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 | 67 +++++++++++++++++++++---
1 file changed, 60 insertions(+), 7 deletions(-)
diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst
index cec780c2f497..d8cd8cd9ce25 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: September 2022
+:Date: October 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 +70,28 @@ 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) {
+ if (abi < 0) {
+ /* Degrades gracefully if Landlock is not handled. */
+ perror("The running kernel does not enable to use Landlock");
+ return 0;
+ }
+ switch (abi) {
+ 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.
@@ -127,8 +140,8 @@ descriptor.
It may also be required to create rules following the same logic as explained
for the ruleset creation, by filtering access rights according to the Landlock
-ABI version. In this example, this is not required because
-``LANDLOCK_ACCESS_FS_REFER`` is not allowed by any rule.
+ABI version. In this example, this is not required because all of the requested
+``allowed_access`` rights are already available in ABI 1.
We now have a ruleset with one rule allowing read access to ``/usr`` while
denying all other handled accesses for the filesystem. The next step is to
@@ -252,6 +265,37 @@ To be allowed to use :manpage:`ptrace(2)` and related syscalls on a target
process, a sandboxed process should have a subset of the target process rules,
which means the tracee must be in a sub-domain of the tracer.
+Truncating files
+----------------
+
+The operations covered by ``LANDLOCK_ACCESS_FS_WRITE_FILE`` and
+``LANDLOCK_ACCESS_FS_TRUNCATE`` both change the contents of a file and sometimes
+overlap in non-intuitive ways. It is recommended to always specify both of
+these together.
+
+A particularly surprising example is :manpage:`creat(2)`. The name suggests
+that this system call requires the rights to create and write files. However,
+it also requires the truncate right if an existing file under the same name is
+already present.
+
+It should also be noted that truncating files does not require the
+``LANDLOCK_ACCESS_FS_WRITE_FILE`` right. Apart from the :manpage:`truncate(2)`
+system call, this can also be done through :manpage:`open(2)` with the flags
+``O_RDONLY | O_TRUNC``.
+
+When opening a file, the availability of the ``LANDLOCK_ACCESS_FS_TRUNCATE``
+right is associated with the newly created file descriptor and will be used for
+subsequent truncation attempts using :manpage:`ftruncate(2)`. The behavior is
+similar to opening a file for reading or writing, where permissions are checked
+during :manpage:`open(2)`, but not during the subsequent :manpage:`read(2)` and
+:manpage:`write(2)` calls.
+
+As a consequence, it is possible to have multiple open file descriptors for the
+same file, where one grants the right to truncate the file and the other does
+not. It is also possible to pass such file descriptors between processes,
+keeping their Landlock properties, even when these processes do not have an
+enforced Landlock ruleset.
+
Compatibility
=============
@@ -398,6 +442,15 @@ Starting with the Landlock ABI version 2, it is now possible to securely
control renaming and linking thanks to the new ``LANDLOCK_ACCESS_FS_REFER``
access right.
+File truncation (ABI < 3)
+-------------------------
+
+File truncation could not be denied before the third Landlock ABI, so it is
+always allowed when using a kernel that only supports the first or second ABI.
+
+Starting with the Landlock ABI version 3, it is now possible to securely control
+truncation thanks to the new ``LANDLOCK_ACCESS_FS_TRUNCATE`` access right.
+
.. _kernel_support:
Kernel support
--
2.38.0
