FreeBSD Capsicum vs. Linux Seccomp Process Sandboxing(vivianvoss.net) |
FreeBSD Capsicum vs. Linux Seccomp Process Sandboxing(vivianvoss.net) |
This article contains a lot of errors, for example Chromium on FreeBSD does NOT use Capsicum, it never has. That was experimental and invasive work done 17 years ago that was NEVER committed to their official ports repository. In fact, not a single browser on FreeBSD uses Capsicum or any form of sandboxing _at all_.
https://github.com/rwatson/chromium-capsicum
https://www.freshports.org/www/chromium/
https://cgit.freebsd.org/ports/log/www/chromium/Makefile?qt=...
Contrast that with OpenBSD, where the Chromium port has used pledge(2) since January 2016, and unveil(2) since 2018. Both are enabled by default. Mozilla Firefox ports also use both pledge and unveil since 2018-2019, with refinements over the years.
https://marc.info/?l=openbsd-ports-cvs&m=145211683609002&w=2
https://marc.info/?l=openbsd-ports-cvs&m=153250162128188&w=2
OpenBSD's fork of tcpdump has been privsep for ~22 years, and its packet parser runs with no privileges. It's pledged tightly "stdio" and has no network/filesystem access, and uses OpenBSD specific innovations like bpf descriptor locking (BIOCLOCK) missing from both FreeBSD/Linux tcpdump today (despite FreeBSD adding the ioctl in 2005).
In the years since it was added, the reason Capsicum has only been applied to a handful of utilities is because it's a tree barren of decades worth of incremental work on privilege separation and security research.
https://github.com/openssh/openssh-portable/blob/master/sshd...
https://github.com/openssh/openssh-portable/blob/master/sand...
https://github.com/openssh/openssh-portable/blob/master/sand...
https://github.com/openssh/openssh-portable/blob/master/sand...
w/ Capsicum, beyond faffing around with some file descriptors, it's unclear what security cap_enter() adds:
https://github.com/openssh/openssh-portable/blob/master/sand...
Archive: https://archive.ph/rLmTq
Sometime ago I wrote a library for a customer that did exactly that: Open a number of resources, e.g., stdin, stdout, stderr, a pipe or two, a socket or two, make the seccomp calls necessary to restrict the use of read/write/etc. to the associated file descriptors, then lock out all other system calls - which includes seccomp-related calls.
Basically, the library took a very Capsicum-like approach of whitelisting specific actions then sealing itself against further changes.
This is a LOT of work, of course, and the available APIs don't make it particularly easy or elegant, but it is definitely doable. I chose this approach because the docker whitelist approach was far too open ended and "uncurated", if you will, for the use-case we were targeting.
In this particular case, I was aided by the fact the library was written to support the very specific use-case of filters running in containers using FIFOs for IPC, logging, and reporting: Every filter saw exactly the same interfaces to the world, so it was relatively easier to lock things down.
Having said that, I wish Linux had a Capsicum-equivalent call, or, even better for the approach I took, a friendlier way to whitelist specific calls.
I already find it very frustrating that most open source projects spawning on HN's front page are resume-boosting AI slop but if blogs start being the same the internet is definitely dead.
Edit: it doesn't even looks like it's resume-boosting in this case, the “person” behind it doesn't even appear to exist. We can only speculate about the intent behind this.
- the profile picture is almost certainly (like 99%, certainty) AI-generated (I can even tell you it's ChatGPT-generated, the style is way too characteristic to miss).
- the LinkedIn profile shows prolific activity for the past few days, but almost nothing before that, I'm not sure the profile existed before.
- the github account is just 2 weeks old.
Having a mutual connection doesn't mean much, the interesting question would be who's the mutual and for how long has he be a connection. It's not hard to get to 500 LinkedIn connections on LinkedIn in a few days, you just need to add headhunters and other hiring specialists, they'll never refuse an invitation from a profile that look interesting. They could also have added someone who interacted with their LinkedIn slop submission, making the person more likely to accept the invitation.
