Hi, Here are a few corrections for you to consider. On 2/24/20 8:02 AM, Mickaël Salaün wrote: > This documentation can be built with the Sphinx framework. > > Another location might be more appropriate, though. > > Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxx> > Reviewed-by: Vincent Dagonneau <vincent.dagonneau@xxxxxxxxxxx> > Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx> > Cc: James Morris <jmorris@xxxxxxxxx> > Cc: Kees Cook <keescook@xxxxxxxxxxxx> > Cc: Serge E. Hallyn <serge@xxxxxxxxxx> > --- > > Changes since v13: > * Rewrote the documentation according to the major revamp. > > Previous version: > https://lore.kernel.org/lkml/20191104172146.30797-8-mic@xxxxxxxxxxx/ > --- > Documentation/security/index.rst | 1 + > Documentation/security/landlock/index.rst | 18 ++ > Documentation/security/landlock/kernel.rst | 44 ++++ > Documentation/security/landlock/user.rst | 233 +++++++++++++++++++++ > 4 files changed, 296 insertions(+) > create mode 100644 Documentation/security/landlock/index.rst > create mode 100644 Documentation/security/landlock/kernel.rst > create mode 100644 Documentation/security/landlock/user.rst > > diff --git a/Documentation/security/landlock/index.rst b/Documentation/security/landlock/index.rst > new file mode 100644 > index 000000000000..dbd33b96ce60 > --- /dev/null > +++ b/Documentation/security/landlock/index.rst > @@ -0,0 +1,18 @@ > +========================================= > +Landlock LSM: unprivileged access control > +========================================= > + > +:Author: Mickaël Salaün > + > +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 > +unexpected/malicious behaviors in user-space applications. Landlock empower any empowers > +process, including unprivileged ones, to securely restrict themselves. > + > +.. toctree:: > + > + user > + kernel > diff --git a/Documentation/security/landlock/kernel.rst b/Documentation/security/landlock/kernel.rst > new file mode 100644 > index 000000000000..b87769909029 > --- /dev/null > +++ b/Documentation/security/landlock/kernel.rst > @@ -0,0 +1,44 @@ > +============================== > +Landlock: kernel documentation > +============================== > + > +Landlock's goal is to create scoped access-control (i.e. sandboxing). To > +harden a whole system, this feature should be available to any process, > +including unprivileged ones. Because such process may be compromised or > +backdoored (i.e. untrusted), Landlock's features must be safe to use from the > +kernel and other processes point of view. Landlock's interface must therefore > +expose a minimal attack surface. > + > +Landlock is designed to be usable by unprivileged processes while following the > +system security policy enforced by other access control mechanisms (e.g. DAC, > +LSM). Indeed, a Landlock rule shall not interfere with other access-controls > +enforced on the system, only add more restrictions. > + > +Any user can enforce Landlock rulesets on their processes. They are merged and > +evaluated according to the inherited ones in a way that ensure that only more ensures > +constraints can be added. > + > + > +Guiding principles for safe access controls > +=========================================== > + > +* A Landlock rule shall be focused on access control on kernel objects instead > + of syscall filtering (i.e. syscall arguments), which is the purpose of > + seccomp-bpf. > +* To avoid multiple kind of side-channel attacks (e.g. leak of security kinds > + policies, CPU-based attacks), Landlock rules shall not be able to > + programmatically communicate with user space. > +* Kernel access check shall not slow down access request from unsandboxed > + processes. > +* Computation related to Landlock operations (e.g. enforce a ruleset) shall > + only impact the processes requesting them. > + > + > +Landlock rulesets and domains > +============================= > + > +A domain is a read-only ruleset tied to a set of subjects (i.e. tasks). A > +domain can transition to a new one which is the intersection of the constraints > +from the current and a new ruleset. The definition of a subject is implicit > +for a task sandboxing itself, which makes the reasoning much easier and helps > +avoid pitfalls. > diff --git a/Documentation/security/landlock/user.rst b/Documentation/security/landlock/user.rst > new file mode 100644 > index 000000000000..cbd7f61fca8c > --- /dev/null > +++ b/Documentation/security/landlock/user.rst > @@ -0,0 +1,233 @@ > +================================= > +Landlock: userspace documentation > +================================= > + > +Landlock rules > +============== > + > +A Landlock rule enables to describe an action on an object. An object is > +currently a file hierarchy, and the related filesystem actions are defined in > +`Access rights`_. A set of rules are aggregated in a ruleset, which can then is > +restricts the thread enforcing it, and its future children. restrict > + > + > +Defining and enforcing a security policy > +---------------------------------------- > + > +Before defining a security policy, an application should first probe for the > +features supported by