1.. SPDX-License-Identifier: GPL-2.0 2.. Copyright © 2017-2020 Mickaël Salaün <mic@digikod.net> 3.. Copyright © 2019-2020 ANSSI 4.. Copyright © 2021-2022 Microsoft Corporation 5 6===================================== 7Landlock: unprivileged access control 8===================================== 9 10:Author: Mickaël Salaün 11:Date: September 2024 12 13The goal of Landlock is to enable to restrict ambient rights (e.g. global 14filesystem or network access) for a set of processes. Because Landlock 15is a stackable LSM, it makes possible to create safe security sandboxes as new 16security layers in addition to the existing system-wide access-controls. This 17kind of sandbox is expected to help mitigate the security impact of bugs or 18unexpected/malicious behaviors in user space applications. Landlock empowers 19any process, including unprivileged ones, to securely restrict themselves. 20 21We can quickly make sure that Landlock is enabled in the running system by 22looking for "landlock: Up and running" in kernel logs (as root): 23``dmesg | grep landlock || journalctl -kb -g landlock`` . 24Developers can also easily check for Landlock support with a 25:ref:`related system call <landlock_abi_versions>`. 26If Landlock is not currently supported, we need to 27:ref:`configure the kernel appropriately <kernel_support>`. 28 29Landlock rules 30============== 31 32A Landlock rule describes an action on an object which the process intends to 33perform. A set of rules is aggregated in a ruleset, which can then restrict 34the thread enforcing it, and its future children. 35 36The two existing types of rules are: 37 38Filesystem rules 39 For these rules, the object is a file hierarchy, 40 and the related filesystem actions are defined with 41 `filesystem access rights`. 42 43Network rules (since ABI v4) 44 For these rules, the object is a TCP port, 45 and the related actions are defined with `network access rights`. 46 47Defining and enforcing a security policy 48---------------------------------------- 49 50We first need to define the ruleset that will contain our rules. 51 52For this example, the ruleset will contain rules that only allow filesystem 53read actions and establish a specific TCP connection. Filesystem write 54actions and other TCP actions will be denied. 55 56The ruleset then needs to handle both these kinds of actions. This is 57required for backward and forward compatibility (i.e. the kernel and user 58space may not know each other's supported restrictions), hence the need 59to be explicit about the denied-by-default access rights. 60 61.. code-block:: c 62 63 struct landlock_ruleset_attr ruleset_attr = { 64 .handled_access_fs = 65 LANDLOCK_ACCESS_FS_EXECUTE | 66 LANDLOCK_ACCESS_FS_WRITE_FILE | 67 LANDLOCK_ACCESS_FS_READ_FILE | 68 LANDLOCK_ACCESS_FS_READ_DIR | 69 LANDLOCK_ACCESS_FS_REMOVE_DIR | 70 LANDLOCK_ACCESS_FS_REMOVE_FILE | 71 LANDLOCK_ACCESS_FS_MAKE_CHAR | 72 LANDLOCK_ACCESS_FS_MAKE_DIR | 73 LANDLOCK_ACCESS_FS_MAKE_REG | 74 LANDLOCK_ACCESS_FS_MAKE_SOCK | 75 LANDLOCK_ACCESS_FS_MAKE_FIFO | 76 LANDLOCK_ACCESS_FS_MAKE_BLOCK | 77 LANDLOCK_ACCESS_FS_MAKE_SYM | 78 LANDLOCK_ACCESS_FS_REFER | 79 LANDLOCK_ACCESS_FS_TRUNCATE | 80 LANDLOCK_ACCESS_FS_IOCTL_DEV, 81 .handled_access_net = 82 LANDLOCK_ACCESS_NET_BIND_TCP | 83 LANDLOCK_ACCESS_NET_CONNECT_TCP, 84 .scoped = 85 LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET, 86 }; 87 88Because we may not know on which kernel version an application will be 89executed, it is safer to follow a best-effort security approach. Indeed, we 90should try to protect users as much as possible whatever the kernel they are 91using. 