Re: [PATCH RFC 0/3] landlock: Add access rights on created files

[Justin Suess <utilityemal77@gmail.com>]

On Sat, Oct 24, 2026 at 05:43:26PM -0400, Justin Suess wrote:
> On Sat, Oct 24, 2026 at 01:39:16PM -0700, Farid Zakaria wrote:
> > This is a proposal for an idea to add support for landlock to work
> > better with build systems such as Nix where they want to apply
> > restrictions but the paths may not yet exist.
> > 
> > TL;DR;
> > ========
> > 
> > Landlock rules attach to inodes that exist when a ruleset is built.
> > That makes a directory shared between trusted and sandboxed tasks hard
> > to confine: the sandbox may create entries there with
> > LANDLOCK_ACCESS_FS_MAKE_*, but no rule can name those entries ahead of
> > time, and granting read access on the directory to cover them exposes
> > every other entry as well. This series adds a rule flag that grants
> > access rights on the entries a domain creates in a directory, instead
> > of on the directory.
> Hi,
> 
> I'll chime in since I run nix and have a little bit of familiarity.
> 
> Looking at my (admittedly bloated) nix store.
> 
> [justin@zenbox ~]$ ls /nix/store | wc -l
> 259356
> 
> That's a lot of inodes. And a nix derivation creates hundreds if not
> thousands potentially throughout it's build.
> 
> My main concern is that rulesets are generally considered
> immutable after enforcement. This allows for well-defined semantics on
> inheritence. This from my understanding would mutate the ruleset after
> it's already enforced, so changes wouldn't be copied to inherited
> domains automatically.

* nvm on this, it looks like you are adding them to a different
heirarchy. The point still stands though on the inode pinning, which you
pointed out.


> 
> My other concern is with the overhead of automatically generating an inserting
> a rule for every new inode that gets created. It seems like a complex
> way to tackle this problem.
> 
> But beyond that, let's go back to your usecase.
> 
> ....
> 
> Basically from what I'm seeing you want to grant some access to certain
> subdirectories (including newly created ones) but exclude others.
> 
> Presumably in the nix case, you want to grant /nix/store/{abc,efg,hij} and NOT
> /nix/store/{xyz}, but still allow /nix/store/{foo} (the derivation
> output) to be created and written to. This would allow the nix build to only
> have access to certain inputs, while being able to write to the /nix/store
> and produce an output. This is a valid usecase that Landlock cannot today
> accomplish.
> 
> There was this NO_INHERIT series I proposed a little while back.
> [1] It's sort of the inverse of this (removes accesses inherited from
> parents rather than granting new access) but would accomplish this
> goal.
> 
> With NO_INHERIT, you could grant whatever access rights you want by default on
> the parent directory, and place NO_INHERIT on any specific entries you
> don't want to have inherit those rights. 
>
It seems like NO_INHERIT too might scale poorly for the usecase. Since then
you are trading O(store) for O(inputs) so not much better (strictly
worse).

Thoughts?

