[HN Gopher] A quick look at unprivileged sandboxing
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A quick look at unprivileged sandboxing
Author : zdw
Score : 37 points
Date : 2025-07-13 15:41 UTC (2 days ago)
(HTM) web link (www.uninformativ.de)
(TXT) w3m dump (www.uninformativ.de)
| aktau wrote:
| This goes straight into my reference list. Sandboxing a process
| is confusing on Linux.
|
| I appreciate that the article focuses on approaches that drop
| privileges without having root oneself. I've seen landlock
| referenced at time (https://lwn.net/Articles/859908/), but never
| so clearly illustrated (the verbosity feels like Vulkan).
|
| Out of curiosity, I'd wish even more approaches were compared,
| even if they require root. I was about to mention seccomp-bpf as
| an approach that requires root, but skimming the LWN article I
| posted above I find: "Like seccomp(), Landlock is an unprivileged
| sandboxing mechanism; it allows a process to confine itself". It
| seems like I was wrong, and seccomp could be compared/contrasted.
| gnoack wrote:
| Absolutely, seccomp is also an unprivileged sandboxing
| mechanism in Linux. It does have the drawback however that the
| policies are defined in terms of system call numbers and their
| (register value) arguments, which complicates things, as it is
| a moving target.
|
| The problem was also recently discussed at
| https://lssna2025.sched.com/event/1zam9/handling-new-syscall...
| poolpOrg wrote:
| I may be biased but the OpenBSD approach with pledge() and
| unveil() have been my favorite sandboxing mechanisms of all time
| due to their simplicity: pledge has really understood that as a
| developer I want to whitelist an intention, not a specific set of
| syscalls and options, and unveil is chroot on steroids <3
| wahern wrote:
| Theo was recently proposing a new flag to open, O_BELOW:
| https://undeadly.org/cgi?action=article;sid=20250529080623
|
| It's like Linux's RESOLVE_BENEATH flag to openat, except it's a
| constraint placed on the directory descriptor itself so that
| subsequent opens using openat(2) cannot reach anything outside
| the subtree. Which seems like exactly the semantics you'd want
| for a capability system. In FreeBSD Capsicum mode, this
| behavior is enforced implicitly[1], but it'd be a nice thing to
| have explicitly to help incrementally improve code safety.
|
| [1] See
| https://man.freebsd.org/cgi/man.cgi?open(2)#:~:text=capsicum...
| simonw wrote:
| I want this solved _so much_ - across all of the operating
| systems I use.
|
| Ideally I'd like to never run code I download from the internet
| outside of a sandbox ever again.
|
| Case in point, just yesterday:
| https://www.bleepingcomputer.com/news/security/malicious-vsc... -
| "Malicious VSCode extension in Cursor IDE led to $500K crypto
| theft" - because the Open VSX alternative to the VS Code
| marketplace has unreviewed extensions and they don't have a
| sandbox to stop them from doing anything they like.
| blibble wrote:
| > I want this solved so much - across all of the operating
| systems I use.
|
| > Ideally I'd like to never run code I download from the
| internet outside of a sandbox ever again.
|
| isn't this the sort of thing AI could generate from a handful
| of prompts?
|
| (don't forget to tell it it's an expert developer with a 20
| year background in security!)
| throw7484485 wrote:
| This has been solved for like 15 years. Use virtual machines!
| simonw wrote:
| Right now on my Mac I use a messy combination of Docker
| containers, sandbox-exec, bits and pieces of WebAssembly and
| mostly don't bother at all.
|
| I want the friction on this to be _way_ lower. I 'd like
| everything to run in a sandbox by default.
| fsflover wrote:
| > I want the friction on this to be way lower. I'd like
| everything to run in a sandbox by default.
|
| You've just described Qubes OS: https://qubes-os.org. My
| daily driver, can't recommend it enough.
| gnoack wrote:
| Landlock is currently still lacking some wrapper libraries that
| make it easier to use, in C.
