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Sat, 24 Oct 2026 15:03:32 -0700 (PDT) Received: from zenbox ([2600:1700:18fb:6011:f15a:4eed:c4ba:3585]) by smtp.gmail.com with ESMTPSA id 00721157ae682-8b166dd01c0sm12800017b3.7.2026.10.24.15.03.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 24 Oct 2026 15:03:31 -0700 (PDT) Date: Sat, 24 Oct 2026 18:03:30 -0400 From: Justin Suess To: Farid Zakaria Cc: =?utf-8?Q?Micka=C3=ABl_Sala=C3=BCn?= , =?utf-8?Q?G=C3=BCnther?= Noack , Paul Moore , James Morris , "Serge E. Hallyn" , Shuah Khan , Jonathan Corbet , John Ericson , linux-security-module@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH RFC 0/3] landlock: Add access rights on created files Message-ID: References: <20261024-landlock-created-rights-v1-0-77c8db3f6bd2@gmail.com> X-Mailing-List: linux-security-module@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 > > > >