92 93To be compatible with older Linux versions, we detect the available Landlock ABI 94version, and only use the available subset of access rights: 95 96.. code-block:: c 97 98 int abi; 99 100 abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION); 101 if (abi < 0) { 102 /* Degrades gracefully if Landlock is not handled. */ 103 perror("The running kernel does not enable to use Landlock"); 104 return 0; 105 } 106 switch (abi) { 107 case 1: 108 /* Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2 */ 109 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER; 110 __attribute__((fallthrough)); 111 case 2: 112 /* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */ 113 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE; 114 __attribute__((fallthrough)); 115 case 3: 116 /* Removes network support for ABI < 4 */ 117 ruleset_attr.handled_access_net &= 118 ~(LANDLOCK_ACCESS_NET_BIND_TCP | 119 LANDLOCK_ACCESS_NET_CONNECT_TCP); 120 __attribute__((fallthrough)); 121 case 4: 122 /* Removes LANDLOCK_ACCESS_FS_IOCTL_DEV for ABI < 5 */ 123 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_IOCTL_DEV; 124 __attribute__((fallthrough)); 125 case 5: 126 /* Removes LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET for ABI < 6 */ 127 ruleset_attr.scoped &= ~LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET; 128 } 129 130This enables to create an inclusive ruleset that will contain our rules. 131 132.. code-block:: c 133 134 int ruleset_fd; 135 136 ruleset_fd = landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0); 137 if (ruleset_fd < 0) { 138 perror("Failed to create a ruleset"); 139 return 1; 140 } 141 142We can now add a new rule to this ruleset thanks to the returned file 143descriptor referring to this ruleset. The rule will only allow reading the 144file hierarchy ``/usr``. Without another rule, write actions would then be 145denied by the ruleset. To add ``/usr`` to the ruleset, we open it with the 146``O_PATH`` flag and fill the &struct landlock_path_beneath_attr with this file 147descriptor. 148 149.. code-block:: c 150 151 int err; 152 struct landlock_path_beneath_attr path_beneath = { 153 .allowed_access = 154 LANDLOCK_ACCESS_FS_EXECUTE | 155 LANDLOCK_ACCESS_FS_READ_FILE | 156 LANDLOCK_ACCESS_FS_READ_DIR, 157 }; 158 159 path_beneath.parent_fd = open("/usr", O_PATH | O_CLOEXEC); 160 if (path_beneath.parent_fd < 0) { 161 perror("Failed to open file"); 162 close(ruleset_fd); 163 return 1; 164 } 165 err = landlock_add_rule(ruleset_fd, LANDLOCK_RULE_PATH_BENEATH, 166 &path_beneath, 0); 167 close(path_beneath.parent_fd); 168 if (err) { 169 perror("Failed to update ruleset"); 170 close(ruleset_fd); 171 return 1; 172 } 173 174It may also be required to create rules following the same logic as explained 175for the ruleset creation, by filtering access rights according to the Landlock 176ABI version. In this example, this is not required because all of the requested 177``allowed_access`` rights are already available in ABI 1. 178 179For network access-control, we can add a set of rules that allow to use a port 180number for a specific action: HTTPS connections. 181 182.. code-block:: c 183 184 struct landlock_net_port_attr net_port = { 185 .allowed_access = LANDLOCK_ACCESS_NET_CONNECT_TCP, 186 .port = 443, 187 }; 188 189 err = landlock_add_rule(ruleset_fd, LANDLOCK_RULE_NET_PORT, 190 &net_port, 0); 191 192The next step is to restrict the current thread from gaining more privileges 193(e.g. through a SUID binary). We now have a ruleset with the first rule 194allowing read access to ``/usr`` while denying all other handled accesses for 195the filesystem, and a second rule allowing HTTPS connections. 196 197.. code-block:: c 198 199 if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) { 200 perror("Failed to restrict privileges"); 201 close(ruleset_fd); 202 return 1; 203 } 204 205The current thread is now ready to sandbox itself with the ruleset. 206 207.. code-block:: c 208 209 if (landlock_restrict_self(ruleset_fd, 0)) { 210 perror("Failed to enforce ruleset"); 211 close(ruleset_fd); 212 return 1; 213 } 214 close(ruleset_fd); 215 216If the ``landlock_restrict_self`` system call succeeds, the current thread is 217now restricted and this policy will be enforced on all its subsequently created 218children as well. Once a thread is landlocked, there is no way to remove its 219security policy; only adding more restrictions is allowed. These threads are 220now in a new Landlock domain, merge of their parent one (if any) with the new 221ruleset. 222 223Full working code can be found in `samples/landlock/sandboxer.c`_. 