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Date:   Mon, 1 Apr 2019 02:52:46 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Christian Brauner <christian@...uner.io>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Daniel Colascione <dancol@...gle.com>,
        Andrew Lutomirski <luto@...nel.org>,
        David Howells <dhowells@...hat.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Linux API <linux-api@...r.kernel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Kees Cook <keescook@...omium.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Michael Kerrisk-manpages <mtk.manpages@...il.com>,
        Jonathan Kowalski <bl0pbl33p@...il.com>,
        "Dmitry V. Levin" <ldv@...linux.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Nagarathnam Muthusamy <nagarathnam.muthusamy@...cle.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH v2 0/5] pid: add pidfd_open()

On Mon, Apr 1, 2019 at 12:33 AM Christian Brauner <christian@...uner.io> wrote:
> On Sun, Mar 31, 2019 at 03:16:47PM -0700, Linus Torvalds wrote:
> > On Sun, Mar 31, 2019 at 3:03 PM Christian Brauner <christian@...uner.io> wrote:
> > > Thanks for the input. The problem Jann and I saw with this is that it
> > > would be awkward to have the kernel open a file in some procfs instance,
> > > since then userspace would have to specify which procfs instance the fd
> > > should come from.
> >
> > I would actually suggest we just make the rules be that the
> > pidfd_open() always return the internal /proc entry regardless of any
> > mount-point (or any "hidepid") but also suggest that exactly *because*
> > it gives you visibility into the target pid, you'd basically require
> > the strictest kind of control of the process you're trying to get the
> > pidfd of.
> >
> > Ie likely something along the lines of
> >
> >         ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS)
>
> I can live with that but I would like to hear what Jann thinks too if
> that's ok.

Ah, yes. That seems reasonable. And, as Linus said, pidfd_open() is
less important if you can just do open("/proc/...") on systems with
procfs instead.

One minor detail to keep in mind for the future is that in a
straightforward implementation of this concept, if a non-capable
process is running in a mount namespace, but in the initial network
namespace, without any reachable /proc mount, it will be able to look
at information about other processes' network connections by first
using pidfd_open() on itself or by using clone(CLONE_PIDFD), then
looking at the "net" directory under the resulting file descriptor.

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