linux-kernel - Re: [PATCH v1 2/4] pid: add pidctl()

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Date:   Tue, 26 Mar 2019 09:42:59 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Christian Brauner <christian@...uner.io>
Cc:     Daniel Colascione <dancol@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        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>,
        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 v1 2/4] pid: add pidctl()

On Tue, Mar 26, 2019 at 9:34 AM Christian Brauner <christian@...uner.io> wrote:
>
> On Tue, Mar 26, 2019 at 05:31:42PM +0100, Christian Brauner wrote:
> > On Tue, Mar 26, 2019 at 05:23:37PM +0100, Christian Brauner wrote:
> > > On Tue, Mar 26, 2019 at 09:17:07AM -0700, Daniel Colascione wrote:
> > > > Thanks for the patch.
> > > >
> > > > On Tue, Mar 26, 2019 at 8:55 AM Christian Brauner <christian@...uner.io> wrote:
> > > > >
> > > > > The pidctl() syscalls builds on, extends, and improves translate_pid() [4].
> > > > > I quote Konstantins original patchset first that has already been acked and
> > > > > picked up by Eric before and whose functionality is preserved in this
> > > > > syscall:
> > > >
> > > > We still haven't had a much-needed conversation about splitting this
> > > > system call into smaller logical operations. It's important that we
> > > > address this point before this patch is merged and becomes permanent
> > > > kernel ABI.
> > >
> > > I don't particularly mind splitting this into an additional syscall like
> > > e.g.  pidfd_open() but then we have - and yes, I know you'll say
> > > syscalls are cheap - translate_pid(), and pidfd_open(). What I like
> > > about this rn is that it connects both apis in a single syscall
> > > and allows pidfd retrieval across pid namespaces. So I guess we'll see
> > > what other people think.
> >
> > There's something to be said for
> >
> > pidfd_open(pid_t pid, int pidfd, unsigned int flags);
> >
> > /* get pidfd */
> > int pidfd = pidfd_open(1234, -1, 0);
> >
> > /* convert to procfd */
> > int procfd = pidfd_open(-1, 4, 0);
> >
> > /* convert to pidfd */
> > int pidfd = pidfd_open(4, -1, 0);
>
> probably rather:
>
> int pidfd = pidfd_open(-1, 4, PIDFD_TO_PROCFD);

Do you mean:

int procrootfd = open("/proc", O_DIRECTORY | O_RDONLY);
int procfd = pidfd_open(procrootfd, pidfd, PIDFD_TO_PROCFD);

or do you have some other solution in mind to avoid the security problem?