[v20,08/12] landlock: Add syscall implementations

diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index b951a87da987..8373fb28eb44 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -1005,6 +1005,14 @@  asmlinkage long sys_pidfd_send_signal(int pidfd, int sig,
 				       siginfo_t __user *info,
 				       unsigned int flags);
 asmlinkage long sys_pidfd_getfd(int pidfd, int fd, unsigned int flags);
+asmlinkage long sys_landlock_get_features(struct landlock_attr_features __user *features_ptr,
+		size_t features_size, __u32 options);
+asmlinkage long sys_landlock_create_ruleset(const struct landlock_attr_ruleset __user *ruleset_ptr,
+		size_t ruleset_size, __u32 options);
+asmlinkage long sys_landlock_add_rule(int ruleset_fd, enum landlock_rule_type rule_type,
+		const void __user *rule_ptr, size_t rule_size, __u32 options);
+asmlinkage long sys_landlock_enforce_ruleset(int ruleset_fd, enum landlock_target_type target_type,
+		int target_fd, __u32 options);
 
 /*
  * Architecture-specific system calls
diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h
index 5141185e6487..eb2a5e0d61a4 100644
--- a/include/uapi/linux/landlock.h
+++ b/include/uapi/linux/landlock.h
@@ -9,6 +9,172 @@ 
 #ifndef _UAPI__LINUX_LANDLOCK_H__
 #define _UAPI__LINUX_LANDLOCK_H__
 
+#include <linux/types.h>
+
+#if 0
+/**
+ * DOC: options_intro
+ *
+ * These options may be used as second argument of sys_landlock().  Each
+ * command have a dedicated set of options, represented as bitmasks.  For two
+ * different commands, their options may overlap.  Each command have at least
+ * one option defining the used attribute type.  This also enables to always
+ * have a usable &struct landlock_attr_features (i.e. filled with bits).
+ */
+#endif
+
+/**
+ * enum landlock_rule_type - Landlock rule type
+ *
+ * Argument of sys_landlock_add_rule().
+ */
+enum landlock_rule_type {
+	/**
+	 * @LANDLOCK_RULE_PATH_BENEATH: Type of a &struct
+	 * landlock_attr_path_beneath .
+	 */
+	LANDLOCK_RULE_PATH_BENEATH = 1,
+};
+
+/**
+ * enum landlock_target_type - Landlock target type
+ *
+ * Argument of sys_landlock_enforce_ruleset().
+ */
+enum landlock_target_type {
+	/**
+	 * @LANDLOCK_TARGET_CURRENT_THREAD: Enforce a ruleset on the thread
+	 * asking for (i.e. seccomp-like).
+	 */
+	LANDLOCK_TARGET_CURRENT_THREAD = 1,
+};
+
+/**
+ * struct landlock_attr_features - Receives the supported features
+ *
+ * This struct should be allocated by user space but it will be filled by the
+ * kernel to indicate the subset of Landlock features effectively handled by
+ * the running kernel.  This enables backward compatibility for applications
+ * which are developed on a newer kernel than the one running the application.
+ * This helps avoid hard errors that may entirely disable the use of Landlock
+ * features because some of them may not be supported.  Indeed, because
+ * Landlock is a security feature, even if the kernel doesn't support all the
+ * requested features, user space applications should still use the subset
+ * which is supported by the running kernel.  Indeed, a partial security policy
+ * can still improve the security of the application and better protect the
+ * user (i.e. best-effort approach).  The %LANDLOCK_CMD_GET_FEATURES command
+ * and &struct landlock_attr_features are future-proof because the future
+ * unknown fields requested by user space (i.e. a larger &struct
+ * landlock_attr_features) can still be filled with zeros.
+ *
+ * The Landlock commands will fail if an unsupported option or access is
+ * requested.  By firstly requesting the supported options and accesses, it is
+ * quite easy for the developer to binary AND these returned bitmasks with the
+ * used options and accesses from the attribute structs (e.g. &struct
+ * landlock_attr_ruleset), and even infer the supported Landlock commands.
+ * Indeed, because each command must support at least one option, the options_*
+ * fields are always filled if the related commands are supported.  The
+ * supported attributes are also discoverable thanks to the size_* fields.  All
+ * this data enable to create applications doing their best to sandbox
+ * themselves regardless of the running kernel.
+ */
+struct landlock_attr_features {
+	/**
+	 * @options_get_features: Options supported by
+	 * sys_landlock_get_features().
+	 */
+	__u32 options_get_features;
+	/**
+	 * @options_create_ruleset: Options supported by
+	 * sys_landlock_create_ruleset().
+	 */
+	__u32 options_create_ruleset;
+	/**
+	 * @options_add_rule: Options supported by sys_landlock_add_rule().
