linux-kernel - seccomp: Delay filter activation

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Date:   Sat, 20 Feb 2021 01:31:57 -0800
From:   Sargun Dhillon <sargun@...gun.me>
To:     Kees Cook <keescook@...omium.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Giuseppe Scrivano <gscrivan@...hat.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Tycho Andersen <tycho@...ho.pizza>,
        Hariharan Ananthakrishnan <hari@...flix.com>,
        Keerti Lakshminarayan <keerti@...flix.com>,
        Kyle Anderson <kylea@...flix.com>
Subject: seccomp: Delay filter activation

We've run into a problem where attaching a filter can be quite messy
business because the filter itself intercepts sendmsg, and other
syscalls related to exfiltrating the listener FD. I believe that this
problem set has been brought up before, and although there are
"simpler" methods of exfiltrating the listener, like clone3 or
pidfd_getfd, but these are still less than ideal.

One of the ideas that's been talked about (I want to say back at LSS
NA) is the idea of "delayed activation". I was thinking that it might
be nice to have a mechanism to do delayed attach, either activated on
execve / fork, or an ioctl on the listenerfd to activate the filter
and have a flag like SECCOMP_FILTER_FLAG_NEW_LISTENER_INACTIVE, which
indicates that the listener should be setup, but not enforcing, and
another ioctl to activate it.

The later approach is preferred due to simplicity, but I can see a
situation where you could accidentally get into a state where the
filter is not being enforced. Additionally, this may have unforeseen
implications with CRIU.

I'm curious whether this is a problem others share, and whether any of
the aforementioned approaches seem reasonable.

-Thanks,
Sargun