Capsudo: Rethinking sudo with object capabilities

https://ariadne.space/2025/12/12/rethinking-sudo-with-object-capabilities.html

by fanf2 • 7 months ago

93 58 7 months ago

That's functionally equivalent to using sudo but only allowing a certain shell script that's a wrapper for what needs to be done by a given user (to avoid the whole syntax mess). But somehow with more boilerplate.

ryukafalz• 7 months ago

If I'm a user who's been given access to run such a wrapper script via sudo, how do I further delegate that access?

thwarted• 7 months ago

What's missing in this writeup is if/how capsudod can be configured to drop/acquire capabilities (in the capabilities(7) sense, in a namespace sense, or using openbsd's pledge(2)) before executing the command. From looking at the code it seems to rely on filesystem permissions to access the capsudod socket; when in order to make mounting USB drives actually safe via this kind of permission proxying method, you'd want to only give it very limited access to operations, like only the ability to make the mount(2) syscall. I generally agree, this is sudo in a different format and with a daemon; perhaps that's better. And it's written in C, which doesn't speak to avoiding the classic C-related privilege escalation risks (although this is a small amount of code compared to sudo).

esseph6• 7 months ago

Every time somebody wants to do something in Linux and they mention Objects, I turn around and go the other way.

These people still do not understand why that userspace became so powerful and so useful.

I also think that's the "solution", which is to craft a new optional userspace experience that leaves traditional unix strings and pipes alone. It's not for me, but I'm sure many would like it. I mean, look at PowerShell on Linux :/

charcircuit11• 7 months ago

You don't need to elevate privileges if you give things the right privileges from the start.

Sudo is just a hack to avoid setting up proper capabilities / permissions in the first place.

kccqzy2• 7 months ago

I am the owner and only user of the computer. Does that mean I should run everything with root? Of course not. It’s simply better to start with little privileges and then elevate when needed. Using any additional privileges should be an intentional act. I also do it the other way: reduce my privileges via sudo -u nobody.

charcircuit11• 7 months ago

The root account shouldn't exist either. Having god accounts is a bad idea security wise. Instead everything should follow the principle of least privilege.

Veserv• 7 months ago

No, you should run every program with only the privileges it needs. The very concept of running your programs with all your privileges as a user by default is wrong-headed to begin with. To strain the "user" model you should have a distinct "user" for every single program which has only the resources and privileges needed by/allocated to that program. The actual user can allocate their resources to these "users" as needed. This is a fairly primitive version of the idea due to having to torture fundamentally incompatible insecure building blocks to fit, but points in the direction of the correct idea.

arialdomartini• 7 months ago

Can you please elaborate on the use of sudo -u nobody? Do you use it interactively? I’m intrigued.

saghm2• 7 months ago

My most recent mini-adventure with sudo was on the Steam Deck; with OS updates, everything (or at least most?) of what's outside of your home directory is replaced. (In fact, you have to manually opt into being able to write to those directories at all, e.g. to use the system package manager instead of flatpak, by running `sudo steamos-readonly disable`). There are a couple sudo settings I change from the defaults, and because `/etc/sudoers and `/etc/sudoers.d/` (as in the entire directory) were restored to the base versions, my custom settings don't get preserved when updates occur. However, I was surprised to find out that using `visudo` to try to update the settings wasn't actually causing the settings I changed to take effect, which I eventually tracked down to some extra configs getting shipped by the base system in `/etc/sudoers.d`. I looked up what the precedence rules are for sudo rules, and apparently the rule is that anything in `/etc/sudoers.d/` will override `/etc/sudoers`, and the files in `/etc/sudoers.d/` are evaluated in lexigraphical order.

That's how I got where I am today, with a file called `zzz` that I copy into `/etc/sudoers.d/` every time there's a system update.

heavyset_go• 7 months ago

There's probably a way you can just set it and forget it with overlayfs.

OpenWRT solved the problem of updates via immutable firmware image flashing while maintaining customizations 15 years ago using it.

saghm2• 7 months ago

Probably! At the end of the day, this is the only change I need to make, and the updates are fairly infrequent, so this solution is good enough for me. I only brought it up as an example of how even the meta-rules around how sudo rules are interpreted might be more complex than people realize.

ninjin2• 7 months ago

I am somewhat cautious to comment as I know the author is way more experienced than I am and I fear that I may be missing something. However, let me try to accomplish the same with my elementary doas(1) knowledge.

Allowing mounting for a specific group is simple with doas.conf(5):

    permit :mountd cmd /sbin/mount
    permit :mountd cmd /sbin/umount

We can of course tighten it further as the author did:

    permit :mount-usb cmd /sbin/mount /dev/sdb1
    permit :umount-usb cmd /sbin/umount /media/usb

If you want to go more complex than specifying arguments, we could of course create a shell script and specify it instead of a binary.

