On Tue, Mar 16, 2021 at 09:42:52PM +0100, Mickaël Salaün wrote:
From: Mickaël Salaün mic@linux.microsoft.com
This documentation can be built with the Sphinx framework.
Well, yes. :) Maybe describe what the documentation covers instead here. Regardless: yay docs! This is great.
[...] +Bind mounts and OverlayFS +-------------------------
+Landlock enables to restrict access to file hierarchies, which means that these +access rights can be propagated with bind mounts (cf. +:doc:`/filesystems/sharedsubtree`) but not with :doc:`/filesystems/overlayfs`.
+A bind mount mirrors a source file hierarchy to a destination. The destination +hierarchy is then composed of the exact same files, on which Landlock rules can +be tied, either via the source or the destination path. These rules restrict +access when they are encountered on a path, which means that they can restrict +access to multiple file hierarchies at the same time, whether these hierarchies +are the result of bind mounts or not.
+An OverlayFS mount point consists of upper and lower layers. These layers are +combined in a merge directory, result of the mount point. This merge hierarchy +may include files from the upper and lower layers, but modifications performed +on the merge hierarchy only reflects on the upper layer. From a Landlock +policy point of view, each OverlayFS layers and merge hierarchies are +standalone and contains their own set of files and directories, which is +different from bind mounts. A policy restricting an OverlayFS layer will not +restrict the resulted merged hierarchy, and vice versa.
Can you include some examples about what a user of landlock should do? i.e. what are some examples of unexpected results when trying to write policy that runs on top of overlayfs, etc?
[...] +File renaming and linking +-------------------------
+Because Landlock targets unprivileged access controls, it is needed to properly +handle composition of rules. Such property also implies rules nesting. +Properly handling multiple layers of ruleset, each one of them able to restrict +access to files, also implies to inherit the ruleset restrictions from a parent +to its hierarchy. Because files are identified and restricted by their +hierarchy, moving or linking a file from one directory to another implies to +propagate the hierarchy constraints. To protect against privilege escalations +through renaming or linking, and for the sack of simplicity, Landlock currently
typo: sack -> sake
[...] +Special filesystems +-------------------
+Access to regular files and directories can be restricted by Landlock, +according to the handled accesses of a ruleset. However, files that do not +come from a user-visible filesystem (e.g. pipe, socket), but can still be +accessed through /proc/self/fd/, cannot currently be restricted. Likewise, +some special kernel filesystems such as nsfs, which can be accessed through +/proc/self/ns/, cannot currently be restricted. For now, these kind of special +paths are then always allowed. Future Landlock evolutions will enable to +restrict such paths with dedicated ruleset flags.
With this series, can /proc (at the top level) be blocked? (i.e. can a landlock user avoid the weirdness by making /proc/$pid/ unavailable?)
+Ruleset layers +--------------
+There is a limit of 64 layers of stacked rulesets. This can be an issue for a +task willing to enforce a new ruleset in complement to its 64 inherited +rulesets. Once this limit is reached, sys_landlock_restrict_self() returns +E2BIG. It is then strongly suggested to carefully build rulesets once in the +life of a thread, especially for applications able to launch other applications +that may also want to sandbox themselves (e.g. shells, container managers, +etc.).
How was this value (64) chosen?