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SubjectRe: [RFC PATCH v14 10/10] landlock: Add user and kernel documentation
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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@digikod.net>
> Reviewed-by: Vincent Dagonneau <vincent.dagonneau@ssi.gouv.fr>
> Cc: Andy Lutomirski <luto@amacapital.net>
> Cc: James Morris <jmorris@namei.org>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Serge E. Hallyn <serge@hallyn.com>
> ---
>
> Changes since v13:
> * Rewrote the documentation according to the major revamp.
>
> Previous version:
> https://lore.kernel.org/lkml/20191104172146.30797-8-mic@digikod.net/
> ---
> 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

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