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.
