[PATCH v8 9/9] landlock: Document Landlock's file truncation support

[Mickaël Salaün]

On 01/10/2022 17:49, 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 at gmail.com>
> ---
>   Documentation/userspace-api/landlock.rst | 66 +++++++++++++++++++++---
>   1 file changed, 59 insertions(+), 7 deletions(-)
> 
> diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst
> index b8ea59493964..44d6f598b63d 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: 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 +70,27 @@ 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) {
> +        perror("The running kernel does not enable to use Landlock");

Please insert in a dedicated line this comment: /* Degrades gracefully 
if Landlock is not handled. */


> +        return 0;  /* Degrade gracefully if Landlock is not handled. */
> +    }
> +    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 +139,8 @@ 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
> @@ -251,6 +263,37 @@ 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
>   =============
>   
> @@ -397,6 +440,15 @@ 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