linux-kernel - Re: [PATCH v2 0/5] pid: add pidfd

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Message-ID: <CAG48ez0KeQU32HV8OGY4-LNPF0Q+PtaH7H47GmKAd1Hhho=jXw@mail.gmail.com>
Date:   Mon, 1 Apr 2019 15:43:13 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Christian Brauner <christian@...uner.io>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...capital.net>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH v2 0/5] pid: add pidfd_open()

On Mon, Apr 1, 2019 at 2:04 PM Christian Brauner <christian@...uner.io> wrote:
> On Sun, Mar 31, 2019 at 08:13:38PM -0600, Andy Lutomirski wrote:
> > > On Mar 31, 2019, at 3:17 PM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> > >> On Sun, Mar 31, 2019 at 2:10 PM Christian Brauner <christian@...uner.io> wrote:
> > >>
> > >> I don't think that we want or can make them equivalent since that would
> > >> mean we depend on procfs.
> > >
> > > Sure we can.
> > >
> > > If /proc is enabled, then you always do that dance YOU ALREADY WROTE
> > > THE CODE FOR to do the stupid ioctl.
> > >
> > > And if /procfs isn't enabled, then you don't do that.
> > >
> > > Ta-daa. Done. No stupid ioctl, and now /proc and pidfd_open() return
> > > the same damn thing.
> > >
> > > And guess what? If /proc isn't enabled, then obviously pidfd_open()
> > > gives you the /proc-less thing, but at least there is no crazy "two
> > > different file descriptors for the same thing" situation, because then
> > > the /proc one doesn't exist.
> > >
> >
> > I wish we could do this, and, in a clean design, it would be a no-brainer.  But /proc has too much baggage.  Just to mention two such things, there’s “net” and “../sys”.  This crud is why we have all kinds of crazy rules that prevent programs in sandboxes from making a new mounts and mounting /proc in it.  If we make it possible to clone a new process and this access /proc without having /proc mounted, we’ll open up a big can of worms.
> >
> > Maybe we could have a sanitized view of /proc and make a pidfd be a directory fd pointing at that.
>
> We can also just create something like an internal bind-mount without a
> parent, i.e. similar to
>
> open_tree(<internal-procfs-mount>, "<pid>", OPEN_TREE_CLONE);
>
> on a clone(CLONE_PIDFD);
>
> that would block any openat(fd, "..");

Or we add a check to follow_dotdot()/follow_dotdot_rcu() that throws
an error if nd->path.mnt->mnt_flags has some new flag for "no dotdot
traversal on this mountpoint", and then set that on the internal procfs
mount... if Al Viro doesn't think that that's too hideous.