Likewise, we can do something similar for a service account:

    permit :www-deployment as www-deployment cmd /var/www/bin/build /var/www/application

The key difference here would be that www-deployment can not delegate as easily to arbitrary users, as they would need to ask someone with root access to add additional users to the www-deployment group. But I am left wondering if this use case (if it is important enough) is not equally well served by specifying a location for non-root users to add permissions akin to what we see in doas.conf(5), but with the constraint that they of course can only allow other users to run commands with their privileges. Yes, it would "bloat" doas(1), but these code paths are not that long as long as you keep your scope constrained (doas(1) has a core of just over 500 lines and with environment handling and configuration format parsing we arrive a a final line count at just over 1,300).

At this point, the main advantage I see with capsudod is that you can more easily drop privileges and put in restrictions like pledge(2) before the binary is ever called upon by whatever user we have granted permissions. While with the doas(1) thinking above you have to run over plenty of code that could be exploited. Still, this feels like a rather minor relative improvement to what we already have.

Am I missing something in my ignorance? Lastly, let me also say that I am sure that sudo(8) has the ability to do the same things I proposed to do with doas(1) above, but I know the latter far better.

srazkvt• 7 months ago

sure, but that doesn't change the fact that doas(1) is a suid binary. everything done would be done as root, from parsing the config file, checking the rights, and finally executing the command.

capsudo here would rely on singular unix sockets with file access rights, so in essence, it would indeed be similar to what you could do with doas, but the idea here is to seperate things. with doas, doas would check if you have the correct group or user to do the command, while with capsudo, the kernel would check it, and reject it if you don't have the right.

davexunit4• 7 months ago

The whole problem is mapping privilege to users and groups, so doas doesn't solve the issues explained in the article.

The key difference here would be that www-deployment can not delegate as easily to arbitrary users, as they would need to ask someone with root access to add additional users to the www-deployment group. But I am left wondering if this use case (if it is important enough)...

Delegation is the killer feature of the object capability model. It's not just important enough, it's the most important. Keep in mind that the ACL model allows delegation, too, it's just unsafe. Users share credentials all the time. Capabilities allow delegation in a way that can be attenuated, revoked, and audited.

ninjin2• 7 months ago

Firstly, thank you for engaging and trying to enlighten me.

I do understand why capability delegation is useful and I am familiar with using Unix sockets to delegate the control of daemons using socket permissions, which feels similar to what we see here with capsudod (I have not read the code sadly, too much other code to read today).

However, I am still puzzled what the advantage of having a herd of capsudod instances running is to say my proposal of allowing users to set up their own doas.conf(5)s to delegate capabilities. Yes, we still need SUID and we will need to be darn sure 1,000 or so lines are properly secured, but it is attenuable, revocable, auditable, and feels (perhaps wrongly, because I have a bias towards text files describing the state of a system?) more natural to me than putting it all into the running state of a daemon.

Is there some other strength/weakness of these approaches that I am failing to see? I am no systems programmer, but I find topics like this interesting and dream of a day when I could be one.

haradion• 7 months ago

If you squint hard enough, this is really a pretty similar idea to Polkit [0] but with a much simpler communication layer.

[0] https://polkit.pages.freedesktop.org/polkit/

qhwudbebd• 7 months ago

Whenever I think about writing a central privileged daemon to grant capabilities to other processes, I'm puzzled by the choice to remove the old version of CAP_SETPCAP in 2.6.24: "grant or remove new capabilities to/from an existing running process" - sadly it still exists but means something else in newer kernels with filesystem capabilities.

(In a sense, not having this capability in processes running as root is theatre anyway: you have /dev/kmem access so could just edit the kernel data structures. It's just doing so cleanly that is no longer possible.)

Being able to briefly escalate my editor to have the capabilities to write /etc/wibble.conf when I started editing it as a non-privileged user, then take away the capability again would be more convenient that always needing to run the editor as root. (So convenient, in fact, that people fake this with little editor helpers that do the equivalent of 'really tee FILE-TO-WRITE >/dev/null', but that's an ugly hack.)

Denvercoder9• 7 months ago

you have /dev/kmem access so could just edit the kernel data structures.

Not anymore: since kernel 2.6.26 /dev/kmem only exists if CONFIG_DEVKMEM is enabled, and it was removed completely in 5.13.

[1] https://lwn.net/Articles/851531/ [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin...

davexunit4• 7 months ago

It's nice to see other people writing about the capability transfer feature of Unix domain sockets. File paths are not object capabilities, but file descriptors are. Using a privileged daemon on top of an ambient authority system like Linux seems to be a good way to retrofit object capabilities onto the operating systems we already use. This is the same approach we took in Goblins[0] for our Unix domain socket netlayer for the OCapN[1] protocol.

[0] https://spritely.institute/news/spritely-goblins-v0-16-0-rel...

[1] https://ocapn.org

nuudlman• 7 months ago

This feels like pfexec on Illumos/Solaris, although I’m too unqualified to say for certain.

Capsudo: Rethinking sudo with object capabilities

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