Re: [PATCH RFC -next 00/12] landlock: Add READ_METADATA and WRITE_METADATA access rights

[Justin Suess <utilityemal77@gmail.com>]

On Thu, Sep 24, 2026 at 06:48:19PM +0800, Cai Xinchen wrote:
> This series adds two new Landlock filesystem access rights,
> LANDLOCK_ACCESS_FS_READ_METADATA and LANDLOCK_ACCESS_FS_WRITE_METADATA,
> which control access to file and directory metadata such as inode
> attributes (mode, ownership, timestamps), extended attributes and POSIX
> ACLs.  It picks up the work from the "landlock: add chmod and chown
> support" series [1] and follows the coarse-grained grouping discussed in
> that thread [2]: instead of separate chmod/chown rights, metadata
> operations are grouped into one read and one write right.
> 
> Landlock evaluates access rights on a per-path basis, but the metadata
> related LSM hooks (inode_getattr, inode_setattr, inode_setxattr,
> inode_getxattr, inode_listxattr, inode_removexattr, inode_set_acl,
> inode_get_acl, inode_remove_acl) only receive the dentry of the accessed
> object.  Patches 1-7 therefore first pass struct path instead of dentry
> through the metadata-related VFS helpers and LSM hooks.  This is a pure
> refactoring with no behavior change, split so that every patch builds
> and works on its own:
>
I like these patches, but is the ability to read metadata already
sorta controlled by LANDLOCK_ACCESS_FS_READ_DIR on the parent
directory?

The one case I see this being different is:

1. if you wanted to grant read access to the file, but not metadata
read access, but I can't think of any usecase for being able to read
the contents of a file, but not the metadata. (see below)

2. If you had the absolute path already and didn't need READ_DIR.

I see introducing this READ_METADATA as causing potential
hard-to-diagnose issues.

Say you handle READ_METADATA and READ_FILE, but only grant READ_FILE.

The program can technically open the file with the READ_FILE permission,
but it may error out because the stat() on it beforehand failed.
It's pretty common for programs to do that kind of thing (stat before
open), like for checking for config files (strace bash and you see it
stat .profile, /etc/profile)

There may be other bugs, because being able to set permissions to read
a file *but not read it's metadata* isn't possible currently in posix
acl and userspace may not work well if that assumption no longer holds.

So maybe WRITE_METADATA is good enough?

