this post was submitted on 09 May 2024
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The article talks about
sudoanddoasbeing SUID binaries and having a larger attack surface thanrun0would. Could someone ELI5 what this means?Basically, the SUID bit makes a program get the permissions of the owner when executed. If you set
/bin/bashas 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
sudoanddoasuse 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.
Sounds good in theory.
But I've had so many issues with D-Bus fucking shit up on my systems that I'd be very reluctant to hinge my only way of recovering from failures upon something so brittle.
Granted, D-Bus hasn't given me any trouble since moving to NixOS. The hell of trying to recover my arch systems from a perpetually failing D-Bus would make me very apprehensive to adopt this. I could see myself using run0 by default, but keeping sudo-rs or doas around with a much stricter configuration as a failsafe until the run0 + D-Bus + PolKit is absolutely stable and bulletproof.
I haven't had D-Bus problems in quite a while but actually run0 should help with some of those issues. Like,
systemctl --userwill actually work when used with run0, or at least systemd-run can.Haven't used it yet so it's all theoretical, but it makes sense to me especially at work. I've used systemd-run to run processes in very precise contexts, it's worth using even if just to smush together schedtool, numactl, nice, taskset and sudo in one command and one syntax. Anything a systemd unit can do, systemd-run and run0 can do as well.
I'm definitely going to keep
suaround just in case because I will break it the same I've broken sudo a few times, but I might give it a shot and see if it's any good just for funsies.Just trying to explain what it does and what it can do as accurately as possible, because out of context "systemd adds sudo clone" people immediately jump to conclusions. It might not be the best idea in the end but it's also worth exploring.
At that point just set a break-glass root password and don’t use sudo or doas.