linux-kernel - Re: [PATCH v4 1/4] seccomp: add a return code to trap to userspace

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Date:   Mon, 25 Jun 2018 19:32:04 -0600
From:   Tycho Andersen <tycho@...ho.ws>
To:     Jann Horn <jannh@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...capital.net>,
        kernel list <linux-kernel@...r.kernel.org>,
        containers@...ts.linux-foundation.org,
        Linux API <linux-api@...r.kernel.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>,
        suda.akihiro@....ntt.co.jp, "Tobin C. Harding" <me@...in.cc>
Subject: Re: [PATCH v4 1/4] seccomp: add a return code to trap to userspace

On Sat, Jun 23, 2018 at 12:27:43AM +0200, Jann Horn wrote:
> On Fri, Jun 22, 2018 at 11:51 PM Kees Cook <keescook@...omium.org> wrote:
> >
> > On Fri, Jun 22, 2018 at 11:09 AM, Andy Lutomirski <luto@...capital.net> wrote:
> > > One possible extra issue: IIRC /proc/.../mem uses FOLL_FORCE, which is not what we want here.
> 
> Uuugh, I forgot about that.
> 
> > > How about just adding an explicit “read/write the seccomp-trapped task’s memory” primitive?  That should be easier than a “open mem fd” primitive.
> >
> > Uuugh. Can we avoid adding another "read/write remote process memory"
> > interface? The point of this series was to provide a lightweight
> > approach to what should normally be possible via the existing
> > seccomp+ptrace interface. I do like Jann's context idea, but I agree
> > with Andy: it can't be a handle to /proc/$pid/mem, since it's
> > FOLL_FORCE. Is there any other kind of process context id we can use
> > for this instead of pid? There was once an idea of pid-fd but it never
> > landed... This would let us get rid of the "id" in the structure too.
> > And if that existed, we could make process_vm_*v() safer too (taking a
> > pid-fd instead of a pid).
> 
> Or make a duplicate of /proc/$pid/mem that only differs in whether it
> sets FOLL_FORCE? The code is basically already there... something like
> this:

But we want more than just memory access, I think. rootfs access, ns
fds, etc. all seem like they might be useful, and racy to open.

I guess I see two options: use the existing id and add something to
seccomp() to ask if it's still valid or independent of this patchset
add some kind of pid id :\

Tycho