On Mon, Sep 12, 2022 at 09:15:06PM +0200, Mickaël Salaün wrote:
>
>
> On 08/09/2022 21:58, 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 | 62 +++++++++++++++++++++---
> > 1 file changed, 54 insertions(+), 8 deletions(-)
> >
> > diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst
> > index b8ea59493964..57802fd1e09b 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: September 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 +70,26 @@ 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) {
> > - ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
> > + switch (abi) {
> > + case -1:
> > + perror("The running kernel does not enable to use Landlock");
> > + return 1;
>
> I think it would be easier to understand to explicitly check for abi < 0
> in a dedicated block as in the sample, instead of case -1, and return 0
> (instead of 1) with a comment to inform that Landlock is not handled but
> it is OK (expected error).
Done.
>
>
> > + 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.
--
