On 29/02/2020 18:23, Randy Dunlap wrote: > 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. > Done. Thanks for this attentive review!