224 225Good practices 226-------------- 227 228It is recommended setting access rights to file hierarchy leaves as much as 229possible. For instance, it is better to be able to have ``~/doc/`` as a 230read-only hierarchy and ``~/tmp/`` as a read-write hierarchy, compared to 231``~/`` as a read-only hierarchy and ``~/tmp/`` as a read-write hierarchy. 232Following this good practice leads to self-sufficient hierarchies that do not 233depend on their location (i.e. parent directories). This is particularly 234relevant when we want to allow linking or renaming. Indeed, having consistent 235access rights per directory enables to change the location of such directory 236without relying on the destination directory access rights (except those that 237are required for this operation, see ``LANDLOCK_ACCESS_FS_REFER`` 238documentation). 239 240Having self-sufficient hierarchies also helps to tighten the required access 241rights to the minimal set of data. This also helps avoid sinkhole directories, 242i.e. directories where data can be linked to but not linked from. However, 243this depends on data organization, which might not be controlled by developers. 244In this case, granting read-write access to ``~/tmp/``, instead of write-only 245access, would potentially allow to move ``~/tmp/`` to a non-readable directory 246and still keep the ability to list the content of ``~/tmp/``. 247 248Layers of file path access rights 249--------------------------------- 250 251Each time a thread enforces a ruleset on itself, it updates its Landlock domain 252with a new layer of policy. Indeed, this complementary policy is stacked with 253the potentially other rulesets already restricting this thread. A sandboxed 254thread can then safely add more constraints to itself with a new enforced 255ruleset. 256 257One policy layer grants access to a file path if at least one of its rules 258encountered on the path grants the access. A sandboxed thread can only access 259a file path if all its enforced policy layers grant the access as well as all 260the other system access controls (e.g. filesystem DAC, other LSM policies, 261etc.). 262 263Bind mounts and OverlayFS 264------------------------- 265 266Landlock enables to restrict access to file hierarchies, which means that these 267access rights can be propagated with bind mounts (cf. 268Documentation/filesystems/sharedsubtree.rst) but not with 269Documentation/filesystems/overlayfs.rst. 270 271A bind mount mirrors a source file hierarchy to a destination. The destination 272hierarchy is then composed of the exact same files, on which Landlock rules can 273be tied, either via the source or the destination path. These rules restrict 274access when they are encountered on a path, which means that they can restrict 275access to multiple file hierarchies at the same time, whether these hierarchies 276are the result of bind mounts or not. 277 278An OverlayFS mount point consists of upper and lower layers. These layers are 279combined in a merge directory, result of the mount point. This merge hierarchy 280may include files from the upper and lower layers, but modifications performed 281on the merge hierarchy only reflects on the upper layer. From a Landlock 282policy point of view, each OverlayFS layers and merge hierarchies are 283standalone and contains their own set of files and directories, which is 284different from bind mounts. A policy restricting an OverlayFS layer will not 285restrict the resulted merged hierarchy, and vice versa. Landlock users should 286then only think about file hierarchies they want to allow access to, regardless 287of the underlying filesystem. 288 289Inheritance 290----------- 291 292Every new thread resulting from a :manpage:`clone(2)` inherits Landlock domain 293restrictions from its parent. This is similar to the seccomp inheritance (cf. 294Documentation/userspace-api/seccomp_filter.rst) or any other LSM dealing with 295task's :manpage:`credentials(7)`. For instance, one process's thread may apply 296Landlock rules to itself, but they will not be automatically applied to other 297sibling threads (unlike POSIX thread credential changes, cf. 298:manpage:`nptl(7)`). 