccomp: add a return code to trap to userspace

On Wed, Feb 14, 2018 at 05:19:52PM +0000, Andy Lutomirski wrote:
> On Wed, Feb 14, 2018 at 3:29 PM, Tycho Andersen <tycho@...ho.ws> wrote:
> > Hey Kees,
> >
> > Thanks for taking a look!
> >
> > On Tue, Feb 13, 2018 at 01:09:20PM -0800, Kees Cook wrote:
> >> On Sun, Feb 4, 2018 at 2:49 AM, Tycho Andersen <tycho@...ho.ws> wrote:
> >> > This patch introduces a means for syscalls matched in seccomp to notify
> >> > some other task that a particular filter has been triggered.
> >> >
> >> > The motivation for this is primarily for use with containers. For example,
> >> > if a container does an init_module(), we obviously don't want to load this
> >> > untrusted code, which may be compiled for the wrong version of the kernel
> >> > anyway. Instead, we could parse the module image, figure out which module
> >> > the container is trying to load and load it on the host.
> >> >
> >> > As another example, containers cannot mknod(), since this checks
> >> > capable(CAP_SYS_ADMIN). However, harmless devices like /dev/null or
> >> > /dev/zero should be ok for containers to mknod, but we'd like to avoid hard
> >> > coding some whitelist in the kernel. Another example is mount(), which has
> >> > many security restrictions for good reason, but configuration or runtime
> >> > knowledge could potentially be used to relax these restrictions.
> >>
> >> Related to the eBPF seccomp thread, can the logic for these things be
> >> handled entirely by eBPF? My assumption is that you still need to stop
> >> the process to do something (i.e. do a mknod, or a mount) before
> >> letting it continue. Is there some "wait for notification" system in
> >> eBPF?
> >
> > I replied in the other thread
> > (https://patchwork.ozlabs.org/cover/872938/#1856642 for those
> > following along at home), but no, at least not that I know of.
> 
> eBPF can call functions.  One of those functions could put the caller
> to sleep.  In fact, I think I once proposed doing this for the seccomp
> logging action as well.

Yes, true. We could always add a bpf_func_map_lookup_wait or
something. I can look into that if it's preferable.