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2021-04-22landlock: Enable user space to infer supported featuresMickaël Salaün1-4/+13
Add a new flag LANDLOCK_CREATE_RULESET_VERSION to landlock_create_ruleset(2). This enables to retreive a Landlock ABI version that is useful to efficiently follow a best-effort security approach. Indeed, it would be a missed opportunity to abort the whole sandbox building, because some features are unavailable, instead of protecting users as much as possible with the subset of features provided by the running kernel. This new flag enables user space to identify the minimum set of Landlock features supported by the running kernel without relying on a filesystem interface (e.g. /proc/version, which might be inaccessible) nor testing multiple syscall argument combinations (i.e. syscall bisection). New Landlock features will be documented and tied to a minimum version number (greater than 1). The current version will be incremented for each new kernel release supporting new Landlock features. User space libraries can leverage this information to seamlessly restrict processes as much as possible while being compatible with newer APIs. This is a much more lighter approach than the previous landlock_get_features(2): the complexity is pushed to user space libraries. This flag meets similar needs as securityfs versions: selinux/policyvers, apparmor/features/*/version* and tomoyo/version. Supporting this flag now will be convenient for backward compatibility. Cc: Arnd Bergmann <arnd@arndb.de> Cc: James Morris <jmorris@namei.org> Cc: Jann Horn <jannh@google.com> Cc: Kees Cook <keescook@chromium.org> Cc: Serge E. Hallyn <serge@hallyn.com> Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com> Link: https://lore.kernel.org/r/20210422154123.13086-14-mic@digikod.net Signed-off-by: James Morris <jamorris@linux.microsoft.com>
2021-04-22landlock: Add syscall implementationsMickaël Salaün2-1/+443
These 3 system calls are designed to be used by unprivileged processes to sandbox themselves: * landlock_create_ruleset(2): Creates a ruleset and returns its file descriptor. * landlock_add_rule(2): Adds a rule (e.g. file hierarchy access) to a ruleset, identified by the dedicated file descriptor. * landlock_restrict_self(2): Enforces a ruleset on the calling thread and its future children (similar to seccomp). This syscall has the same usage restrictions as seccomp(2): the caller must have the no_new_privs attribute set or have CAP_SYS_ADMIN in the current user namespace. All these syscalls have a "flags" argument (not currently used) to enable extensibility. Here are the motivations for these new syscalls: * A sandboxed process may not have access to file systems, including /dev, /sys or /proc, but it should still be able to add more restrictions to itself. * Neither prctl(2) nor seccomp(2) (which was used in a previous version) fit well with the current definition of a Landlock security policy. All passed structs (attributes) are checked at build time to ensure that they don't contain holes and that they are aligned the same way for each architecture. See the user and kernel documentation for more details (provided by a following commit): * Documentation/userspace-api/landlock.rst * Documentation/security/landlock.rst Cc: Arnd Bergmann <arnd@arndb.de> Cc: James Morris <jmorris@namei.org> Cc: Jann Horn <jannh@google.com> Cc: Kees Cook <keescook@chromium.org> Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com> Acked-by: Serge Hallyn <serge@hallyn.com> Link: https://lore.kernel.org/r/20210422154123.13086-9-mic@digikod.net Signed-off-by: James Morris <jamorris@linux.microsoft.com>
2021-04-22landlock: Support filesystem access-controlMickaël Salaün8-2/+781
Using Landlock objects and ruleset, it is possible to tag inodes according to a process's domain. To enable an unprivileged process to express a file hierarchy, it first needs to open a directory (or a file) and pass this file descriptor to the kernel through landlock_add_rule(2). When checking if a file access request is allowed, we walk from the requested dentry to the real root, following the different mount layers. The access to each "tagged" inodes are collected according to their rule layer level, and ANDed to create access to the requested file hierarchy. This makes possible to identify a lot of files without tagging every inodes nor modifying the filesystem, while still following the view and understanding the user has from the filesystem. Add a new ARCH_EPHEMERAL_INODES for UML because it currently does not keep the same struct inodes for the same inodes whereas these inodes are in use. This commit adds a minimal set of supported filesystem access-control which doesn't enable to restrict all file-related actions. This is the result of multiple discussions to minimize the code of Landlock to ease review. Thanks to the Landlock design, extending this access-control without breaking user space will not be a problem. Moreover, seccomp filters can be used to restrict the use of syscall families which may not be currently handled by Landlock. Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: Anton Ivanov <anton.ivanov@cambridgegreys.com> Cc: James Morris <jmorris@namei.org> Cc: Jann Horn <jannh@google.com> Cc: Jeff Dike <jdike@addtoit.com> Cc: Kees Cook <keescook@chromium.org> Cc: Richard Weinberger <richard@nod.at> Cc: Serge E. Hallyn <serge@hallyn.com> Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com> Link: https://lore.kernel.org/r/20210422154123.13086-8-mic@digikod.net Signed-off-by: James Morris <jamorris@linux.microsoft.com>
