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

[Daniel Colascione]

On Sat, Apr 20, 2019 at 12:14 AM Kevin Easton <kevin@xxxxxxxxxxx> wrote:
> On Mon, Apr 15, 2019 at 01:29:23PM -0700, Andy Lutomirski wrote:
> > 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.
>
> The idea of a system call that takes an infinitely-extendable laundry
> list of operations to perform in kernel space seems quite inelegant, if
> only for the error-reporting reason.
>
> Instead, I suggest that what you'd want is a way to create a new
> embryonic process that has no address space and isn't yet schedulable.
> You then just need other-process-directed variants of all the normal
> setup functions - so pr_openat(pidfd, dirfd, pathname, flags, mode),
> pr_sigaction(pidfd, signum, act, oldact), pr_dup2(pidfd, oldfd, newfd)
> etc.

Providing process-directed versions of these functions would be useful
for a variety of management tasks anyway,

> Then when it's all set up you pr_execve() to kick it off.

Yes. That's the right general approach.