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

[Christian Brauner]

On April 20, 2019 9:14:06 AM GMT+02:00, 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.
>
>Then when it's all set up you pr_execve() to kick it off.
>
>    - Kevin

I proposed a version of this a while back when we first started talking about this.