299 300When a thread sandboxes itself, we have the guarantee that the related security 301policy will stay enforced on all this thread's descendants. This allows 302creating standalone and modular security policies per application, which will 303automatically be composed between themselves according to their runtime parent 304policies. 305 306Ptrace restrictions 307------------------- 308 309A sandboxed process has less privileges than a non-sandboxed process and must 310then be subject to additional restrictions when manipulating another process. 311To be allowed to use :manpage:`ptrace(2)` and related syscalls on a target 312process, a sandboxed process should have a subset of the target process rules, 313which means the tracee must be in a sub-domain of the tracer. 314 315IPC scoping 316----------- 317 318Similar to the implicit `Ptrace restrictions`_, we may want to further restrict 319interactions between sandboxes. Each Landlock domain can be explicitly scoped 320for a set of actions by specifying it on a ruleset. For example, if a 321sandboxed process should not be able to :manpage:`connect(2)` to a 322non-sandboxed process through abstract :manpage:`unix(7)` sockets, we can 323specify such restriction with ``LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET``. 324 325A sandboxed process can connect to a non-sandboxed process when its domain is 326not scoped. If a process's domain is scoped, it can only connect to sockets 327created by processes in the same scope. 328 329A connected datagram socket behaves like a stream socket when its domain is 330scoped, meaning if the domain is scoped after the socket is connected , it can 331still :manpage:`send(2)` data just like a stream socket. However, in the same 332scenario, a non-connected datagram socket cannot send data (with 333:manpage:`sendto(2)`) outside its scope. 334 335A process with a scoped domain can inherit a socket created by a non-scoped 336process. The process cannot connect to this socket since it has a scoped 337domain. 338 339IPC scoping does not support exceptions, so if a domain is scoped, no rules can 340be added to allow access to resources or processes outside of the scope. 341 342Truncating files 343---------------- 344 345The operations covered by ``LANDLOCK_ACCESS_FS_WRITE_FILE`` and 346``LANDLOCK_ACCESS_FS_TRUNCATE`` both change the contents of a file and sometimes 347overlap in non-intuitive ways. It is recommended to always specify both of 348these together. 349 350A particularly surprising example is :manpage:`creat(2)`. The name suggests 351that this system call requires the rights to create and write files. However, 352it also requires the truncate right if an existing file under the same name is 353already present. 354 355It should also be noted that truncating files does not require the 356``LANDLOCK_ACCESS_FS_WRITE_FILE`` right. Apart from the :manpage:`truncate(2)` 357system call, this can also be done through :manpage:`open(2)` with the flags 358``O_RDONLY | O_TRUNC``. 359 360The truncate right is associated with the opened file (see below). 361 362Rights associated with file descriptors 363--------------------------------------- 364 365When opening a file, the availability of the ``LANDLOCK_ACCESS_FS_TRUNCATE`` and 366``LANDLOCK_ACCESS_FS_IOCTL_DEV`` rights is associated with the newly created 367file descriptor and will be used for subsequent truncation and ioctl attempts 368using :manpage:`ftruncate(2)` and :manpage:`ioctl(2)`. The behavior is similar 369to opening a file for reading or writing, where permissions are checked during 370:manpage:`open(2)`, but not during the subsequent :manpage:`read(2)` and 371:manpage:`write(2)` calls. 372 373As a consequence, it is possible that a process has multiple open file 374descriptors referring to the same file, but Landlock enforces different things 375when operating with these file descriptors. This can happen when a Landlock 376ruleset gets enforced and the process keeps file descriptors which were opened 377both before and after the enforcement. It is also possible to pass such file 378descriptors between processes, keeping their Landlock properties, even when some 379of the involved processes do not have an enforced Landlock ruleset. 