Interested in your thoughts,
Justin
>   1: notify_change() and its callers
>   2: inode_setsecctx hook (must come before 3: the SELinux and Smack
>      implementations call __vfs_setxattr_locked internally)
>   3: xattr helpers, which also drops a redundant EVM xattr size sanity
>      check whose vfs_getxattr() call only has a dentry and therefore
>      cannot be migrated to the new path-based signature
>   4: POSIX ACL helpers
>   5: inode_setattr hook
>   6: inode xattr hooks
>   7: inode POSIX ACL hooks
> 
> Two deliberate scoping decisions for this refactor:
> 
> - The hooks consistently take struct path rather than struct file.  The
>   VFS call sites involved (chmod(2), chown(2), utimensat(2), xattr(2)
>   and ACL syscalls) operate on paths, and several of them (lstat(2),
>   lchown(2), llistxattr(2), ...) have no struct file to begin with.
> 
> - struct inode_operations->setattr still receives (idmap, dentry, attr).
>   Only the VFS boundary (notify_change()) and the LSM hook layer see the
>   path, which keeps the refactor contained to fs/attr.c and the LSM
>   infrastructure instead of touching every filesystem.
> 
> Patches 8-12 then implement the new rights, their tests, the sandboxer
> sample and the documentation.  Semantics:
> 
> - READ_METADATA covers stat(2) and friends, getxattr(2) and friends,
>   listxattr(2) and friends, and POSIX ACL reads.
> - WRITE_METADATA covers chmod(2), chown(2), utimensat(2), setxattr(2),
>   removexattr(2) and friends, and POSIX ACL set and remove.
> - Only explicit metadata changes requested by user space are restricted.
>   Implicit changes performed by the kernel (e.g. timestamp updates on
>   write(2), size changes on truncate(2)) are not, and neither are
>   chmod(2)/chown(2) calls that change nothing (e.g. chown(2) with
>   (-1, -1), which never reaches the hook), matching the SELinux
>   inode_setattr behavior.
> - Kernel-internal accesses performed with override_creds() (e.g.
>   overlayfs, cachefiles) and kernel threads without a Landlock domain
>   (e.g. nfsd, ksmbd) are not restricted.
> 
> The Landlock ABI version is incremented from 11 to 12.
> 
> The series is based on linux-next commit 5c4d4169604b ("Add linux-next
> specific files for 20260921").
> 
> Testing: each patch has been built for aarch64 (gcc, -Werror) and the
> landlock selftests (445 tests, including the new ones) pass in QEMU on
> aarch64; base_test reports ABI v12.
> 
> [1] https://lore.kernel.org/all/20220827111215.131442-1-xiujianfeng@huawei.com/
> [2] https://lore.kernel.org/all/abc960a1-e66e-792e-6869-cfd201c29dbe@digikod.net/
> 
> Assisted-by: opencode: glm-5.3
> 
> Cai Xinchen (12):
>   fs: pass struct path to notify_change()
>   LSM: pass struct path to the inode_setsecctx hook
>   fs: pass struct path to xattr helpers
>   fs: pass struct path to POSIX ACL helpers
>   LSM: pass struct path to the inode_setattr hook
>   LSM: pass struct path to the inode xattr hooks
>   LSM: pass struct path to the inode posix acl hooks
>   landlock: Add READ_METADATA and WRITE_METADATA access rights
>   landlock: Implement metadata access hooks
>   selftests/landlock: Add tests for metadata access rights
>   samples/landlock: Add metadata rights to sandboxer
>   Documentation: Update landlock doc for metadata rights
> 
>  Documentation/userspace-api/landlock.rst     |  11 +-
>  drivers/base/devtmpfs.c                      |   6 +-
>  drivers/block/zloop.c                        |   4 +-
>  fs/attr.c                                    |  20 +-
>  fs/cachefiles/interface.c                    |   6 +-
>  fs/cachefiles/xattr.c                        |  32 +-
>  fs/coredump.c                                |   2 +-
>  fs/ecryptfs/inode.c                          |  34 +-
>  fs/exfat/file.c                              |   3 +-
>  fs/fat/file.c                                |   3 +-
>  fs/inode.c                                   |   7 +-
>  fs/internal.h                                |  17 +-
>  fs/namei.c                                   |   7 +-
>  fs/nfsd/nfs4ctl.h                            |   4 +-
>  fs/nfsd/nfs4state.c                          |  14 +-
>  fs/nfsd/nfs4xdr.c                            |   2 +-
>  fs/nfsd/state.h                              |   2 +-
>  fs/nfsd/vfs.c                                |  73 +++--
>  fs/open.c                                    |  18 +-
>  fs/overlayfs/copy_up.c                       |   4 +-
>  fs/overlayfs/inode.c                         |   4 +-
>  fs/overlayfs/overlayfs.h                     |  39 ++-
>  fs/overlayfs/xattrs.c                        |  13 +-
>  fs/posix_acl.c                               |  46 +--
>  fs/smb/server/smb2pdu.c                      |  77 ++---
>  fs/smb/server/smb_common.c                   |   2 -
>  fs/smb/server/smbacl.c                       |  21 +-
>  fs/smb/server/tests/smbacl_kunit.c           |   6 +-
>  fs/smb/server/vfs.c                          | 111 +++----
>  fs/smb/server/vfs.h                          |  39 +--
>  fs/smb/server/vfs_cache.c                    |   3 +-
>  fs/utimes.c                                  |   3 +-
>  fs/xattr.c                                   |  96 +++---
>  include/linux/fs.h                           |   6 +-
>  include/linux/landlock.h                     |   4 +-
>  include/linux/lsm_hook_defs.h                |  29 +-
>  include/linux/posix_acl.h                    |  21 +-
>  include/linux/security.h                     |  65 ++--
>  include/linux/xattr.h                        |  22 +-
>  include/uapi/linux/landlock.h                |  26 +-
>  samples/landlock/sandboxer.c                 |  17 +-
>  security/commoncap.c                         |  22 +-
>  security/integrity/evm/evm_crypto.c          |   8 +-
>  security/integrity/evm/evm_main.c            |  36 ++-
>  security/integrity/ima/ima_appraise.c        |  17 +-
>  security/landlock/fs.c                       |  86 ++++++
>  security/landlock/limits.h                   |   2 +-
>  security/landlock/syscalls.c                 |   2 +-
>  security/security.c                          |  99 +++---
>  security/selinux/hooks.c                     |  49 +--
>  security/smack/smack_lsm.c                   |  62 ++--
>  tools/testing/selftests/landlock/base_test.c |   2 +-
>  tools/testing/selftests/landlock/fs_test.c   | 309 ++++++++++++++++++-
>  53 files changed, 999 insertions(+), 614 deletions(-)
> 
> -- 
> 2.18.0.huawei.25
> 
>