+	 */
+	__u32 options_add_rule;
+	/**
+	 * @options_enforce_ruleset: Options supported by
+	 * sys_landlock_enforce_ruleset().
+	 */
+	__u32 options_enforce_ruleset;
+	/**
+	 * @access_fs: Subset of file system access supported by the running
+	 * kernel, used in &landlock_attr_ruleset.handled_access_fs and
+	 * &landlock_attr_path_beneath.allowed_access .  Cf. `Filesystem
+	 * flags`_.
+	 */
+	__u64 access_fs;
+	/**
+	 * @size_attr_features: Size of the &struct landlock_attr_features
+	 * (current struct) as known by the kernel (i.e. ``sizeof(struct
+	 * landlock_attr_features)``).
+	 */
+	__u16 size_attr_features;
+	/**
+	 * @size_attr_ruleset: Size of the &struct landlock_attr_ruleset as
+	 * known by the kernel (i.e. ``sizeof(struct
+	 * landlock_attr_ruleset)``).
+	 */
+	__u16 size_attr_ruleset;
+	/**
+	 * @size_attr_path_beneath: Size of the &struct
+	 * landlock_attr_path_beneath as known by the kernel (i.e.
+	 * ``sizeof(struct landlock_attr_path_beneath)``).
+	 */
+	__u16 size_attr_path_beneath;
+	/**
+	 * @last_rule_type: Indicate the last entry of &enum
+	 * landlock_rule_type.
+	 */
+	__u8 last_rule_type;
+	/**
+	 * @last_target_type: Indicate the last entry of &enum
+	 * landlock_target_type.
+	 */
+	__u8 last_target_type;
+};
+
+/**
+ * struct landlock_attr_ruleset- Defines a new ruleset
+ *
+ * Used as first attribute for the %LANDLOCK_CMD_CREATE_RULESET command and
+ * with the %LANDLOCK_OPT_CREATE_RULESET option.
+ */
+struct landlock_attr_ruleset {
+	/**
+	 * @handled_access_fs: Bitmask of actions (cf. `Filesystem flags`_)
+	 * that is handled by this ruleset and should then be forbidden if no
+	 * rule explicitly allow them.  This is needed for backward
+	 * compatibility reasons.  The user space code should check the
+	 * effectively supported actions thanks to %LANDLOCK_CMD_GET_FEATURES
+	 * and &struct landlock_attr_features, and then adjust the arguments of
+	 * the next calls to sys_landlock() accordingly.
+	 */
+	__u64 handled_access_fs;
+};
+
+/**
+ * struct landlock_attr_path_beneath - Defines a path hierarchy
+ */
+struct landlock_attr_path_beneath {
+	/**
+	 * @allowed_access: Bitmask of allowed actions for this file hierarchy
+	 * (cf. `Filesystem flags`_).
+	 */
+	__u64 allowed_access;
+	/**
+	 * @parent_fd: File descriptor, open with ``O_PATH``, which identify
+	 * the parent directory of a file hierarchy, or just a file.
+	 */
+	__s32 parent_fd;
+	/*
+	 * This struct is packed to enable to append future members without
+	 * requiring to have dummy reserved members.
+	 * Cf. security/landlock/syscall.c:build_check_abi()
+	 */
+} __attribute__((packed));
+
 /**
  * DOC: fs_access
  *
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index 3b69a560a7ac..d0f3cf333d86 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -264,6 +264,12 @@  COND_SYSCALL(request_key);
 COND_SYSCALL(keyctl);
 COND_SYSCALL_COMPAT(keyctl);
 
+/* security/landlock/syscall.c */
+COND_SYSCALL(landlock_get_features);
+COND_SYSCALL(landlock_create_ruleset);
+COND_SYSCALL(landlock_add_rule);
+COND_SYSCALL(landlock_enforce_ruleset);
+
 /* arch/example/kernel/sys_example.c */
 
 /* mm/fadvise.c */
diff --git a/security/landlock/Makefile b/security/landlock/Makefile
index 92e3d80ab8ed..4388494779ec 100644
--- a/security/landlock/Makefile
+++ b/security/landlock/Makefile
@@ -1,4 +1,4 @@ 
 obj-$(CONFIG_SECURITY_LANDLOCK) := landlock.o
 
-landlock-y := setup.o object.o ruleset.o \
+landlock-y := setup.o syscall.o object.o ruleset.o \
 	cred.o ptrace.o fs.o
diff --git a/security/landlock/syscall.c b/security/landlock/syscall.c
new file mode 100644
index 000000000000..7bf4dc175dee
--- /dev/null
+++ b/security/landlock/syscall.c
@@ -0,0 +1,554 @@ 
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Landlock LSM - System call and user space interfaces
+ *
+ * Copyright © 2016-2020 Mickaël Salaün <mic@digikod.net>
+ * Copyright © 2018-2020 ANSSI
+ */
+
+#include <asm/current.h>
+#include <linux/anon_inodes.h>
+#include <linux/build_bug.h>
+#include <linux/capability.h>
+#include <linux/compiler_types.h>
+#include <linux/dcache.h>
+#include <linux/err.h>
+#include <linux/errno.h>
+#include <linux/fs.h>
+#include <linux/limits.h>
+#include <linux/mount.h>
+#include <linux/path.h>
+#include <linux/sched.h>
+#include <linux/security.h>
+#include <linux/stddef.h>
+#include <linux/syscalls.h>
+#include <linux/types.h>
+#include <linux/uaccess.h>
+#include <uapi/linux/landlock.h>
+
+#include "cred.h"
+#include "fs.h"
+#include "ruleset.h"
+#include "setup.h"
+
+/**
+ * copy_struct_if_any_from_user - Safe future-proof argument copying
+ *
+ * Extend copy_struct_from_user() to handle NULL @src, which allows for future
+ * use of @src even if it is not used right now.