380 381Compatibility 382============= 383 384Backward and forward compatibility 385---------------------------------- 386 387Landlock is designed to be compatible with past and future versions of the 388kernel. This is achieved thanks to the system call attributes and the 389associated bitflags, particularly the ruleset's ``handled_access_fs``. Making 390handled access right explicit enables the kernel and user space to have a clear 391contract with each other. This is required to make sure sandboxing will not 392get stricter with a system update, which could break applications. 393 394Developers can subscribe to the `Landlock mailing list 395<https://subspace.kernel.org/lists.linux.dev.html>`_ to knowingly update and 396test their applications with the latest available features. In the interest of 397users, and because they may use different kernel versions, it is strongly 398encouraged to follow a best-effort security approach by checking the Landlock 399ABI version at runtime and only enforcing the supported features. 400 401.. _landlock_abi_versions: 402 403Landlock ABI versions 404--------------------- 405 406The Landlock ABI version can be read with the sys_landlock_create_ruleset() 407system call: 408 409.. code-block:: c 410 411 int abi; 412 413 abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION); 414 if (abi < 0) { 415 switch (errno) { 416 case ENOSYS: 417 printf("Landlock is not supported by the current kernel.\n"); 418 break; 419 case EOPNOTSUPP: 420 printf("Landlock is currently disabled.\n"); 421 break; 422 } 423 return 0; 424 } 425 if (abi >= 2) { 426 printf("Landlock supports LANDLOCK_ACCESS_FS_REFER.\n"); 427 } 428 429The following kernel interfaces are implicitly supported by the first ABI 430version. Features only supported from a specific version are explicitly marked 431as such. 432 433Kernel interface 434================ 435 436Access rights 437------------- 438 439.. kernel-doc:: include/uapi/linux/landlock.h 440 :identifiers: fs_access net_access scope 441 442Creating a new ruleset 443---------------------- 444 445.. kernel-doc:: security/landlock/syscalls.c 446 :identifiers: sys_landlock_create_ruleset 447 448.. kernel-doc:: include/uapi/linux/landlock.h 449 :identifiers: landlock_ruleset_attr 450 451Extending a ruleset 452------------------- 453 454.. kernel-doc:: security/landlock/syscalls.c 455 :identifiers: sys_landlock_add_rule 456 457.. kernel-doc:: include/uapi/linux/landlock.h 458 :identifiers: landlock_rule_type landlock_path_beneath_attr 459 landlock_net_port_attr 460 461Enforcing a ruleset 462------------------- 463 464.. kernel-doc:: security/landlock/syscalls.c 465 :identifiers: sys_landlock_restrict_self 466 467Current limitations 468=================== 469 470Filesystem topology modification 471-------------------------------- 472 473Threads sandboxed with filesystem restrictions cannot modify filesystem 474topology, whether via :manpage:`mount(2)` or :manpage:`pivot_root(2)`. 475However, :manpage:`chroot(2)` calls are not denied. 476 477Special filesystems 478------------------- 479 480Access to regular files and directories can be restricted by Landlock, 481according to the handled accesses of a ruleset. However, files that do not 482come from a user-visible filesystem (e.g. pipe, socket), but can still be 483accessed through ``/proc/<pid>/fd/*``, cannot currently be explicitly 484restricted. Likewise, some special kernel filesystems such as nsfs, which can 485be accessed through ``/proc/<pid>/ns/*``, cannot currently be explicitly 486restricted. However, thanks to the `ptrace restrictions`_, access to such 487sensitive ``/proc`` files are automatically restricted according to domain 488hierarchies. Future Landlock evolutions could still enable to explicitly 489restrict such paths with dedicated ruleset flags. 