Justin
> So for nix use case, you could grant no_inherit on everything except the
> inputs you need, then add READ/WRITE/MKDIR etc on /nix/store. The new
> directories would inherit this by default, while the ones you added
> NO_INHERIT on would be restricted.
> 
> Additionally, you should also examine the mutable domains proposal by
> Tingmao Wang, which could also facilitate this usecase as well [2] (i.e by
> dynamically adding and removing rules with some fanotify/inotify glue).
> 
> That being said, neither of these two patchsets / proposals are merged,
> and the playing field is still open on this.
> 
> So the problem you are pointing to is legitamate, but the solution still
> needs to be considered carefully to avoid complexity.
> 
> Justin
> 
> [1] https://lore.kernel.org/linux-security-module/20260621035223.2651547-1-utilityemal77@gmail.com/
> [2] https://lore.kernel.org/linux-security-module/cover.1741047969.git.m@maowtm.org/
> > Motivation
> > ==========
> > 
> > The motivating user is the Nix build sandbox. Every Nix build runs in
> > its own sandbox, and a build may only read its declared inputs in
> > /nix/store and must be able to write and read back the outputs it
> > creates there. Nix enforces this with a private mount namespace: it
> > creates an empty store directory and bind-mounts each input path into
> > it, one mount per input. Nix has paid this per-input cost since the
> > sandbox was introduced [1].
> >
> > The cost is paid per build and grows with the size of the input
> > closure. Nix is moving towards much finer-grained builds (many small
> > derivations instead of a few large ones), so the number of sandboxes
> > grows while each build's own work shrinks, and the sandbox setup is
> > becoming the bottleneck. A typical build has hundreds of inputs; large
> > ones have thousands.
> > 
> > A microbenchmark sets up and tears down a Nix-like sandbox (user,
> > mount, PID, IPC, UTS and network namespaces, pivot_root into a chroot,
> > removing the chroot afterwards). It compares one bind mount per input
> > with a single bind mount of the whole store plus one Landlock rule per
> > input. Median latency per sandbox, one sandbox at a time, Linux 7.1,
> > ext4, 16 cores:
> > 
> >   inputs   bind mount per input   one bind mount + Landlock   speedup
> >       10         3.2 ms                    2.3 ms               1.4x
> >      100        11.6 ms                    2.8 ms               4.1x
> >     1000        79.3 ms                    6.4 ms              12.4x
> >     5000       400.1 ms                   24.4 ms              16.4x
> > 
> > With Landlock, an input costs an O_PATH open and a landlock_add_rule()
> > call: there is a single mount, and no mountpoint directory to create
> > before the build and remove after it (removing the 1000 empty
> > mountpoints alone took 17 ms).
> > 
> > The single bind mount is only sound if the store listing and the
> > undeclared store paths stay unreadable while the build can still use
> > its outputs. Without this series, covering the outputs requires a rule
> > on /nix/store itself, which makes every store path readable again.
> > 
> > [1] https://github.com/NixOS/nix/issues/179
> > 
> > Series
> > ======
> > 
> > Patch 1 lets an LSM observe inode creation through the
> > inode_init_security hook when no LSM provides an xattr. Patch 2 adds
> > the LANDLOCK_ADD_RULE_CREATED flag for path-beneath rules and bumps the
> > ABI to 13. Patch 3 adds selftests.
> > 
> > Testing
> > =======
> > 
> > The series applies to the Landlock next branch (3e271a27c46f) and was
> > tested on Linux 7.3 with that branch applied.
> > 
> > Without patch 2, the five new tests and the ABI version check fail.
> > 
> > Open questions
> > ==============
> > 
> > - A created inode stays pinned until the layer's hierarchy is freed, so a
> >   sandbox that creates many entries in such a directory keeps them all in
> >   memory. Should this be bounded?
> > 
> > - Creation is detected in inode_init_security, so on filesystems that do
> >   not call security_inode_init_security() created entries get no rights.
> >   Is there a better hook?
> > 
> > - Through overlayfs, created entries get no rights either, because the
> >   upper inode is created with the mounter's credentials. Should overlayfs
> >   be handled, e.g. via security_dentry_create_files_as()?
> > 
> > - A file renamed or linked in keeps only the rights it had at its source.
> >   Should it get none instead?
> > 
> > ---
> > Farid Zakaria (3):
> >       lsm: Call inode_init_security hooks without xattrs
> >       landlock: Add access rights on created files
> >       selftests/landlock: Test LANDLOCK_ADD_RULE_CREATED
> > 
> >  Documentation/userspace-api/landlock.rst     |  42 +++++
> >  include/uapi/linux/landlock.h                |  18 +++
> >  security/landlock/domain.c                   | 182 ++++++++++++++++++++-
> >  security/landlock/domain.h                   |  28 ++++
> >  security/landlock/fs.c                       | 176 +++++++++++++++++++--
> >  security/landlock/object.h                   |   6 +
> >  security/landlock/ruleset.c                  |   6 +
> >  security/landlock/ruleset.h                  |  14 ++
> >  security/landlock/syscalls.c                 |  20 ++-
> >  security/security.c                          |   9 +-
> >  tools/testing/selftests/landlock/base_test.c |   2 +-
> >  tools/testing/selftests/landlock/fs_test.c   | 227 +++++++++++++++++++++++++++
> >  12 files changed, 703 insertions(+), 27 deletions(-)
> > ---
> > base-commit: 3e271a27c46fd591d1011c7de5a644a41e2010f1
> > change-id: 20261024-landlock-created-rights-9340b00a8c4d
> > 
> > Best regards,
> > --  
> > Farid Zakaria <farid.m.zakaria@gmail.com>
> > 
> >