Sorry, please ignore this thread -- I messed it up and accidentally sent it with the wrong Reply-To headers.
—Günther
On Sat, Oct 08, 2022 at 01:13:36PM +0200, 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 | 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
>
--
