[PATCH v2] landlock: Explain file descriptor access rights

[Günther Noack]

On Fri, Dec 09, 2022 at 08:38:13PM +0100, Mickaël Salaün wrote:
> Starting with LANDLOCK_ACCESS_FS_TRUNCATE, it is worth explaining why we
> choose to restrict access checks at open time.  This new "File
> descriptor access rights" section is complementary to the existing
> "Inode access rights" section.  Add a new guiding principle related to
> this section.
> 
> Cc: Günther Noack <gnoack3000 at gmail.com>
> Signed-off-by: Mickaël Salaün <mic at digikod.net>
> Link: https://lore.kernel.org/r/20221209193813.972012-1-mic@digikod.net
> ---
> 
> Changes since v1:
> https://lore.kernel.org/r/20221205112621.3530557-1-mic@digikod.net
> * Reworded the new section based on Günther suggestions.
> * Added a new guiding principle.
> * Update date.
> ---
>  Documentation/security/landlock.rst | 33 ++++++++++++++++++++++++++---
>  1 file changed, 30 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/security/landlock.rst b/Documentation/security/landlock.rst
> index c0029d5d02eb..95a0e4726dc5 100644
> --- a/Documentation/security/landlock.rst
> +++ b/Documentation/security/landlock.rst
> @@ -7,7 +7,7 @@ Landlock LSM: kernel documentation
>  ==================================
>  
>  :Author: Mickaël Salaün
> -:Date: September 2022
> +:Date: December 2022
>  
>  Landlock's goal is to create scoped access-control (i.e. sandboxing).  To
>  harden a whole system, this feature should be available to any process,
> @@ -41,12 +41,15 @@ Guiding principles for safe access controls
>    processes.
>  * Computation related to Landlock operations (e.g. enforcing a ruleset) shall
>    only impact the processes requesting them.
> +* Resources (e.g. file descriptors) directly obtained from the kernel by a
> +  sandboxed process shall retain their scoped accesses whatever process use

Optional nit: Maybe add "at the time of resource acquisition" to stress that part?

> +  them.  Cf. `File descriptor access rights`_.
>  
>  Design choices
>  ==============
>  
> -Filesystem access rights
> -------------------------
> +Inode access rights
> +-------------------
>  
>  All access rights are tied to an inode and what can be accessed through it.
>  Reading the content of a directory does not imply to be allowed to read the
> @@ -57,6 +60,30 @@ directory, not the unlinked inode.  This is the reason why
>  ``LANDLOCK_ACCESS_FS_REMOVE_FILE`` or ``LANDLOCK_ACCESS_FS_REFER`` are not
>  allowed to be tied to files but only to directories.
>  
> +File descriptor access rights
> +-----------------------------
> +
> +Access rights are checked and tied to file descriptors at open time.  The
> +underlying principle is that equivalent sequences of operations should lead to
> +the same results, when they are executed under the same Landlock domain.
> +
> +Taking the ``LANDLOCK_ACCESS_FS_TRUNCATE`` right as an example, it may be
> +allowed to open a file for writing without being allowed to
> +:manpage:`ftruncate` the resulting file descriptor if the related file
> +hierarchy doesn't grant such access right.  The following sequences of
> +operations have the same semantic and should then have the same result:
> +
> +* ``truncate(path);``
> +* ``int fd = open(path, O_WRONLY); ftruncate(fd); close(fd);``
> +
> +Similarly to file access modes (e.g. ``O_RDWR``), Landlock access rights
> +attached to file descriptors are retained even if they are passed between
> +processes (e.g. through a Unix domain socket).  Such access rights will then be
> +enforced even if the receiving process is not sandboxed by Landlock.  Indeed,
> +this is required to keep a consistent access control over the whole system, and
> +avoid unattended bypasses through file descriptor passing (i.e. confused deputy
> +attack).
> +
>  Tests
>  =====
>  
> 
> base-commit: 0b4ab8cd635e8b21e42c14b9e4810ca701babd11
> -- 
> 2.38.1
> 

Reviewed-by: Günther Noack <gnoack3000 at gmail.com>

Thank you!

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