Linux

I never understood the hate, tbh. A lot of users don't even care if Sysd is used, as long as it works. So... Since the majority of distros use it... I think it works enough.

Basically, the SUID bit makes a program get the permissions of the owner when executed. If you set /bin/bash as SUID, suddenly every bash shell would be a root shell, kind of. Processes on Linux have a real user ID, an effective user ID, and also a saved user ID that can be used to temporarily drop privileges and gain them back again later.

So tools like sudo and doas use this mechanism to temporarily become root, then run checks to make sure you're allowed to use sudo, then run your command. But that process is still in your user's session and process group, and you're still its real user ID. If anything goes wrong between sudo being root and checking permissions, that can lead to a root shell when you weren't supposed to, and you have a root exploit. Sudo is entirely responsible for cleaning the environment before launching the child process so that it's safe.

Run0/systemd-run acts more like an API client. The client, running as your user, asks systemd to create a process and give you its inputs and outputs, which then creates it on your behalf on a clean process tree completely separate from your user session's process tree and group. The client never ever gets permissions, never has to check for the permissions, it's systemd that does over D-Bus through PolKit which are both isolated and unprivileged services. So there's no dangerous code running anywhere to exploit to gain privileges. And it makes run0 very non-special and boring in the process, it really does practically nothing. Want to make your own in Python? You can, safely and quite easily. Any app can easily integrate sudo functionnality fairly safely, and it'll even trigger the DE's elevated permission prompt, which is a separate process so you can grant sudo access to an app without it being able to know about your password.

Run0 takes care of interpreting what you want to do, D-Bus passes the message around, PolKit adds its stamp of approval to it, systemd takes care of spawning of the process and only the spawning of the process. Every bit does its job in isolation from the others so it's hard to exploit.

SUID stands for Set User ID. An SUID binary is a file that is always run with the UID of the owner user (almost always root). Note that this does not require that the user running them has root permissions, the UID is always changed. For instance, the ping command needs to set up network sockets, which requires root permissions, but is also often used by non-root users to check their network connections. Instead of having to sudo ping, any normal user is able to just run ping, as it uses SUID to run as the root user. sudo and doas also require functions that necessitate them running as root, and so if you can find out how to exploit these commands to run some arbitrary code without having to authenticate (since authentication happens after the binary has started running), there is a potential for vulnerabilities. Specifically, there is the privilege escalation, which is one of the most severe types of vulnerabilities.

run0 starts using systemd-run, which does not use SUID. Instead, it runs with the permissions of the current user, and then authenticates to the root user after the binary has already started to run. systemd-run contacts polkit for authentication, and if it succeeds, it creates a root PTY (pseudo-terminal/virtual terminal), and sends information between your session and the root PTY. So this means that in order to achieve privilege escalation with run0 as root, you have to actually authenticate first, removing the "before authentication" attack surface of sudo and doas.

TL;DR SUID binaries will always run as the owner (usually root), even before any form of authentication. run0 will start with the permissions of the current user, and then authenticate before running anything with root permissions.

Hear me out, breathes in, "Linux defender"

I’d just like to interject for a moment. What you’re referring to as Linux, is in fact, SystemD/Linux, or as I’ve recently taken to calling it, SystemD plus Linux. Linux is not an operating system unto itself, but rather another free component of a fully functioning SystemD system made useful by the SystemD corelibs, shell utilities and vital system components comprising a full OS as defined by POSIX.

Many computer users run a modified version of the SystemD system every day, without realizing it. Through a peculiar turn of events, the version of SystemD which is widely used today is often called Linux, and many of its users are not aware that it is basically the SystemD system, developed by the SystemD Project.

There really is a Linux, and these people are using it, but it is just a part of the system they use. Linux is the kernel: the program in the system that allocates the machine’s resources to the other programs that you run. The kernel is an essential part of an operating system, but useless by itself; it can only function in the context of a complete operating system. Linux is normally used in combination with the SystemD operating system: the whole system is basically SystemD with Linux added, or SystemD/Linux. All the so-called Linux distributions are really distributions of SystemD/Linux!

Systemd does a lot of things that could probably be separate projects, but run0 is an example of something that benefits from being a part of systemd. It ties directly into the existing service manager to spawn new processes.

It seems a fairly explicit goal of systemd to redefine Linux as a unified platform rather than as a kernel that can run any one of many implementations of many different services. I assume this is not just the systemd lead but also a goal of Red Hat.

Personally, while I am ok with systemd defining itself as a single source for all this functionality, I hate that they are taking away ( or making it hard at least ) to have independent implementations of these services.

What Chinera is doing with dinit and turnstile is really interesting. It would be nice to have feature comparable approaches to the systemd monolith that distributions could choose from.

You can uninstall the sudo application and add sudo as an alias for run0 in your shell initialization script. That's better than them renaming run0 to sudo, because that will prevent people from running the real sudo if they want it.

I'll just use an alias; sudo has been around for to long for me to change it and not be stressed about it.

Well, since doas has a Linux implementation, stealing that name would cause lots of issues to users who already use it or want to use doas instead of run0. This will be a default part of systemd; not a new package. The reason it's called run0 is because it's just a symbolic link to systemd-run, and instead of executing as an SUID binary, like sudo or doas, it runs using the current user's UID.

it's just a link to systemd-run which is a part of systemd, i doubt it works separately.

but, if you use s6 as an alternative init system, s6-sudo is a somewhat equivalent aproach to how run0 works (instead of systemd-run it calls s6-ipcclient)

The article says it works by messaging systemd to run the process as the given user, rather than being a SUID binary. So it wouldn't work without systemd.

That wasn't the "sudo", MS just named something else "sudo".