I hate becoming the old person shaking their fist at the sky but the AI bros have just gone too far. I don't know why there isn't a bigger political and social movement against them. I would sign up in an instant to see their companies and practices regulated out of existence.
On Linux I understand that Landlock is the way to go.
Between file system, bind/connect, and sending signals, that covers most of it. Probably the biggest remaining risk is any unpatched bugs in the kernel itself.
So one would need to first gain execution in the process, and then elevate that access inside the kernel, in a way that doesn't just grant you root but still Landlocked, and with a much smaller effective syscall attack surface. Like even if there's a kernel bug in ioctl on devs, landlock can turn that off too.
The UI is fun but unreadable, but content is solid. Explain how this is slop please.
But hey, it's a game!
Also, what is well-known piece of software that uses Capsicum on FreeBSD ? Can someone name a few ?
They also operate under pretty fundamentally different philosophies. Seccomp is based on a program dropping its own permissions. SELinux is based on a system integrator writing an ahead of time policy restricting what a program can do.
78.46.78.181
$ curl -v https://vivianvoss.net/ 2>&1 | tail -3
* OpenSSL/3.0.13: error:0A00010B:SSL routines::wrong version number
* Closing connection
curl: (35) OpenSSL/3.0.13: error:0A00010B:SSL routines::wrong version number
$ curl -v http://vivianvoss.net/ 2>&1 | grep Location
< Location: https://www.safebrowse.io/warn.html?url=http://vivianvoss.ne...
$ whois 78.46.78.181 | grep -i netname
netname: HETZNER-RZ-NBG-NET
$ host 78.46.78.181
181.78.46.78.in-addr.arpa domain name pointer min2max.run.
The domain's authoritative nameserver (Infomaniak) points vivianvoss.net at 78.46.78.181 — a Hetzner box in Germany with rDNS min2max.run. That server redirects HTTP to SafeBrowse.io and responds to TLS handshakes with garbage. Not a local issue, not a DNS hijack — the A record itself is wrong.
I'm not sure what glibc's latest policy is on linking statically, but at least it used to be basically unsupported and bugs about it were ignored. But even if supported, you can't know if it under some configurations or runtime circumstances uses dlopen for something.
Or maybe once you juggle more than X file descriptors some code switches from using `poll()` to using `select()` (or `epoll()`).
My thoughts last time I looked at seccomp: https://blog.habets.se/2022/03/seccomp-unsafe-at-any-speed.h...
That would break capsicum, too, so I don’t see how that’s a problem when “comparing Capsicum to using seccomp in the same way”.
> Why trust a program to set its own capabilities?
An example may be that a program starts needing a wide range of capabilties but can then ratchet down to a reduced set once running, aka "privdrop".
> why isn't there a way to set capabilities from the parent process when execing?
There have been replies on other systems so just to stick with pledge which provides the abiliy to set "execpromises" to do this.
[1] https://man.openbsd.org/pledge
[2] https://www.openbsd.org/papers/eurobsdcon2017-pledge.pdf
[3] https://www.openbsd.org/papers/BeckPledgeUnveilBSDCan2018.pd...
[1] https://bsdb0y.github.io/posts/openbsd-intro-to-update-on-pl...
The way capabilities usually work is you more or less turn off the usual do whatever you want syscalls, and have to do restricted things through FDs that have the capability to do them. So like, no more open any path, you have to use openat with a FD in your directory of interest. But that requires the program to understand how to use the capabilities and how to be passed them. It's not something that you can just impose.
My understanding of SELinux, is it can be imposed on a program without the knowledge of the program, because it's more or less matching rules for syscalls... rather than giving a restricted FD to use with openat, you restrict the options for open.
