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
