On 22/08/2022 23:53, Casey Schaufler wrote:
On 8/22/2022 2:21 PM, Mickaël Salaün wrote:
On 22/08/2022 23:18, Günther Noack wrote:
On Mon, Aug 22, 2022 at 12:35:18PM -0700, Casey Schaufler wrote:
On 8/22/2022 12:17 PM, Günther Noack wrote:
Hi!
Very exciting to see! Thank you for sending this! :)
I'm just throwing in some comments based on the very similar truncate patch set, in the hope that it helps. (But obviously, Mickaël Salaün has the last word on this code.)
Slightly higher level question: Should we start to group the functionality of multiple LSM hooks under one Landlock flag? (Will it be harder to change the LSM hook interface in the future if we continue to add one flag per hook? Or is this structure already exposed to userspace by other LSMs?)
I'm not a landlock expert. The question is nonsensical, yet somewhat frightening nonetheless. Could you put just a touch more context into what you're asking for?
By "Landlock flags", I meant the integer that Landlock uses to represent the set of possible operations on a file hierarchy:
Landlock's file system access rights (access_mode_t on the kernel side) are defined with an integer with flags (LANDLOCK_ACCESS_FS_*) for different operations that one might do with files. They get used from userspace to control what is permitted on which parts of the file system. (Docs: https://docs.kernel.org/userspace-api/landlock.html)
Currently most of the available Landlock flags map pretty closely to one of the file- and path-related LSM hooks. (See various hook implementations in security/landlock/fs.c)
The file system operations that Landlock doesn't cover yet (as of kernel 5.19) are listed below, and there are potentially a few more that might be missing. I suspect/hope that there will be more patches in the style of the truncate/chmod/chown patches, which will add that coverage.
The question is basically: When these patches get added, how should the userspace-exposed Landlock file system access rights map to the LSM hooks for these upcoming Landlock features? Should each of the newly covered operations have its own flag, or is it better to group them?
(It's well possible that the right answer is "one flag per feature", but I feel it still makes sense to ask this before all these patches get written?)
Landlock is not strictly tied to the current LSM hooks, but they fit well (because they are designed to be flexible enough to be use by multiple access control systems). In fact, Landlock already uses orthogonal access rights such as LANDLOCK_ACCESS_FS_REFER (using the path_link or path_rename hooks), LANDLOCK_ACCESS_FS_MAKE_* (using the path_mknod and path_mkdir hooks)…
Anyway, the LSM framework is evolving, we can add new hooks and modify others (e.g. see the security_path_rename hook modification for FS_REFER) as long as mainline access control systems don't break and subsystem maintainers are OK with such changes. Like any kernel API, the LSM API is not stable, but this is not an issue for mainline code.
Landlock's goal is to find the sweet spot between flexibility for different sandboxing use cases and an understandable/simple-enough access control system. The access rights should then be meaningful for users, which are already familiar with the UAPI/syscalls, hence the current Landlock access rights (which are not very original, and that is a good thing). This is why I'm wondering if it is worth it to differentiate between chmod and chgrp (and add a dedicated access right per action or only one for both).
The lesson from capabilities is that differentiating between chmod, chown and chgrp is pointless, and CAP_DAC_CHMOD, CAP_DAC_CHOWN and CAP_DAC_CHGRP should have just been CAP_DAC_OVERRIDE. On the other hand, those who argue that SELinux proves the value of fine granularity would likely have you go with separate rights. What's important is that you don't tie your rights too tightly to the underlying implementation. That has the potential to expose details of how the code work that user-space has no business basing decisions on.
Indeed, for a sandboxing feature like Landlock, it may not be useful to duplicate other access rights. From a user point of view, I think it would make sense to split the file metadata modification into potentially-security related or not. That would means three access rights: - modify user/informative metadata (e.g. dates, user.* xattr); - modify security-related metadata (e.g. chown, chmod, chgrp, any other xattr); - read any metadata.
This require some LSM hook changes to handle paths instead of inodes (e.g. security_inode_setattr, security_inode_setxattr…).