Openbsd had a neat external syscall sandboxing system at one point (systrace ) it was removed for reasons I don't fully understand. But I think it boils down to "optional security isn't". hard to maintain, problematic, external policies, the first thing you do is disable them (cough selinux cough)
Child processes inherit the restrictions from the parent. You can therefore have the parent fork, setup it's rules, then exec. This is exactly how syscall filtering (and a bunch of other lockdowns) are implemented in SystemD
But I am pretty sure you CAN get your capabilities from a patent process using capsicum, since they are just file descriptors.
Landlock is one of my favorite linux-only APIs almost feels like it was FreeBSD's answer to some Linux feature.
It reminds me the pinnacle of design - Microsoft Authenticator. On Android, out of the blue, it displays global overlay to select one of the 3 numbers to confirm login.
The overlay is ... transparent.
1. The post mainly reiterates a single idea (Capsicum enumerates what the process can do, seccomp provides a configurable filter) in many different ways. There is not much actual depth, code samples notwithstanding. Nothing on why different designs were chosen, how easy each is to use, outcomes besides the Chrome example, etc.
2. There are a lot of AI writing tells, like staccato sentences, parallelism ("Same browser. Same threat model. Same problem."), pointless summary tables, "it's not X, it's Y" contradiction ("This is not a bug. It is the original Unix security model"), etc.
3. The author has roughly a blog post a day, all with similar style and on widely varied topics, and in the same writing style. Unless the author has deep expertise on a remarkably wide range of topics and spends all their time writing, these can't reflect deep insight or experience, but minimal editing of AI output.
So yes, it's pretty sloppy.
$ curl -v https://vivianvoss.net/ 2>&1 | tail -3
<script src="/assets/scripts/perf.js"></script>
</body>
</html>
* Established connection to vivianvoss.net (78.46.78.181 port 443) from 172.16.245.55 port 36208
--
Unrelated, but the site also returns an AAAA record for an ipv6 address that does not work. So they've misconfigured their server in that regard.
$ drill vivianvoss.net AAAA @1.1.1.1
[...]
vivianvoss.net. 3600 IN AAAA 2a01:4f8:120:34ad::1
[...]
$ curl --header 'Host: vivianvoss.net' 'https://[2a01:4f8:120:34ad::1]:443'
<hangs forever>
$ curl https://ipv6.google.com
<works immediately>-------
$ dig vivianvoss.net A +short @8.8.8.8
78.46.78.181
$ curl -v4 https://vivianvoss.net/ 2>&1 | grep -E "Connected|error"
* Connected to vivianvoss.net (78.46.78.181) port 443
* OpenSSL/3.0.13: error:0A00010B:SSL routines::wrong version number
$ curl -s https://ipinfo.io | grep org
"org": "AS7922 Comcast Cable Communications, LLC",
Same IP you're hitting, same port, but Comcast's xFi Advanced Security seems to be MITMing the connection before TLS completes.
I hate Comcast so much ...
Your example may be true, but I'm guessing it's not a guarantee. Not to mention if one wants to be portable to musl or cosmopolitan libc. The others inherently are more likely to work in a way that any libc would be "unsurprised" by.
“The goal of Landlock is to enable restriction of ambient rights (e.g. global filesystem or network access) for a set of processes. Because Landlock is a stackable LSM [(Linux Security Model)], it makes it possible to create safe security sandboxes as new security layers in addition to the existing system-wide access-controls. ... Landlock empowers any process, including unprivileged ones, to securely restrict themselves.”
The one restriction I'm not sure about is that you can't say "~/ except ~/.gnupg". You have to actually enumerate everything you do want to allow. But maybe that's for the best. Both because it mandates rules not becoming too complex to reason about, and because that's a weird requirement in general. Like did you really mean to give access to ~/.gnupg.backup/? Probably not. Probably best to enumerate the allowlist.
And if you really want to, I guess you can listdir() and compose the exhaustive list manually, after subtracting the "except X".
I find seccomp unusable and not fit for purpose, but landlock closes many doors.
Maybe you know better? I'd love to hear your take.