On Tue, Sep 29, 2026 at 02:12:42PM +0200, Günther Noack wrote: > On Mon, Sep 28, 2026 at 01:13:35PM -0400, Justin Suess wrote: > > 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. > > I am wary of situations where the handling of access rights implicity > implies other access rights. We have discussed such schemes in the > past (e.g. we considered a design for RESOLVE_UNIX where we'd have > both a "scoped" and a "access_fs" right that would interact with each > other), but in the end we always settled for approaches where such > interactions would not be necessary. One of the concerns was that it > would complicate "best effort" fallback logic in all Landlock > libraries and in the countless places where people use the syscalls > directly. > > To throw another option in the mix. (To be clear, I have only 70% > confidence, so feel free to push back, but it feels like it might > work?): > > Is this similar to the "truncate" right? > ---------------------------------------- > > With "truncate", there was an existing common operation (open(2) with > O_TRUNC, a.k.a. creat(2)) which called the truncation hook and checked > for the truncation access right. But that was in fact OK. The way we > resolved it at the time was by documenting very loudly that WRITE_FILE > and TRUNCATE access rights should always be requested in lockstep, if > TRUNCATE is handled. > > If READ_DIR and READ_FILE do in fact read and return metadata to the > user, maybe the right thing would be to do it the same way here and > *require* that we have READ_METADATA to do these operations? > > (READ_METADATA is automatically allowed as long as it's not handled, > so that approach does not break existing programs. Programs who > consciously start handling READ_METADATA must simply take into account > that reading directories and opening files for reading requires > READ_METADATA.) > > (BTW, I can see it for reading directories, but I am not sure I fully > follow the argument why opening files for reading means that you can > read the metadata? Can't that be guarded on fstat()-like operations?) > Many standard libraries will call stat before / after opening a file to allocate a buffer matching the file size. Or to figure out if mmap is more efficient than opening it directly.
So Python's open().read() or Go's os.ReadFile they may throw an error when opening the file, making it look as if the file is inaccessible when its contents are. They make the assumption that if a file is readable, the metadata is too. There's also a lot of metadata that is already leaked by just having READ_FILE. FS_IOC_GETFLAGS/FS_IOC_FSGETXATTR ioctl / fileattr_get are unrestrictable and allow you to see inode attributes. There's also access(2) which isn't restricted here and allows you to see your rights on the file. The size can be found by simply seeking to the beginning and end. So much of the metadata is obtainable with just READ_DIR/READ_FILE. ... Another problem I'm just realizing is that this READ_METADATA right is inconsistent with open file descriptor behavior. With most rights, already open file descriptors are exempted, but here, fstat, (and the other stat-family calls which takes a file descriptor) become denied even on already open files. It's a catch 22 here, if you change the fstat to not apply to opened files, then READ_FILE becomes a bypass for READ_METADATA. But if you leave it as is, then it's inconsistent with the other Landlock rights wrt already opened files. Which isn't the end of the world, but just shows that either approach is going to have it's quirks. I think it would be better to just avoid the cat and mouse game here of trying to seperate READ_FILE/READ_METADATA and either require READ_METADATA be specified with READ_FILE if READ_METADATA is handled like Gunther proposed, or have READ_FILE imply READ_METADATA. Documenting it strongly is OK too, but really there are really zero usecases where you'd want to grant READ_FILE without READ_METADATA so it would need to be made extremely clear. Thanks, Justin > —Günther
