linux-kernel - Re: [PATCH 1/2] seccomp: notify user trap about unused filter

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Message-ID: <202005271849.F34CE6A@keescook>
Date:   Wed, 27 May 2020 18:50:56 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Tycho Andersen <tycho@...ho.ws>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
        Matt Denton <mpdenton@...gle.com>,
        Sargun Dhillon <sargun@...gun.me>,
        Jann Horn <jannh@...gle.com>, Chris Palmer <palmer@...gle.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Robert Sesek <rsesek@...gle.com>,
        Jeffrey Vander Stoep <jeffv@...gle.com>,
        Linux Containers <containers@...ts.linux-foundation.org>
Subject: Re: [PATCH 1/2] seccomp: notify user trap about unused filter

On Wed, May 27, 2020 at 04:56:00PM -0600, Tycho Andersen wrote:
> On Wed, May 27, 2020 at 03:36:09PM -0700, Kees Cook wrote:
> > On Wed, May 27, 2020 at 03:52:03PM -0600, Tycho Andersen wrote:
> > > On Wed, May 27, 2020 at 02:43:49PM -0700, Kees Cook wrote:
> > > > (While I'm here -- why can there be only one listener per task? The
> > > > notifications are filter-specific, not task-specific?)
> > > 
> > > Not sure what you mean here?
> > 
> > tatic struct file *init_listener(struct seccomp_filter *filter)
> > {
> >         struct file *ret = ERR_PTR(-EBUSY);
> >         struct seccomp_filter *cur;
> > 
> >         for (cur = current->seccomp.filter; cur; cur = cur->prev) {
> >                 if (cur->notif)
> >                         goto out;
> >         }
> > 
> > ...
> > 
> >         /* Installing a second listener in the chain should EBUSY */
> >         EXPECT_EQ(user_trap_syscall(__NR_getpid,
> >                                     SECCOMP_FILTER_FLAG_NEW_LISTENER),
> >                   -1);
> >         EXPECT_EQ(errno, EBUSY);
> > 
> > 
> > Why does this limit exist? Since the fd is tied to a specific filter,
> > I don't see conflicts about having multiple USER_NOTIF filters on one
> > task -- the monitor's response will either fake it or continue it, so
> > there is no "composition" needed? I must be missing something.
> 
> It exists because Andy asked for it :)
> 
> I agree that there's no technical reason for it to be there. I think
> it's just that the semantics were potentially confusing, and it wasn't
> a requirement anyone had to have multiples attached.

Okay, sounds good. It just seems seccomp continues to grow "layers", so
I'm eyeing this aspect of user_notif. i.e. what if systemd decides to
add a user_notif for something and now suddenly the containers can't use
it. Or if some complex thing inside a container tries to use user_notif
and it can't because the container manager is doing it, etc.

Future work! :)

-- 
Kees Cook