[PATCH v9 12/12] landlock: Document Landlock's network support

[Konstantin Meskhidze]

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>
---

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 | 72 ++++++++++++++++++------
 1 file changed, 56 insertions(+), 16 deletions(-)

diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst
index d8cd8cd9ce25..980558b879d6 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.
 
@@ -30,18 +30,20 @@ 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
-the thread enforcing it, and its future children.
+rights`_.  Since ABI version 4 a port data appears with related network actions
+for TCP socket families.  A set of rules is aggregated in a ruleset, which
+can then restrict the thread enforcing it, and its future children.
 
 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 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.
+the need to be explicit about the denied-by-default access rights.  Also ruleset
+will have network rules for specific ports, so it should handle network actions.
 
 .. code-block:: c
 
@@ -62,6 +64,9 @@ 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
@@ -70,14 +75,18 @@ 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 the ``LANDLOCK_ACCESS_FS_REFER`` or ``LANDLOCK_ACCESS_FS_TRUNCATE`` or
+network access rights, which are only supported starting with the second,
+third and fourth version of the ABI.
 
 .. code-block:: c
 
     int abi;
 
+    #define ACCESS_NET_BIND_CONNECT ( \
+        LANDLOCK_ACCESS_NET_BIND_TCP | \
+        LANDLOCK_ACCESS_NET_CONNECT_TCP)
+
     abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
     if (abi < 0) {
         /* Degrades gracefully if Landlock is not handled. */
@@ -92,6 +101,11 @@ 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 +157,24 @@ 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. All ports values must be defined in network byte
+order.
+
+.. code-block:: c
+
+    struct landlock_net_service_attr net_service = {
+        .allowed_access = LANDLOCK_ACCESS_NET_BIND_TCP,
+        .port = htons(8080),
+    };
+
+    err = landlock_add_rule(ruleset_fd, LANDLOCK_RULE_NET_SERVICE,
+                            &net_service, 0);
+
+The next step is to restrict the current thread from gaining more privileges
+(e.g. thanks to 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 TCP binding on port 8080.
 
 .. code-block:: c
 
@@ -355,7 +383,7 @@ Access rights
 -------------
 
 .. kernel-doc:: include/uapi/linux/landlock.h
-    :identifiers: fs_access
+    :identifiers: fs_access net_access
 
 Creating a new ruleset
 ----------------------
@@ -374,6 +402,7 @@ 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 +480,12 @@ 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 +504,11 @@ 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