490 491Ruleset layers 492-------------- 493 494There is a limit of 16 layers of stacked rulesets. This can be an issue for a 495task willing to enforce a new ruleset in complement to its 16 inherited 496rulesets. Once this limit is reached, sys_landlock_restrict_self() returns 497E2BIG. It is then strongly suggested to carefully build rulesets once in the 498life of a thread, especially for applications able to launch other applications 499that may also want to sandbox themselves (e.g. shells, container managers, 500etc.). 501 502Memory usage 503------------ 504 505Kernel memory allocated to create rulesets is accounted and can be restricted 506by the Documentation/admin-guide/cgroup-v1/memory.rst. 507 508IOCTL support 509------------- 510 511The ``LANDLOCK_ACCESS_FS_IOCTL_DEV`` right restricts the use of 512:manpage:`ioctl(2)`, but it only applies to *newly opened* device files. This 513means specifically that pre-existing file descriptors like stdin, stdout and 514stderr are unaffected. 515 516Users should be aware that TTY devices have traditionally permitted to control 517other processes on the same TTY through the ``TIOCSTI`` and ``TIOCLINUX`` IOCTL 518commands. Both of these require ``CAP_SYS_ADMIN`` on modern Linux systems, but 519the behavior is configurable for ``TIOCSTI``. 520 521On older systems, it is therefore recommended to close inherited TTY file 522descriptors, or to reopen them from ``/proc/self/fd/*`` without the 523``LANDLOCK_ACCESS_FS_IOCTL_DEV`` right, if possible. 524 525Landlock's IOCTL support is coarse-grained at the moment, but may become more 526fine-grained in the future. Until then, users are advised to establish the 527guarantees that they need through the file hierarchy, by only allowing the 528``LANDLOCK_ACCESS_FS_IOCTL_DEV`` right on files where it is really required. 529 530Previous limitations 531==================== 532 533File renaming and linking (ABI < 2) 534----------------------------------- 535 536Because Landlock targets unprivileged access controls, it needs to properly 537handle composition of rules. Such property also implies rules nesting. 538Properly handling multiple layers of rulesets, each one of them able to 539restrict access to files, also implies inheritance of the ruleset restrictions 540from a parent to its hierarchy. Because files are identified and restricted by 541their hierarchy, moving or linking a file from one directory to another implies 542propagation of the hierarchy constraints, or restriction of these actions 543according to the potentially lost constraints. To protect against privilege 544escalations through renaming or linking, and for the sake of simplicity, 545Landlock previously limited linking and renaming to the same directory. 546Starting with the Landlock ABI version 2, it is now possible to securely 547control renaming and linking thanks to the new ``LANDLOCK_ACCESS_FS_REFER`` 548access right. 549 550File truncation (ABI < 3) 551------------------------- 552 553File truncation could not be denied before the third Landlock ABI, so it is 554always allowed when using a kernel that only supports the first or second ABI. 555 556Starting with the Landlock ABI version 3, it is now possible to securely control 557truncation thanks to the new ``LANDLOCK_ACCESS_FS_TRUNCATE`` access right. 558 559Network support (ABI < 4) 560------------------------- 561 562Starting with the Landlock ABI version 4, it is now possible to restrict TCP 563bind and connect actions to only a set of allowed ports thanks to the new 564``LANDLOCK_ACCESS_NET_BIND_TCP`` and ``LANDLOCK_ACCESS_NET_CONNECT_TCP`` 565access rights. 566 567IOCTL (ABI < 5) 568--------------- 569 570IOCTL operations could not be denied before the fifth Landlock ABI, so 571:manpage:`ioctl(2)` is always allowed when using a kernel that only supports an 572earlier ABI. 573 574Starting with the Landlock ABI version 5, it is possible to restrict the use of 575:manpage:`ioctl(2)` using the new ``LANDLOCK_ACCESS_FS_IOCTL_DEV`` right. 