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

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Date:   Wed, 27 May 2020 18:49:21 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
        Tycho Andersen <tycho@...ho.ws>,
        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 Thu, May 28, 2020 at 12:45:01AM +0200, Christian Brauner wrote:
> On Wed, May 27, 2020 at 03:37:58PM -0700, Kees Cook wrote:
> > But there's a mapping between pidfd and task struct that is separate
> > from task struct itself, yes? I.e. keeping a pidfd open doesn't pin
> > struct task in memory forever, right?
> 
> No, but that's an implementation detail and we discussed that. It pins
> struct pid instead of task_struct. Once the process is fully gone you
> just get ESRCH.

Oh right! struct pid, yes. Okay, that's quite a bit smaller.

> For example, fds to /proc/<pid>/<tid>/ fds aren't just closed once the
> task has gone away, userspace will just get ESRCH when it tries to open
> files under there but the fd remains valid until close() is called.
> 
> In addition, of all the anon inode fds, none of them have the "close the
> file behind userspace back" behavior: io_uring, signalfd, timerfd, btf,
> perf_event, bpf-prog, bpf-link, bpf-map, pidfd, userfaultfd, fanotify,
> inotify, eventpoll, fscontext, eventfd. These are just core kernel ones.
> I'm pretty sure that it'd be very odd behavior if we did that. I'd
> rather just notify userspace and leave the close to them. But maybe I'm
> missing something.

Well, they have a "you are now disconnected" state, which I was thinking
could be done entirely entirely on the VFS side of things, but it looks
like it's not.

So, yes, okay, thank you for walking me through all that. I still want
to take a closer look at all the notify calls in here. It seems strange
that seccomp has to do all the wakeups (but I guess there are no
"generic" poll helpers?)

-- 
Kees Cook