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

[Kevin Easton]

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.

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

    - Kevin