[PATCH v4 7/7] landlock: Document IOCTL support
Günther Noack
gnoack at google.com
Fri Nov 3 15:57:17 UTC 2023
In the paragraph above the fallback logic, use the shorter phrasing
from the landlock(7) man page.
Signed-off-by: Günther Noack <gnoack at google.com>
---
Documentation/userspace-api/landlock.rst | 74 +++++++++++++++++++-----
include/uapi/linux/landlock.h | 28 +++++++--
2 files changed, 83 insertions(+), 19 deletions(-)
diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst
index 2e3822677061..c64f315d5a2e 100644
--- a/Documentation/userspace-api/landlock.rst
+++ b/Documentation/userspace-api/landlock.rst
@@ -75,7 +75,8 @@ to be explicit about the denied-by-default access rights.
LANDLOCK_ACCESS_FS_MAKE_BLOCK |
LANDLOCK_ACCESS_FS_MAKE_SYM |
LANDLOCK_ACCESS_FS_REFER |
- LANDLOCK_ACCESS_FS_TRUNCATE,
+ LANDLOCK_ACCESS_FS_TRUNCATE |
+ LANDLOCK_ACCESS_FS_IOCTL,
.handled_access_net =
LANDLOCK_ACCESS_NET_BIND_TCP |
LANDLOCK_ACCESS_NET_CONNECT_TCP,
@@ -84,10 +85,10 @@ to be explicit about the denied-by-default access rights.
Because we may not know on which kernel version an application will be
executed, it is safer to follow a best-effort security approach. Indeed, we
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 access rights which are only supported in higher versions of the ABI.
+using.
+
+To be compatible with older Linux versions, we detect the available Landlock ABI
+version, and only use the available subset of access rights:
.. code-block:: c
@@ -113,6 +114,10 @@ remove access rights which are only supported in higher versions of the ABI.
ruleset_attr.handled_access_net &=
~(LANDLOCK_ACCESS_NET_BIND_TCP |
LANDLOCK_ACCESS_NET_CONNECT_TCP);
+ __attribute__((fallthrough));
+ case 4:
+ /* Removes LANDLOCK_ACCESS_FS_IOCTL for ABI < 5 */
+ ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_IOCTL;
}
This enables to create an inclusive ruleset that will contain our rules.
@@ -224,6 +229,7 @@ access rights per directory enables to change the location of such directory
without relying on the destination directory access rights (except those that
are required for this operation, see ``LANDLOCK_ACCESS_FS_REFER``
documentation).
+
Having self-sufficient hierarchies also helps to tighten the required access
rights to the minimal set of data. This also helps avoid sinkhole directories,
i.e. directories where data can be linked to but not linked from. However,
@@ -317,18 +323,24 @@ It should also be noted that truncating files does not require the
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
+The truncate right is associated with the opened file (see below).
+
+Rights associated with file descriptors
+---------------------------------------
+
+When opening a file, the availability of the ``LANDLOCK_ACCESS_FS_TRUNCATE`` and
+``LANDLOCK_ACCESS_FS_IOCTL`` rights is associated with the newly created file
+descriptor and will be used for subsequent truncation and ioctl attempts using
+:manpage:`ftruncate(2)` and :manpage:`ioctl(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.
+As a consequence, it is possible to have multiple open file descriptors
+referring to the same file, where one grants the truncate or ioctl right 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
=============
@@ -457,6 +469,28 @@ 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.
+
Previous limitations
====================
@@ -494,6 +528,16 @@ bind and connect actions to only a set of allowed ports thanks to the new
``LANDLOCK_ACCESS_NET_BIND_TCP`` and ``LANDLOCK_ACCESS_NET_CONNECT_TCP``
access rights.
+Ioctl (ABI < 5)
+---------------
+
+IOCTL operations could not be denied before the fourth Landlock ABI, so
+:manpage:`ioctl(2)` is always allowed when using a kernel that only supports an
+earlier ABI.
+
+Starting with the Landlock ABI version 4, it is possible to restrict the use of
+:manpage:`ioctl(2)` using the new ``LANDLOCK_ACCESS_FS_IOCTL`` access right.
+
.. _kernel_support:
Kernel support
diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h
index 6d41c059e910..3af0b1590f1b 100644
--- a/include/uapi/linux/landlock.h
+++ b/include/uapi/linux/landlock.h
@@ -205,11 +205,31 @@ struct landlock_net_port_attr {
* file or directory.
*
* This access right applies to all :manpage:`ioctl(2)` commands, except of
- * ``FIOCLEX``, ``FIONCLEX``, ``FIONBIO``, ``FIOASYNC`` and ``FIONREAD``.
- * These commands continue to be invokable independent of the
- * %LANDLOCK_ACCESS_FS_IOCTL access right.
+ * ``FIOCLEX``, ``FIONCLEX``, ``FIONBIO`` and ``FIOASYNC``. These commands
+ * continue to be invokable independent of the %LANDLOCK_ACCESS_FS_IOCTL
+ * access right.
*
- * This access right is available since the fourth version of the Landlock
+ * When certain other access rights are handled in the ruleset, in addition to
+ * %LANDLOCK_ACCESS_FS_IOCTL, granting these access rights will unlock access
+ * to additional groups of IOCTL commands, on the affected files:
+ *
+ * * %LANDLOCK_ACCESS_FS_READ_FILE unlocks access to ``FIOQSIZE``,
+ * ``FS_IOC_FIEMAP``, ``FIBMAP``, ``FIGETBSZ``, ``FIONREAD``,
+ * ``FIDEDUPRANGE``.
+ *
+ * * %LANDLOCK_ACCESS_FS_WRITE_FILE unlocks access to ``FIOQSIZE``,
+ * ``FS_IOC_FIEMAP``, ``FIBMAP``, ``FIGETBSZ``, ``FICLONE``,
+ * ``FICLONERANGE``, ``FS_IOC_RESVSP``, ``FS_IOC_RESVSP64``,
+ * ``FS_IOC_UNRESVSP``, ``FS_IOC_UNRESVSP64``, ``FS_IOC_ZERO_RANGE``.
+ *
+ * * %LANDLOCK_ACCESS_FS_READ_DIR unlocks access to ``FIOQSIZE``,
+ * ``FS_IOC_FIEMAP``, ``FIBMAP``, ``FIGETBSZ``.
+ *
+ * When these access rights are handled in the ruleset, the availability of
+ * the affected IOCTL commands is not governed by %LANDLOCK_ACCESS_FS_IOCTL
+ * any more, but by the respective access right.
+ *
+ * This access right is available since the fifth version of the Landlock
* ABI.
*
* .. warning::
--
2.42.0.869.gea05f2083d-goog
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