[PATCH v9 11/11] landlock: Document Landlock's file truncation support

[Günther Noack]

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 at gmail.com>
> ---
>  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
> 

--