+ *
+ * @dst: Kernel space pointer or NULL.
+ * @ksize: Actual size of the data pointed to by @dst.
+ * @ksize_min: Minimal required size to be copied.
+ * @src: User space pointer or NULL.
+ * @usize: (Alleged) size of the data pointed to by @src.
+ */
+static int copy_struct_if_any_from_user(void *const dst, const size_t ksize,
+		const size_t ksize_min, const void __user *const src,
+		const size_t usize)
+{
+	int ret;
+
+	/* Checks kernel buffer size inconsistencies. */
+	if (dst) {
+		if (WARN_ON_ONCE(ksize == 0))
+			return -EFAULT;
+	} else {
+		if (WARN_ON_ONCE(ksize != 0))
+			return -EFAULT;
+	}
+
+	/* Checks minimal size. */
+	if (WARN_ON_ONCE(ksize < ksize_min))
+		return -EFAULT;
+	if (usize < ksize_min)
+		return -EINVAL;
+
+	/* Handles empty user buffer. */
+	if (!src) {
+		if (usize != 0)
+			return -EFAULT;
+		if (dst)
+			memset(dst, 0, ksize);
+		return 0;
+	}
+
+	/* Checks user buffer size inconsistency and limit. */
+	if (usize == 0)
+		return -ENODATA;
+	if (usize > PAGE_SIZE)
+		return -E2BIG;
+
+	/* Copies user buffer and fills with zeros. */
+	if (dst)
+		return copy_struct_from_user(dst, ksize, src, usize);
+
+	/* Checks unknown user data. */
+	ret = check_zeroed_user(src, usize);
+	if (ret <= 0)
+		return ret ?: -E2BIG;
+	return 0;
+}
+
+/* Features */
+
+/*
+ * This function only contains arithmetic operations with constants, leading to
+ * BUILD_BUG_ON().  The related code is evaluated and checked at build time,
+ * but it is then ignored thanks to compiler optimizations.
+ */
+static void build_check_abi(void)
+{
+	size_t size_features, size_ruleset, size_path_beneath;
+
+	/*
+	 * For each user space ABI structures, first checks that there is no
+	 * hole in them, then checks that all architectures have the same
+	 * struct size.
+	 */
+	size_features = sizeof_field(struct landlock_attr_features, options_get_features);
+	size_features += sizeof_field(struct landlock_attr_features, options_create_ruleset);
+	size_features += sizeof_field(struct landlock_attr_features, options_add_rule);
+	size_features += sizeof_field(struct landlock_attr_features, options_enforce_ruleset);
+	size_features += sizeof_field(struct landlock_attr_features, access_fs);
+	size_features += sizeof_field(struct landlock_attr_features, size_attr_features);
+	size_features += sizeof_field(struct landlock_attr_features, size_attr_ruleset);
+	size_features += sizeof_field(struct landlock_attr_features, size_attr_path_beneath);
+	size_features += sizeof_field(struct landlock_attr_features, last_rule_type);
+	size_features += sizeof_field(struct landlock_attr_features, last_target_type);
+	BUILD_BUG_ON(sizeof(struct landlock_attr_features) != size_features);
+	BUILD_BUG_ON(sizeof(struct landlock_attr_features) != 32);
+
+	size_ruleset = sizeof_field(struct landlock_attr_ruleset, handled_access_fs);
+	BUILD_BUG_ON(sizeof(struct landlock_attr_ruleset) != size_ruleset);
+	BUILD_BUG_ON(sizeof(struct landlock_attr_ruleset) != 8);
+
+	size_path_beneath = sizeof_field(struct landlock_attr_path_beneath, allowed_access);
+	size_path_beneath += sizeof_field(struct landlock_attr_path_beneath, parent_fd);
+	BUILD_BUG_ON(sizeof(struct landlock_attr_path_beneath) != size_path_beneath);
+	BUILD_BUG_ON(sizeof(struct landlock_attr_path_beneath) != 12);
+}
+
+/**
+ * sys_landlock_get_features - Identify the supported Landlock features
+ *
+ * @features_ptr: Pointer to a &struct landlock_attr_features to be filled by
+ *		  the supported features.
