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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 1 Apr 2019 14:30:19 -0700
Message-ID: <CAHk-=wgjndAqzMBxgXZxbSRXLRqdXtNM3aHc9X-xj+Px1fsG-Q@mail.gmail.com>
Subject: Re: [PATCH v2 0/5] pid: add pidfd_open()
To:     Christian Brauner <christian@brauner.io>
Cc:     Jann Horn <jannh@google.com>,
        Daniel Colascione <dancol@google.com>,
        Aleksa Sarai <cyphar@cyphar.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Andrew Lutomirski <luto@kernel.org>,
        David Howells <dhowells@redhat.com>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        Linux API <linux-api@vger.kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
        Kees Cook <keescook@chromium.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Michael Kerrisk-manpages <mtk.manpages@gmail.com>,
        Jonathan Kowalski <bl0pbl33p@gmail.com>,
        "Dmitry V. Levin" <ldv@altlinux.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Nagarathnam Muthusamy <nagarathnam.muthusamy@oracle.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Joel Fernandes <joel@joelfernandes.org>
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On Mon, Apr 1, 2019 at 12:42 PM Christian Brauner <christian@brauner.io> wrote:
>
> From what I gather from this thread we are still best of with using fds
> to /proc/<pid> as pidfds. Linus, do you agree or have I misunderstood?

That does seem to be the most flexible option.

> Yes, we can have an internal mount option to restrict access to various
> parts of procfs from such pidfds

I suspect you'd find that other parties might want such a "restrict
proc" mount option too, so I don't think it needs to be anything
internal.

But it would be pretty much independent of the pidfd issue, of course.

> One thing is that we also need something to disable access to the
> "/proc/<pid>/net". One option could be to give the files in "net/" an
> ->open-handler which checks that our file->f_path.mnt is not one of our
> special clone() mounts and if it is refuse the open.

I would expect that that would be part of the "restrict proc" mount options, no?

> Basically, if you have a system without CONFIG_PROC_FS it makes sense
> that clone gives back an anon inode file descriptor as pidfds because
> you can still signal threads in a race-free way. But it doesn't make a
> lot of sense to have pidfd_open() in this scenario because you can't
> really do anything with that pidfd apart from sending signals.

Well, people might want that.

But realistically, everybody enables /proc support anyway. Even if you
don't actually fully *mount* it in some restricted area, the support
is pretty much always there in any kernel config.

But yes, in general I agree that that also most likely means that a
separate system call for "open_pidfd()" isn't worth it.

Because if the main objection to /proc is that it exposes too much,
then I think a much better option is to see how to sanely restrict the
"too much" parts.

Because I think there might be a lot of people who want a restricted
/proc, in various container models etc.

                   Linus