2021-04-22landlock: Add ptrace restrictionsMickaël Salaün4-1/+137
Using ptrace(2) and related debug features on a target process can lead to a privilege escalation. Indeed, ptrace(2) can be used by an attacker to impersonate another task and to remain undetected while performing malicious activities. Thanks to ptrace_may_access(), various part of the kernel can check if a tracer is more privileged than a tracee. A landlocked process has fewer privileges than a non-landlocked process and must then be subject to additional restrictions when manipulating processes. To be allowed to use ptrace(2) and related syscalls on a target process, a landlocked process must have a subset of the target process's rules (i.e. the tracee must be in a sub-domain of the tracer). Cc: James Morris <jmorris@namei.org> Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com> Reviewed-by: Jann Horn <jannh@google.com> Acked-by: Serge Hallyn <serge@hallyn.com> Reviewed-by: Kees Cook <keescook@chromium.org> Link: https://lore.kernel.org/r/20210422154123.13086-5-mic@digikod.net Signed-off-by: James Morris <jamorris@linux.microsoft.com>
2021-04-22landlock: Set up the security framework and manage credentialsMickaël Salaün6-1/+173
Process's credentials point to a Landlock domain, which is underneath implemented with a ruleset. In the following commits, this domain is used to check and enforce the ptrace and filesystem security policies. A domain is inherited from a parent to its child the same way a thread inherits a seccomp policy. Cc: James Morris <jmorris@namei.org> Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com> Reviewed-by: Jann Horn <jannh@google.com> Acked-by: Serge Hallyn <serge@hallyn.com> Reviewed-by: Kees Cook <keescook@chromium.org> Link: https://lore.kernel.org/r/20210422154123.13086-4-mic@digikod.net Signed-off-by: James Morris <jamorris@linux.microsoft.com>
2021-04-22landlock: Add ruleset and domain managementMickaël Salaün4-1/+652
A Landlock ruleset is mainly a red-black tree with Landlock rules as nodes. This enables quick update and lookup to match a requested access, e.g. to a file. A ruleset is usable through a dedicated file descriptor (cf. following commit implementing syscalls) which enables a process to create and populate a ruleset with new rules. A domain is a ruleset tied to a set of processes. This group of rules defines the security policy enforced on these processes and their future children. A domain can transition to a new domain which is the intersection of all its constraints and those of a ruleset provided by the current process. This modification only impact the current process. This means that a process can only gain more constraints (i.e. lose accesses) over time. Cc: James Morris <jmorris@namei.org> Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com> Acked-by: Serge Hallyn <serge@hallyn.com> Reviewed-by: Kees Cook <keescook@chromium.org> Reviewed-by: Jann Horn <jannh@google.com> Link: https://lore.kernel.org/r/20210422154123.13086-3-mic@digikod.net Signed-off-by: James Morris <jamorris@linux.microsoft.com>
2021-04-22landlock: Add object managementMickaël Salaün4-0/+182
A Landlock object enables to identify a kernel object (e.g. an inode). A Landlock rule is a set of access rights allowed on an object. Rules are grouped in rulesets that may be tied to a set of processes (i.e. subjects) to enforce a scoped access-control (i.e. a domain). Because Landlock's goal is to empower any process (especially unprivileged ones) to sandbox themselves, we cannot rely on a system-wide object identification such as file extended attributes. Indeed, we need innocuous, composable and modular access-controls. The main challenge with these constraints is to identify kernel objects while this identification is useful (i.e. when a security policy makes use of this object). But this identification data should be freed once no policy is using it. This ephemeral tagging should not and may not be written in the filesystem. We then need to manage the lifetime of a rule according to the lifetime of its objects. To avoid a global lock, this implementation make use of RCU and counters to safely reference objects. A following commit uses this generic object management for inodes. Cc: James Morris <jmorris@namei.org> Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com> Reviewed-by: Jann Horn <jannh@google.com> Acked-by: Serge Hallyn <serge@hallyn.com> Reviewed-by: Kees Cook <keescook@chromium.org> Link: https://lore.kernel.org/r/20210422154123.13086-2-mic@digikod.net Signed-off-by: James Morris <jamorris@linux.microsoft.com>