landlock: Document Landlock's file truncation support

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@gmail.com>
Link: https://lore.kernel.org/r/20221018182216.301684-12-gnoack3000@gmail.comSigned-off-by: Mickaël Salaün <mic@digikod.net>

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