linux-kernel - Re: [PATCH v7 1/6] seccomp: add a return code to trap to userspace

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Date:   Tue, 9 Oct 2018 09:29:21 -0700
From:   Tycho Andersen <tycho@...ho.ws>
To:     Christian Brauner <christian@...uner.io>
Cc:     Kees Cook <keescook@...omium.org>, Jann Horn <jannh@...gle.com>,
        Linux API <linux-api@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Akihiro Suda <suda.akihiro@....ntt.co.jp>,
        Oleg Nesterov <oleg@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH v7 1/6] seccomp: add a return code to trap to userspace

On Tue, Oct 09, 2018 at 06:24:14PM +0200, Christian Brauner wrote:
> On Tue, Oct 09, 2018 at 07:28:33AM -0700, Tycho Andersen wrote:
> > On Mon, Oct 08, 2018 at 04:58:05PM +0200, Christian Brauner wrote:
> > > On Thu, Sep 27, 2018 at 04:48:39PM -0600, Tycho Andersen wrote:
> > > > On Thu, Sep 27, 2018 at 02:31:24PM -0700, Kees Cook wrote:
> > > > > I have to say, I'm vaguely nervous about changing the semantics here
> > > > > for passing back the fd as the return code from the seccomp() syscall.
> > > > > Alternatives seem less appealing, though: changing the meaning of the
> > > > > uargs parameter when SECCOMP_FILTER_FLAG_NEW_LISTENER is set, for
> > > > > example. Hmm.
> > > > 
> > > > From my perspective we can drop this whole thing. The only thing I'll
> > > > ever use is the ptrace version. Someone at some point (I don't
> > > > remember who, maybe stgraber) suggested this version would be useful
> > > > as well.
> > > 
> > > So I think we want to have the ability to get an fd via seccomp().
> > > Especially, if we all we worry about are weird semantics. When we
> > > discussed this we knew the whole patchset was going to be weird. :)
> > > 
> > > This is a seccomp feature so seccomp should - if feasible - equip you
> > > with everything to use it in a meaningful way without having to go
> > > through a different kernel api. I know ptrace and seccomp are
> > > already connected but I still find this cleaner. :)
> > > 
> > > Another thing is that the container itself might be traced for some
> > > reason while you still might want to get an fd out.
> > 
> > Sure, I don't see the problem here.
> 
> How'd you to PTRACE_ATTACH in that case?

Oh, you mean if someone has *ptrace*'d the task, and a third party
wants to get a seccomp fd? I think "too bad" is the answer; I don't
really mind not supporting this case.

> Anyway, the whole point is as we've discusses in the other thread we
> really want a one-syscall-only, purely-seccomp() based way of getting
> the fd. There are multiple options to get the fd even when you block
> sendmsg()/socket() whatever and there's no good reason to only be able
> to get the fd via a three-syscall-ptrace dance. :)

Ok, I'll leave these bits in then for v8.

Tycho