Re: [PATCH v30 12/12] landlock: Add user and kernel documentation

[Kees Cook <keescook@chromium.org>]

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?

-- 
Kees Cook