Linux Capabilities Revisited
16 comments
·October 22, 2025Nifty3929
M95D
For this to work, Linux needs a centralized way of managing caps. Review (or diff) the file and know immediately what's changed, instead of looking at ACLs all over the place.
Traditional unix /etc/group style.
surajrmal
A shared global namespace ultimately makes it very difficult to have a decent capability based security system. Namespaces limited to the set of actions you have and a hierarchy of capabilities whereby children can only be given access to capabilities their parents have is required for a sane view of how things work. Much like encapsulation makes it easier to reason about abstractions in a program, this nested hierarchy of capabilities makes it easier to reason about the privilege of various parts of the system. Instead we have soup where no one can quite reason about what has access to what.
PaulHoule
Note it is just a set of flags that subdivide the privileges root has which is potentially an improvement over what we had before but it's nothing like the real capability-based security
https://en.wikipedia.org/wiki/Capability-based_security
that you had in AS/400 or the iAPX 432 where a "capability" is a reference to a system object with associated privileges. It is possible to get this into a POSIX-like system
https://en.wikipedia.org/wiki/Capsicum_(Unix)
It reminds me of using a VAX-11/730 with the VMS operating system in high school where there was a long list of privileges a process could have
https://hunter.goatley.com/vax-professional-articles/vax-pro...
and it was a common game to investigate paths such as "if you have privilege A, B, and C you can get SETPRV and take over the machine"
nullpoint420
A very similar process happens in security research for macOS and iOS w/ the mach kernel. Researchers look for open mach ports that are ripe for privilege escalation.
rootnod3
Even if not super fine grained, I think that OpenBSD’s pledge is really nicely done.
Next after that I’d vote for FreeBSD’s capsicum.
hypeatei
OpenBSDs pledge is so simple and nice to use. I really wish Linux would incorporate it. Seccomp is a nightmare to implement.
rootnod3
I'd rather have a simple coarse-grained mechanism than whatever feverdream that seccomp, selinux and apparmor are. A convoluted mess incorporating almost Turing complete languages that are just asking to shoot yourself in the foot a mile deep.
The simplicity of pledge is good enough for 99% of use-cases I'd wager AND easy to add to existing code.
eikenberry
There is a port...
https://github.com/jart/pledge
https://justine.lol/pledge/charcircuit
I disagree. Pledge requires every app to OPT IN to security. This means that most apps won't do it, and the ones that do will likely be lazy and restrict their usage to what they use before and won't do the work of rearchitecting things.
hypeatei
So we shouldn't provide simple hardening tools because it won't be used or applied how you'd like in 100% of software? This mindset in security circles really needs to stop.
If a piece of important or foundational software wants to lock itself down today, look at the myriad of convoluted "solutions" mentioned in a sibling comment. If you wanted to discourage progress in this area, that's how you'd design something. I'm not assuming malice, obviously, but it's certainly a product of the endless nitpicking and "not good enough, doesn't cover <niche usecase>" type of thinking.
EDIT:
> and the ones that do will likely be lazy
I'd argue the opposite, any developer taking the time to add some pledge calls to their code is probably mindful of security and wants to improve it. If you wanted to be lazy, you'd just... not implement pledge at all since it'd get in your way and be too restrictive.
cyberax
The thing is, it works better. A simple API like pledge/unveil allows apps to significantly improve the security level without much of time investment.
Meanwhile, complex external systems like SELinux end up being unused because they are complex and external (and thus can just be ignored).
monocasa
Linux capabilities are fairly unrelated to the traditional concept of object capability based security.
They're closer to apple entitlements, but inherited through through forks rather than being attached to a binary.
marcosdumay
> whereby children can only be given access to capabilities their parents have
The one thing that makes capabilities usable is that they don't need to follow that rule.
If you don't have processes that let your programs get capabilities from any source other than their creation, you are better just adding your program names into your ACLs.
candiddevmike
I don't think you can bolt something like what you're describing onto an existing kernel (like linux did with capabilities). You'd have to design it this way from the ground up. I think I've read about some experimental OSs exploring that kind of capability by design, almost like a type 1 hypervisor but for processes.
HugoTea
Fascinating, thank you for sharing.
One problem that I have with fine-grained ACLs is that they can unintentionally add security risk, because sometimes those finer grained controls can be exploited to gain additional privledges.
If I grant something root, I know what that means and I'll be very careful. But if I grant something permission X thinking I'm safe, and then it can be used to gain permission Y, or even root, then I can be accidentally exposed.
There is just a much larger surface area to guard against, ensuring that each granular permission can't be so exploited.