On Fri, Dec 08, 2023 at 04:51:21PM +0100, Günther Noack wrote: > In the paragraph above the fallback logic, use the shorter phrasing > from the landlock(7) man page. > > Signed-off-by: Günther Noack <gnoack@xxxxxxxxxx> > --- > Documentation/userspace-api/landlock.rst | 119 ++++++++++++++++++++--- > 1 file changed, 104 insertions(+), 15 deletions(-) > > +Restricting IOCTL commands > +-------------------------- > + > +When the ``LANDLOCK_ACCESS_FS_IOCTL`` access right is handled, Landlock will I only use "right" (instead of "access right") when LANDLOCK_ACCESS_* precede to avoid repetition. > +restrict the invocation of IOCTL commands. However, to *permit* these IOCTL This patch introduces the "permit*" wording instead of the currently used "allowed", which is inconsistent. > +commands again, some of these IOCTL commands are then granted through other, > +preexisting access rights. > + > +For example, consider a program which handles ``LANDLOCK_ACCESS_FS_IOCTL`` and > +``LANDLOCK_ACCESS_FS_READ_FILE``. The program *permits* > +``LANDLOCK_ACCESS_FS_READ_FILE`` on a file ``foo.log``. > + > +By virtue of granting this access on the ``foo.log`` file, it is now possible to > +use common and harmless IOCTL commands which are useful when reading files, such > +as ``FIONREAD``. > + > +On the other hand, if the program permits ``LANDLOCK_ACCESS_FS_IOCTL`` on > +another file, ``FIONREAD`` will not work on that file when it is opened. As > +soon as ``LANDLOCK_ACCESS_FS_READ_FILE`` is *handled* in the ruleset, the IOCTL > +commands affected by it can not be reenabled though ``LANDLOCK_ACCESS_FS_IOCTL`` > +any more, but are then governed by ``LANDLOCK_ACCESS_FS_READ_FILE``. > + > +The following table illustrates how IOCTL attempts for ``FIONREAD`` are > +filtered, depending on how a Landlock ruleset handles and permits the > +``LANDLOCK_ACCESS_FS_IOCTL`` and ``LANDLOCK_ACCESS_FS_READ_FILE`` access rights: > + > ++------------------------+-------------+-------------------+-------------------+ > +| | ``IOCTL`` | ``IOCTL`` handled | ``IOCTL`` handled | I was a bit confused at first read, wondering why IOCTL was quoted, then I realized that it was in fact LANDLOCK_ACCESS_FS_IOCTL. Maybe using the "FS_" prefix would avoid this kind of misreading (same for READ_FILE)? > +| | not handled | and permitted | and not permitted | > ++------------------------+-------------+-------------------+-------------------+ > +| ``READ_FILE`` not | allow | allow | deny | > +| handled | | | | > ++------------------------+ +-------------------+-------------------+ > +| ``READ_FILE`` handled | | allow | > +| and permitted | | | > ++------------------------+ +-------------------+-------------------+ > +| ``READ_FILE`` handled | | deny | > +| and not permitted | | | If it makes the raw text easier to read, it should be OK to extend this table to 100 columns (I guess checkpatch.pl will not complain). > ++------------------------+-------------+-------------------+-------------------+ > + > +The full list of IOCTL commands and the access rights which affect them is > +documented below. > > Compatibility > ============= > @@ -457,6 +514,28 @@ Memory usage > Kernel memory allocated to create rulesets is accounted and can be restricted > by the Documentation/admin-guide/cgroup-v1/memory.rst. > > +IOCTL support > +------------- > + > +The ``LANDLOCK_ACCESS_FS_IOCTL`` access right restricts the use of > +:manpage:`ioctl(2)`, but it only applies to newly opened files. This means > +specifically that pre-existing file descriptors like stdin, stdout and stderr > +are unaffected. > + > +Users should be aware that TTY devices have traditionally permitted to control > +other processes on the same TTY through the ``TIOCSTI`` and ``TIOCLINUX`` IOCTL > +commands. It is therefore recommended to close inherited TTY file descriptors, > +or to reopen them from ``/proc/self/fd/*`` without the > +``LANDLOCK_ACCESS_FS_IOCTL`` right, if possible. The :manpage:`isatty(3)` > +function checks whether a given file descriptor is a TTY. > + > +Landlock's IOCTL support is coarse-grained at the moment, but may become more > +fine-grained in the future. Until then, users are advised to establish the > +guarantees that they need through the file hierarchy, by only permitting the > +``LANDLOCK_ACCESS_FS_IOCTL`` right on files where it is really harmless. In > +cases where you can control the mounts, the ``nodev`` mount option can help to > +rule out that device files can be accessed. > +
