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Sat, 24 Oct 2026 14:43:28 -0700 (PDT) Received: from zenbox ([2600:1700:18fb:6011:f15a:4eed:c4ba:3585]) by smtp.gmail.com with ESMTPSA id 00721157ae682-8b166dcf529sm13199467b3.12.2026.10.24.14.43.27 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 24 Oct 2026 14:43:27 -0700 (PDT) Date: Sat, 24 Oct 2026 17:43:26 -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: <20261024-landlock-created-rights-v1-0-77c8db3f6bd2@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 > >