the running kernel, which is important to be compatible > +with older kernels. This can be done thanks to the `landlock` syscall (cf. > +:ref:`syscall`). > + > +.. code-block:: c > + > + struct landlock_attr_features attr_features; > + > + if (landlock(LANDLOCK_CMD_GET_FEATURES, LANDLOCK_OPT_GET_FEATURES, > + sizeof(attr_features), &attr_features)) { > + perror("Failed to probe the Landlock supported features"); > + return 1; > + } > + > +Then, we need to create the ruleset that will contains our rules. For this contain > +example, the ruleset will contains rules which only allow read actions, but contain > +write actions will be denied. The ruleset then needs to handle both of these > +kind of actions. To have a backward compatibility, these actions should be > +ANDed with the supported ones. > + > +.. code-block:: c > + > + int ruleset_fd; > + struct landlock_attr_ruleset ruleset = { > + .handled_access_fs = > + LANDLOCK_ACCESS_FS_READ | > + LANDLOCK_ACCESS_FS_READDIR | > + LANDLOCK_ACCESS_FS_EXECUTE | > + LANDLOCK_ACCESS_FS_WRITE | > + LANDLOCK_ACCESS_FS_TRUNCATE | > + LANDLOCK_ACCESS_FS_CHMOD | > + LANDLOCK_ACCESS_FS_CHOWN | > + LANDLOCK_ACCESS_FS_CHGRP | > + LANDLOCK_ACCESS_FS_LINK_TO | > + LANDLOCK_ACCESS_FS_RENAME_FROM | > + LANDLOCK_ACCESS_FS_RENAME_TO | > + LANDLOCK_ACCESS_FS_RMDIR | > + LANDLOCK_ACCESS_FS_UNLINK | > + LANDLOCK_ACCESS_FS_MAKE_CHAR | > + LANDLOCK_ACCESS_FS_MAKE_DIR | > + LANDLOCK_ACCESS_FS_MAKE_REG | > + LANDLOCK_ACCESS_FS_MAKE_SOCK | > + LANDLOCK_ACCESS_FS_MAKE_FIFO | > + LANDLOCK_ACCESS_FS_MAKE_BLOCK | > + LANDLOCK_ACCESS_FS_MAKE_SYM, > + }; > + > + ruleset.handled_access_fs &= attr_features.access_fs; > + ruleset_fd = landlock(LANDLOCK_CMD_CREATE_RULESET, > + LANDLOCK_OPT_CREATE_RULESET, sizeof(ruleset), &ruleset); > + if (ruleset_fd < 0) { > + perror("Failed to create a ruleset"); > + return 1; > + } > + > +We can now add a new rule to this ruleset thanks to the returned file > +descriptor referring to this ruleset. The rule will only enable to read the > +file hierarchy ``/usr``. Without other rule, write actions would then be Without other rules, or Without another rule, > +denied by the ruleset. To add ``/usr`` to the ruleset, we open it with the > +``O_PATH`` flag and fill the &struct landlock_attr_path_beneath with this file > +descriptor. > + > +.. code-block:: c > + > + int err; > + struct landlock_attr_path_beneath path_beneath = { > + .ruleset_fd = ruleset_fd, > + .allowed_access = > + LANDLOCK_ACCESS_FS_READ | > + LANDLOCK_ACCESS_FS_READDIR | > + LANDLOCK_ACCESS_FS_EXECUTE, > + }; > + > + path_beneath.allowed_access &= attr_features.access_fs; > + path_beneath.parent_fd = open("/usr", O_PATH | O_CLOEXEC); > + if (path_beneath.parent_fd < 0) { > + perror("Failed to open file"); > + close(ruleset_fd); > + return 1; > + } > + err = landlock(LANDLOCK_CMD_ADD_RULE, LANDLOCK_OPT_ADD_RULE_PATH_BENEATH, > + sizeof(path_beneath), &path_beneath); > + close(path_beneath.parent_fd); > + if (err) { > + perror("Failed to update ruleset"); > + close(ruleset_fd); > + return 1; > + } > + > +We now have a ruleset with one rule allowing read access to ``/usr`` while > +denying all accesses featured in ``attr_features.access_fs`` to everything else > +on the filesystem. The next step is to restrict the current thread from > +gaining more privileges (e.g. thanks to a SUID binary). > + > +.. code-block:: c > + > + if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) { > + perror("Failed to restrict privileges"); > + close(ruleset_fd); > + return 1; > + } > + > +The current thread is now ready to sandbox itself with the ruleset. > + > +.. code-block:: c > + > + struct landlock_attr_enforce attr_enforce = { > + .ruleset_fd = ruleset_fd, > + }; > + > + if (landlock(LANDLOCK_CMD_ENFORCE_RULESET, LANDLOCK_OPT_ENFORCE_RULESET, > + sizeof(attr_enforce), &attr_enforce)) { > + perror("Failed to enforce ruleset"); > + close(ruleset_fd); > + return 1; > + } > + close(ruleset_fd); > + > +If this last system call succeeds, the current thread is now restricted and If this last landlock system call succeeds, [because close() is the last system call] > +this policy will be enforced on all its subsequently created children as well. > +Once a thread is landlocked, there is no way to remove its security policy, preferably: policy; > +only adding more restrictions is allowed. These threads are now in a new > +Landlock domain, merge of their parent one (if any) with the new ruleset. > + > +A full working code can be found in `samples/landlock/sandboxer.c`_. Full working code > + > + > +Inheritance > +----------- > + > +Every new thread resulting from a :manpage:`clone(2)` inherits Landlock program > +restrictions from its parent. This is similar to the seccomp inheritance (cf. > +:doc:`/userspace-api/seccomp_filter`) or any other LSM dealing with task's > +:manpage:`credentials(7)`. For instance, one process' thread may apply process's > +Landlock rules to itself, but they will not be automatically applied to other > +sibling threads (unlike POSIX thread credential changes, cf. > +:manpage:`nptl(7)`). [snip] thanks for the documentation. -- ~Randy