576 577Abstract UNIX socket scoping (ABI < 6) 578-------------------------------------- 579 580Starting with the Landlock ABI version 6, it is possible to restrict 581connections to an abstract :manpage:`unix(7)` socket by setting 582``LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET`` to the ``scoped`` ruleset attribute. 583 584.. _kernel_support: 585 586Kernel support 587============== 588 589Build time configuration 590------------------------ 591 592Landlock was first introduced in Linux 5.13 but it must be configured at build 593time with ``CONFIG_SECURITY_LANDLOCK=y``. Landlock must also be enabled at boot 594time as the other security modules. The list of security modules enabled by 595default is set with ``CONFIG_LSM``. The kernel configuration should then 596contains ``CONFIG_LSM=landlock,[...]`` with ``[...]`` as the list of other 597potentially useful security modules for the running system (see the 598``CONFIG_LSM`` help). 599 600Boot time configuration 601----------------------- 602 603If the running kernel does not have ``landlock`` in ``CONFIG_LSM``, then we can 604enable Landlock by adding ``lsm=landlock,[...]`` to 605Documentation/admin-guide/kernel-parameters.rst in the boot loader 606configuration. 607 608For example, if the current built-in configuration is: 609 610.. code-block:: console 611 612 $ zgrep -h "^CONFIG_LSM=" "/boot/config-$(uname -r)" /proc/config.gz 2>/dev/null 613 CONFIG_LSM="lockdown,yama,integrity,apparmor" 614 615...and if the cmdline doesn't contain ``landlock`` either: 616 617.. code-block:: console 618 619 $ sed -n 's/.*\(\<lsm=\S\+\).*/\1/p' /proc/cmdline 620 lsm=lockdown,yama,integrity,apparmor 621 622...we should configure the boot loader to set a cmdline extending the ``lsm`` 623list with the ``landlock,`` prefix:: 624 625 lsm=landlock,lockdown,yama,integrity,apparmor 626 627After a reboot, we can check that Landlock is up and running by looking at 628kernel logs: 629 630.. code-block:: console 631 632 # dmesg | grep landlock || journalctl -kb -g landlock 633 [ 0.000000] Command line: [...] lsm=landlock,lockdown,yama,integrity,apparmor 634 [ 0.000000] Kernel command line: [...] lsm=landlock,lockdown,yama,integrity,apparmor 635 [ 0.000000] LSM: initializing lsm=lockdown,capability,landlock,yama,integrity,apparmor 636 [ 0.000000] landlock: Up and running. 637 638The kernel may be configured at build time to always load the ``lockdown`` and 639``capability`` LSMs. In that case, these LSMs will appear at the beginning of 640the ``LSM: initializing`` log line as well, even if they are not configured in 641the boot loader. 642 643Network support 644--------------- 645 646To be able to explicitly allow TCP operations (e.g., adding a network rule with 647``LANDLOCK_ACCESS_NET_BIND_TCP``), the kernel must support TCP 648(``CONFIG_INET=y``). Otherwise, sys_landlock_add_rule() returns an 649``EAFNOSUPPORT`` error, which can safely be ignored because this kind of TCP 650operation is already not possible. 651 652Questions and answers 653===================== 654 655What about user space sandbox managers? 656--------------------------------------- 657 658Using user space process to enforce restrictions on kernel resources can lead 659to race conditions or inconsistent evaluations (i.e. `Incorrect mirroring of 660the OS code and state 661<https://www.ndss-symposium.org/ndss2003/traps-and-pitfalls-practical-problems-system-call-interposition-based-security-tools/>`_). 662 663What about namespaces and containers? 664------------------------------------- 665 666Namespaces can help create sandboxes but they are not designed for 667access-control and then miss useful features for such use case (e.g. no 668fine-grained restrictions). Moreover, their complexity can lead to security 669issues, especially when untrusted processes can manipulate them (cf. 670`Controlling access to user namespaces <https://lwn.net/Articles/673597/>`_). 671 672Additional documentation 673======================== 674 675* Documentation/security/landlock.rst 676* https://landlock.io 677 678.. Links 679.. _samples/landlock/sandboxer.c: 680 https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/samples/landlock/sandboxer.c 681