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.
