Re: RFC: on adding new CLONE_* flags [WAS Re: [PATCH 0/4] clone: add CLONE_PIDFD]

[Andy Lutomirski]

On Mon, Apr 15, 2019 at 12:59 PM Aleksa Sarai <cyphar@xxxxxxxxxx> wrote:
>
> On 2019-04-15, Enrico Weigelt, metux IT consult <lkml@xxxxxxxxx> wrote:
> > > This patchset makes it possible to retrieve pid file descriptors at
> > > process creation time by introducing the new flag CLONE_PIDFD to the
> > > clone() system call as previously discussed.
> >
> > Sorry, for highjacking this thread, but I'm curious on what things to
> > consider when introducing new CLONE_* flags.
> >
> > The reason I'm asking is:
> >
> > I'm working on implementing plan9-like fs namespaces, where unprivileged
> > processes can change their own namespace at will. For that, certain
> > traditional unix'ish things have to be disabled, most notably suid.
> > As forbidding suid can be helpful in other scenarios, too, I thought
> > about making this its own feature. Doing that switch on clone() seems
> > a nice place for that, IMHO.
>
> Just spit-balling -- is no_new_privs not sufficient for this usecase?
> Not granting privileges such as setuid during execve(2) is the main
> point of that flag.
>

I would personally *love* it if distros started setting no_new_privs
for basically all processes.  And pidfd actually gets us part of the
way toward a straightforward way to make sudo and su still work in a
no_new_privs world: su could call into a daemon that would spawn the
privileged task, and su would get a (read-only!) pidfd back and then
wait for the fd and exit.  I suppose that, done naively, this might
cause some odd effects with respect to tty handling, but I bet it's
solveable.  I suppose it would be nifty if there were a way for a
process, by mutual agreement, to reparent itself to an unrelated
process.

Anyway, clone(2) is an enormous mess.  Surely the right solution here
is to have a whole new process creation API that takes a big,
extensible struct as an argument, and supports *at least* the full
abilities of posix_spawn() and ideally covers all the use cases for
fork() + do stuff + exec().  It would be nifty if this API also had a
way to say "add no_new_privs and therefore enable extra functionality
that doesn't work without no_new_privs".  This functionality would
include things like returning a future extra-privileged pidfd that
gives ptrace-like access.

As basic examples, the improved process creation API should take a
list of dup2() operations to perform, fds to remove the O_CLOEXEC flag
from, fds to close (or, maybe even better, a list of fds to *not*
close), a list of rlimit changes to make, a list of signal changes to
make, the ability to set sid, pgrp, uid, gid (as in
setresuid/setresgid), the ability to do capset() operations, etc.  The
posix_spawn() API, for all that it's rather complicated, covers a
bunch of the basics pretty well.

Sharing the parent's VM, signal set, fd table, etc, should all be
options, but they should default to *off*.

(Many other operating systems allow one to create a process and gain a
capability to do all kinds of things to that process.  It's a
generally good idea.)

--Andy