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

[Justin Suess <utilityemal77@gmail.com>]

On Sat, Sep 26, 2026 at 09:56:28AM +0200, Günther Noack wrote:
> Hello!
> 
> On Fri, Sep 25, 2026 at 02:03:05PM -0400, Justin Suess wrote:
> > 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.
> >
* posix acl and linux DAC.

> > So maybe WRITE_METADATA is good enough?
> 
> The existing use cases are the combinations of (a) READ_DIR
> allowed/denied and (b) READ_METADATA allowed/denied.  Because these
> two access rights overlap slightly, it seems likely that for a given
> directory or file, users will want to either grant both, or deny both.
> 
> At the moment, where the (not yet existing) READ_METADATA is
> implicitly always allowed, the problematic case is the one where the
> Landlock user wants to deny READ_DIR, but where much of the same
> metadata is still available through stat() and the various
> get-attribute syscalls.  (c.f. the warning box in the Landlock docs
> [1])
> 
> In my view the READ_METADATA right closes a gap that READ_DIR left
> open (which is also potentially surprising to callers if they did not
> read the docs closely).  Also, if its implementation is symmetric to
> WRITE_METADATA, I feel that it's worth having it in the same patch
> set.
> 
> –Günther
> 
> P.S.: I know, even after we can control stat(), there are likely ways
> to infer the presence of a file by observing Landlock error codes.
> This would be nice to fix as well, but is harder to do without
> controlling the path walk itself [2].  But also, the fact that this is
> currently not controllable is not an excuse for leaving READ_METADATA
> open IMHO.
>
I'm still sort of concerned about the case where read access is allowed,
but metadata isn't, due to how many applications will not cleanly handle
such an unexpected condition. You can reproduce this with a seccomp
policy forbidding stat().

Would it make sense to have the existing READ rights
(READ_DIR/READ_FILE) imply READ_METADATA on the files/directories 
if READ_METADATA is handled? Reading a file's contents should always
imply that you can read the metadata.

Thanks,
Justin
> [1] https://docs.kernel.org/userspace-api/landlock.html#filesystem-flags
> [2] https://github.com/landlock-lsm/linux/issues/9