[PATCH v13 12/12] landlock: Document Landlock's network support
Konstantin Meskhidze
konstantin.meskhidze at huawei.com
Mon Oct 16 01:50:30 UTC 2023
Describe network access rules for TCP sockets. Add network access
example in the tutorial. Add kernel configuration support for network.
Signed-off-by: Konstantin Meskhidze <konstantin.meskhidze at huawei.com>
Link: https://lore.kernel.org/r/20230920092641.832134-13-konstantin.meskhidze@huawei.com
Signed-off-by: Mickaël Salaün <mic at digikod.net>
---
Changes since v12:
* None.
Changes since v11:
* Fixes documentaion as suggested in Günther's and Mickaёl's reviews:
https://lore.kernel.org/netdev/3ad02c76-90d8-4723-e554-7f97ef115fc0@digikod.net/
Changes since v10:
* Fixes documentaion as Mickaёl suggested:
https://lore.kernel.org/linux-security-module/ec23be77-566e-c8fd-179e-f50e025ac2cf@digikod.net/
Changes since v9:
* Minor refactoring.
Changes since v8:
* Minor refactoring.
Changes since v7:
* Fixes documentaion logic errors and typos as Mickaёl suggested:
https://lore.kernel.org/netdev/9f354862-2bc3-39ea-92fd-53803d9bbc21@digikod.net/
Changes since v6:
* Adds network support documentaion.
---
Documentation/userspace-api/landlock.rst | 87 ++++++++++++++++++------
1 file changed, 66 insertions(+), 21 deletions(-)
diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst
index f6a7da21708a..affadd9ac662 100644
--- a/Documentation/userspace-api/landlock.rst
+++ b/Documentation/userspace-api/landlock.rst
@@ -11,10 +11,10 @@ Landlock: unprivileged access control
: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
-LSM, it makes possible to create safe security sandboxes as new security layers
-in addition to the existing system-wide access-controls. This kind of sandbox
-is expected to help mitigate the security impact of bugs or
+filesystem or network access) for a set of processes. Because Landlock
+is a stackable LSM, it makes possible to create safe security sandboxes as new
+security layers in addition to the existing system-wide access-controls. This
+kind of sandbox is expected to help mitigate the security impact of bugs or
unexpected/malicious behaviors in user space applications. Landlock empowers
any process, including unprivileged ones, to securely restrict themselves.
@@ -28,20 +28,34 @@ appropriately <kernel_support>`.
Landlock rules
==============
-A Landlock rule describes an action on an object. An object is currently a
-file hierarchy, and the related filesystem actions are defined with `access
-rights`_. A set of rules is aggregated in a ruleset, which can then restrict
+A Landlock rule describes an action on an object which the process intends to
+perform. A set of rules is aggregated in a ruleset, which can then restrict
the thread enforcing it, and its future children.
+The two existing types of rules are:
+
+Filesystem rules
+ For these rules, the object is a file hierarchy,
+ and the related filesystem actions are defined with
+ `filesystem access rights`.
+
+Network rules (since ABI v4)
+ For these rules, the object is currently a TCP port,
+ and the related actions are defined with `network access rights`.
+
Defining and enforcing a security policy
----------------------------------------
-We first need to define the ruleset that will contain our rules. For this
-example, the ruleset will contain rules that only allow read actions, but write
-actions will be denied. The ruleset then needs to handle both of these kind of
-actions. This is required for backward and forward compatibility (i.e. the
-kernel and user space may not know each other's supported restrictions), hence
-the need to be explicit about the denied-by-default access rights.
+We first need to define the ruleset that will contain our rules.
+
+For this example, the ruleset will contain rules that only allow filesystem
+read actions and establish a specific TCP connection. Filesystem write
+actions and other TCP actions will be denied.
+
+The ruleset then needs to handle both of these kind of actions. This is
+required for backward and forward compatibility (i.e. the kernel and user
+space may not know each other's supported restrictions), hence the need
+to be explicit about the denied-by-default access rights.
.. code-block:: c
@@ -62,6 +76,9 @@ the need to be explicit about the denied-by-default access rights.
LANDLOCK_ACCESS_FS_MAKE_SYM |
LANDLOCK_ACCESS_FS_REFER |
LANDLOCK_ACCESS_FS_TRUNCATE,
+ .handled_access_net =
+ LANDLOCK_ACCESS_NET_BIND_TCP |
+ LANDLOCK_ACCESS_NET_CONNECT_TCP,
};
Because we may not know on which kernel version an application will be
@@ -70,9 +87,7 @@ 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`` or ``LANDLOCK_ACCESS_FS_TRUNCATE``
-access rights, which are only supported starting with the second and third
-version of the ABI.
+remove access rights which are only supported in higher versions of the ABI.
.. code-block:: c
@@ -92,6 +107,11 @@ version of the ABI.
case 2:
/* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */
ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE;
+ case 3:
+ /* Removes network support for ABI < 4 */
+ ruleset_attr.handled_access_net &=
+ ~(LANDLOCK_ACCESS_NET_BIND_TCP |
+ LANDLOCK_ACCESS_NET_CONNECT_TCP);
}
This enables to create an inclusive ruleset that will contain our rules.
@@ -143,10 +163,23 @@ for the ruleset creation, by filtering access rights according to the Landlock
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
-restrict the current thread from gaining more privileges (e.g. thanks to a SUID
-binary).
+For network access-control, we can add a set of rules that allow to use a port
+number for a specific action: HTTPS connections.
+
+.. code-block:: c
+
+ struct landlock_net_port_attr net_port = {
+ .allowed_access = LANDLOCK_ACCESS_NET_CONNECT_TCP,
+ .port = 443,
+ };
+
+ err = landlock_add_rule(ruleset_fd, LANDLOCK_RULE_NET_PORT,
+ &net_port, 0);
+
+The next step is to restrict the current thread from gaining more privileges
+(e.g. through a SUID binary). We now have a ruleset with the first rule allowing
+read access to ``/usr`` while denying all other handled accesses for the filesystem,
+and a second rule allowing HTTPS connections.
.. code-block:: c
@@ -355,7 +388,7 @@ Access rights
-------------
.. kernel-doc:: include/uapi/linux/landlock.h
- :identifiers: fs_access
+ :identifiers: fs_access net_access
Creating a new ruleset
----------------------
@@ -374,6 +407,7 @@ Extending a ruleset
.. kernel-doc:: include/uapi/linux/landlock.h
:identifiers: landlock_rule_type landlock_path_beneath_attr
+ landlock_net_service_attr
Enforcing a ruleset
-------------------
@@ -451,6 +485,12 @@ 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.
+Network support (ABI < 4)
+-------------------------
+
+Starting with the Landlock ABI version 4, it is now possible to restrict TCP
+bind and connect actions to only a set of allowed ports.
+
.. _kernel_support:
Kernel support
@@ -469,6 +509,11 @@ still enable it by adding ``lsm=landlock,[...]`` to
Documentation/admin-guide/kernel-parameters.rst thanks to the bootloader
configuration.
+To be able to explicitly allow TCP operations (e.g., adding a network rule with
+``LANDLOCK_ACCESS_NET_TCP_BIND``), the kernel must support TCP (``CONFIG_INET=y``).
+Otherwise, sys_landlock_add_rule() returns an ``EAFNOSUPPORT`` error, which can
+safely be ignored because this kind of TCP operation is already not possible.
+
Questions and answers
=====================
--
2.25.1
More information about the Linux-security-module-archive
mailing list