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From:   Jann Horn <jannh@google.com>
Date:   Mon, 25 Mar 2019 22:15:22 +0100
Message-ID: <CAG48ez2Fy1tFyZqy7bVwG=aBJnwm1mdZgHzchFiguzufctw03Q@mail.gmail.com>
Subject: Re: [PATCH 0/4] pid: add pidctl()
To:     Jonathan Kowalski <bl0pbl33p@gmail.com>
Cc:     Daniel Colascione <dancol@google.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Christian Brauner <christian@brauner.io>,
        Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
        Andy Lutomirski <luto@kernel.org>,
        David Howells <dhowells@redhat.com>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Linux API <linux-api@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Kees Cook <keescook@chromium.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Michael Kerrisk-manpages <mtk.manpages@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>,
        Aleksa Sarai <cyphar@cyphar.com>,
        Al Viro <viro@zeniv.linux.org.uk>
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On Mon, Mar 25, 2019 at 9:40 PM Jonathan Kowalski <bl0pbl33p@gmail.com> wrote:
> On Mon, Mar 25, 2019 at 8:34 PM Jann Horn <jannh@google.com> wrote:
> >
> >  [...SNIP...]
> >
> > 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.)
>
> There is no new entry. What I was saying (and I should have been
> clearer) is that the existing entry for the fd when open'd with
> O_DIRECTORY makes the kernel resolve the symlink to /proc/<PID> of the
> process it maps to, so it would become:
>
>   int dirfd = open("/proc/self/fd/3", O_DIRECTORY|O_CLOEXEC);

That still seems really weird. This magically overloads O_DIRECTORY,
which means "fail if the thing is not a directory", to suddenly have
an entirely different meaning for one magical special type of file. On
top of that, unlike an ioctl or a new syscall, it doesn't convey
explicit intent and increases the risk of confused deputy issues.

> This also means you cannot cross the filesystem boundry, the said
> process needs to have a visible entry (which would mean hidepid= and
> gid= based access controls are honored), and you can only open the
> dirfd of a process in the current ns (as the PID will not map to an
> existent process if the pidfd maps to a process not in the same or
> children pid ns, in fdinfo it lists -1 in the pid field (we might not
> even need fdinfo anymore)).

AFAICS that doesn't have anything to do with whether you do this as a
syscall, as an ioctl, or as a jumped symlink. The kernel would have to
do the same security checks in any of those cases - only a classic,
non-jumped symlink would implicitly go through the existing permission
checks. And if you implement this with a non-jumped symlink, you get
races.