On 08/09/2022 21:58, Günther Noack wrote:
Use the LANDLOCK_ACCESS_FS_TRUNCATE flag in the tutorial.
Adapt the backwards compatibility example and discussion to remove the
truncation flag where needed.
Point out potential surprising behaviour related to truncate.
Signed-off-by: Günther Noack <gnoack3000@xxxxxxxxx>
---
Documentation/userspace-api/landlock.rst | 62 +++++++++++++++++++++---
1 file changed, 54 insertions(+), 8 deletions(-)
diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst
index b8ea59493964..57802fd1e09b 100644
--- a/Documentation/userspace-api/landlock.rst
+++ b/Documentation/userspace-api/landlock.rst
@@ -8,7 +8,7 @@ Landlock: unprivileged access control
=====================================
:Author: Mickaël Salaün
-:Date: May 2022
+:Date: September 2022
The goal of Landlock is to enable to restrict ambient rights (e.g. global
filesystem access) for a set of processes. Because Landlock is a stackable
@@ -60,7 +60,8 @@ the need to be explicit about the denied-by-default access rights.
LANDLOCK_ACCESS_FS_MAKE_FIFO |
LANDLOCK_ACCESS_FS_MAKE_BLOCK |
LANDLOCK_ACCESS_FS_MAKE_SYM |
- LANDLOCK_ACCESS_FS_REFER,
+ LANDLOCK_ACCESS_FS_REFER |
+ LANDLOCK_ACCESS_FS_TRUNCATE,
};
Because we may not know on which kernel version an application will be
@@ -69,16 +70,26 @@ should try to protect users as much as possible whatever the kernel they are
using. To avoid binary enforcement (i.e. either all security features or
none), we can leverage a dedicated Landlock command to get the current version
of the Landlock ABI and adapt the handled accesses. Let's check if we should
-remove the `LANDLOCK_ACCESS_FS_REFER` access right which is only supported
-starting with the second version of the ABI.
+remove the `LANDLOCK_ACCESS_FS_REFER` or `LANDLOCK_ACCESS_FS_TRUNCATE` access
+rights, which are only supported starting with the second and third version of
+the ABI.
.. code-block:: c
int abi;
abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
- if (abi < 2) {
- ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
+ switch (abi) {
+ case -1:
+ perror("The running kernel does not enable to use Landlock");
+ return 1;
I think it would be easier to understand to explicitly check for abi < 0
in a dedicated block as in the sample, instead of case -1, and return 0
(instead of 1) with a comment to inform that Landlock is not handled but
it is OK (expected error).
+ case 1:
+ /* Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2 */
+ ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
+ __attribute__((fallthrough));
+ case 2:
+ /* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */
+ ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE;
}
This enables to create an inclusive ruleset that will contain our rules.