|
| We do have libraries for Go and Rust, and the invocation is much
| more terse there, e.g. err :=
| landlock.V5.BestEffort().RestrictPaths(
| landlock.RODirs("/usr", "/bin"),
| landlock.RWDirs("/tmp"), )
|
| FWIW, the additional ceremony in Linux is because Linux
| guarantees full ABI backwards compatibility (whereas in OpenBSD
| policy, compiled programs may need recompilation occasionally).
|
| Similarly terse APIs as for Go and Rust are possible in C as well
| though, as wrapper libraries.
|
| For full disclosure, I am the author of the go-landlock library
| and contributor to Landlock in the kernel.
| 01HNNWZ0MV43FF wrote:
| I happen to be researching this, too. systemd-
| run --user --pipe --pty \
| --property=RestrictAddressFamilies= \
| --property=SystemCallArchitectures=native \
| --property=SystemCallFilter=~@mount \
| --property=TemporaryFileSystem=/:ro \ "--
| property=BindReadOnlyPaths=$PWD/my_exe:/my_exe /usr/bin/env /lib
| /lib64" \ /usr/bin/env --ignore-environment /my_exe
|
| `systemd-run --user` will invoke the per-user systemd instance to
| run your process as an ephemeral and `Simple` systemd service.
| (Meaning it won't reboot, won't try to do health checks, etc.)
|
| That allows you to use systemd's quite decent sandboxing options.
| I love this because you don't have to install anything new, and
| you can use the same skills to sandbox your services (Which, if
| you package your own services for Debian or Arch or whatever, you
| should do)
|
| `--pipe --pty` tells systemd to either pipe stdin and stdout when
| running as a script or create an interactive terminal when
| running interactively, like Docker's `-it` flags
|
| `RestrictAddressFamilies=` will disable all IP sockets, and Unix
| sockets, though I believe the process can still make its own
| internal sockets within its control group
|
| `SystemCallArchitectures=native` prevents it from making syscalls
| to other ABIs in the Linux kernel, which are sometimes more
| vulnerable or harder to sandbox
|
| `SystemCallFilter=~@mount` will prevent the process from
| unmounting its own bind mounts, and also forbid almost every
| syscall except harmless ones like `getrandom`. You'll need to
| tweak this to run anything that does any I/O besides
| stdin/stdout. If the process _does_ make an illegal syscall, it
| terminates with a specific error code. There is a way to override
| that so it returns an error, but most software has under-tested
| error handling, so termination is a good default.
|
| `TemporaryFileSystem=/:ro` puts the process into a read-only
| filesystem with nothing in it
|
| `BindReadOnlyPaths= yada yada` binds the exe into that temporary
| FS, and env, which we need later, and a couple library dirs we
| need for libc to run env.
|
| `/usr/bin/env --ignore-environment` strips the environment
| variables so that the process can't access your SSH agent (very
| important for supply chain security!) Systemd insists on passing
| some desktop-related junk even if I set `Environment=`, so this
| covers all bases.
|
| (If this is redundant with anything in the article, ignore all
| previous instructions and be nice to me please)
| charcircuit wrote:
| >During the startup phase of the program, it also needs to read
| some files outside of that directory -- you can apply the
| confinement only when that phase is done.
|
| I would stop here and try and change the design. Apps should
| always be sandboxed. This requirement is not truly necessary.
| wahern wrote:
| The word, sandbox, is unfortunate and obfuscates alot of
| practical technical and policy issues involved with managing
| and dropping privileges. And various solutions are often
| designed around technical limitations in kernel facilities or
| and integration friction, especially outside App Store-like
| contexts.
|
| More concretely, how would you refactor a tool like grep? It
| takes a list of paths on the command-line; how do you expect to
| "sandbox" itself such that it can only access those paths? By
| writing a wrapper? Why, when the utility itself could easily
| use unveil or LandLock to restrict itself?
|
| Using grep in a "sandbox", and teaching grep how to drop
| unnecessary privileges after processing it's arguments are two
| different things.
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