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@xxxxxxxxxx> --- Changes since v10: * Fixes documentaion as Mickaёl suggested: https://lore.kernel.org/linux-security-module/ec23be77-566e-c8fd-179e-f50e025ac2cf@xxxxxxxxxxx/ 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@xxxxxxxxxxx/ Changes since v6: * Adds network support documentaion. --- Documentation/userspace-api/landlock.rst | 83 ++++++++++++++++++------ 1 file changed, 62 insertions(+), 21 deletions(-) diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst index f6a7da21708a..f185dbaa726a 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 +28,24 @@ 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 -the thread enforcing it, and its future children. +A Landlock rule describes an action on a kernel object. Filesystem +objects can be defined with a file hierarchy. Since the fourth ABI +version, TCP ports enable to identify inbound or outbound connections. +Actions on these kernel objects are defined according to `access +rights`_. 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. 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. +example, the ruleset will contain rules that only allow filesystem read actions +and establish a specific TCP connection, but 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 +66,9 @@ 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 +77,18 @@ 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 +103,11 @@ 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 +159,23 @@ 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_service_attr net_service = { + .allowed_access = NET_CONNECT_TCP, + .port = 443, + }; + + 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. 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 +384,7 @@ Access rights ------------- .. kernel-doc:: include/uapi/linux/landlock.h - :identifiers: fs_access + :identifiers: fs_access net_access Creating a new ruleset ---------------------- @@ -374,6 +403,7 @@ 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 +481,12 @@ 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 +505,11 @@ 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
