Re: Landlock support in setpriv(1)

Thank you Thomas!  I am excited to see this :)

On Sat, Dec 09, 2023 at 10:18:50AM +0100, Thomas Wei=C3=9Fschuh wrote:
> Hi everybody,
>=20
> For your information:
>=20
> There is a proposal to add landlock support to setpriv(1) from
> util-linux.
> While landlock is meant for self-sandboxing it can also be used to
> sandbox third party executables which makes it a nice fit for setpriv.
>=20
> If you have any remarks let me know.
>=20
> Link to the PR:
> https://github.com/util-linux/util-linux/pull/2628

For inspiration, here are some other existing tools which have a similar in=
terface:

* landlock-restrict (example tool for go-landlock):
  https://github.com/landlock-lsm/go-landlock/blob/main/cmd/landlock-restri=
ct/main.go
  (This one was written by me)

* sandboxer.c (sample tool from the kernel tree):
  https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/s=
amples/landlock/sandboxer.c

* sandboxer.rs (example tool for rust-landlock):
  https://github.com/landlock-lsm/rust-landlock/blob/main/examples/sandboxe=
r.rs

In my personal opinion, when I was sandboxing a few programs "from the outs=
ide"
with the Go tool, I often found that I would start out with a more coarse
ruleset that uses the predefined "convenience" groups of access rights, whi=
ch
are called "ROFiles", "RWFiles", "RODirs" and "RWDirs" in that tool, and wh=
ich
subsume all rights except for the "Refer" right, which is a bit more specia=
l.

I think there is some value in having a mechanism that let you use such
abbreviations for larger sets of rights, to make these tools more approacha=
ble
and less verbose for simple (coarse-grained) use cases.

Another main concern to take into account is the question of backwards
compatibility across different kernels: When a user attempts to sandbox a
program on a kernel that does not support that specific set of access right=
s
yet, what is the fallback strategy?  For the Go and Rust libraries, we have
found that it might often be advisable to fall back to a "best effort" mode
where we restrict as much as we can of what the user asked for, instead of
failing altogether.  But it depends on the use case.  I imagine that an
installation of util-linux will need to work with older kernels as well and=
 does
not have a strict dependency that ensures that it'll only run on new kernel=
s.

Backwards compatibility is also discussed in the landlock(7) man page,
specifically under VERSIONS and EXAMPLE.  I have also talked about it on my
personal weblog at https://blog.gnoack.org/post/landlock-best-effort/ in th=
e
past.  Specifically double check that you are getting it right for the "ref=
er"
right, because that one has unusual semantics compared to the other rights.

Thanks,
=E2=80=94G=C3=BCnther