Date:   Mon, 25 Mar 2019 21:34:00 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Daniel Colascione <dancol@...gle.com>
Cc:     Jonathan Kowalski <bl0pbl33p@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Christian Brauner <christian@...uner.io>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Andy Lutomirski <luto@...nel.org>,
        David Howells <dhowells@...hat.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Kees Cook <keescook@...omium.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Michael Kerrisk-manpages <mtk.manpages@...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>
Subject: Re: [PATCH 0/4] pid: add pidctl()

On Mon, Mar 25, 2019 at 9:15 PM Daniel Colascione <dancol@...gle.com> wrote:
> On Mon, Mar 25, 2019 at 12:42 PM Jonathan Kowalski <bl0pbl33p@...il.com> wrote:
> > On Mon, Mar 25, 2019 at 6:57 PM Daniel Colascione <dancol@...gle.com> wrote:
[...]
> > Yes, but everything in /proc is not equivalent to an attribute, or an
> > option, and depending on its configuration, you may not want to allow
> > processes to even be able to see /proc for any PIDs other than those
> > running as their own user (hidepid). This means, even if this new
> > system call is added, to respect hidepid, it must, depending on if
> > /proc is mounted (and what hidepid is set to, and what gid= is set
> > to), return EPERM, because then there is a discrepancy between how the
> > two entrypoints to acquire a process handle do access control.
>
> That's why I proposed that this translation mechanism accept a procfs
> root directory --- so you'd specify *which* procfs you want and let
> the kernel apply whatever hidepid access restrictions it wants.
[...]
> > > and 2) it's
> > > "fail unsafe": IMHO, most users in practice will skip the line marked
> > > "LIVENESS CHECK", and as a result, their code will appear to work but
> > > contain subtle race conditions. An explicit interface to translate
> > > from a (PIDFD, PROCFS_ROOT) tuple to a /proc/pid directory file
> > > descriptor would be both more efficient and fail-safe.
> > >
> > > [1] as a separate matter, it'd be nice to have a batch version of close(2).
> >
> > Since /proc is full of gunk,
>
> People keep saying /proc is bad, but I haven't seen any serious
> proposals for a clean replacement. :-)
>
> > how about adding more to it and making
> > the magic symlink of /proc/self/fd for the pidfd to lead to the dirfd
> > of the /proc entry of the process it maps to, when one uses
> > O_DIRECTORY while opening it? Otherwise, it behaves as it does today.
> > It would be equivalent to opening the proc entry with usual access
> > restrictions (and hidepid made to work) but without the races, and
> > because for processes outside your and children pid ns, it shouldn't
> > work anyway, and since they wouldn't have their entry on this procfs
> > instance, it would all just fit in nicely?
>
> Thanks. That'll work. It's a bit magical, but /proc/self/fd is magical
> anyway, so that's okay.

Please don't do that. /proc/$pid/fd refers to the set of file
descriptors the process has open, and semantically doesn't have much
to do with the identity of the process. If you want to have a procfs
directory entry for getting a pidfd, please add a new entry. (Although
I don't see the point in adding a new procfs entry for this when you
could instead have an ioctl or syscall operating on the procfs
directory fd.)

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