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

[Justin Suess <utilityemal77@gmail.com>]

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.

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. 

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>
> 
>