+ * @features_size: Size of the pointed &struct landlock_attr_features (needed
+ *		   for backward and forward compatibility).
+ * @options: Must be 0.
+ *
+ * This system call enables to ask the kernel for supported Landlock features.
+ * This is important to build user space code compatible with older and newer
+ * kernels.
+ *
+ * Possible returned errors are:
+ *
+ * - EOPNOTSUPP: Landlock is supported by the kernel but disabled at boot time;
+ * - EINVAL: @options is not 0;
+ * - ENODATA, E2BIG or EFAULT: @features_ptr or @feature_size inconsistencies.
+ */
+SYSCALL_DEFINE3(landlock_get_features,
+		struct landlock_attr_features __user *const, features_ptr,
+		const size_t, features_size, const __u32, options)
+{
+	size_t data_size, fill_size;
+	const struct landlock_attr_features supported = {
+		.options_get_features = 0,
+		.options_create_ruleset = 0,
+		.options_add_rule = 0,
+		.options_enforce_ruleset = 0,
+		.access_fs = _LANDLOCK_ACCESS_FS_MASK,
+		.size_attr_features = sizeof(struct landlock_attr_features),
+		.size_attr_ruleset = sizeof(struct landlock_attr_ruleset),
+		.size_attr_path_beneath = sizeof(struct landlock_attr_path_beneath),
+		.last_rule_type = LANDLOCK_RULE_PATH_BENEATH,
+		.last_target_type = LANDLOCK_TARGET_CURRENT_THREAD,
+	};
+
+	BUILD_BUG_ON(!__same_type(supported.access_fs,
+		((struct landlock_attr_ruleset *)NULL)->handled_access_fs));
+	BUILD_BUG_ON(!__same_type(supported.access_fs,
+		((struct landlock_attr_path_beneath *)NULL)->allowed_access));
+	build_check_abi();
+
+	/*
+	 * Enables user space to identify if Landlock is disabled, thanks to a
+	 * specific error code.
+	 */
+	if (!landlock_initialized)
+		return -EOPNOTSUPP;
+
+	/* No option for now. */
+	if (options)
+		return -EINVAL;
+
+	/* Checks argument consistency. */
+	if (features_size == 0)
+		return -ENODATA;
+	if (features_size > PAGE_SIZE)
+		return -E2BIG;
+
+	/* Copy a subset of features to user space. */
+	data_size = min(sizeof(supported), features_size);
+	if (copy_to_user(features_ptr, &supported, data_size))
+		return -EFAULT;
+
+	/* Fills with zeros. */
+	fill_size = features_size - data_size;
+	if (fill_size > 0 && clear_user((void __user *)features_ptr + data_size, fill_size))
+		return -EFAULT;
+	return 0;
+}
+
+/* Ruleset handling */
+
+static int fop_ruleset_release(struct inode *const inode,
+		struct file *const filp)
+{
+	struct landlock_ruleset *ruleset = filp->private_data;
+
+	landlock_put_ruleset(ruleset);
+	return 0;
+}
+
+static ssize_t fop_dummy_read(struct file *const filp, char __user *const buf,
+		const size_t size, loff_t *const ppos)
+{
+	/* Dummy handler to enable FMODE_CAN_READ. */
+	return -EINVAL;
+}
+
+static ssize_t fop_dummy_write(struct file *const filp,
+		const char __user *const buf, const size_t size,
+		loff_t *const ppos)
+{
+	/* Dummy handler to enable FMODE_CAN_WRITE. */
+	return -EINVAL;
+}
+
+/*
+ * A ruleset file descriptor enables to build a ruleset by adding (i.e.
+ * writing) rule after rule, without relying on the task's context.  This
+ * reentrant design is also used in a read way to enforce the ruleset on the
+ * current task.
+ */
+static const struct file_operations ruleset_fops = {
+	.release = fop_ruleset_release,
+	.read = fop_dummy_read,
+	.write = fop_dummy_write,
+};
+
+/**
+ * sys_landlock_create_ruleset - Create a new ruleset
+ *
+ * @ruleset_ptr: Pointer to a &struct landlock_attr_ruleset identifying the
+ *		 scope of the new ruleset.
+ * @ruleset_size: Size of the pointed &struct landlock_attr_ruleset (needed for
+ *		  backward and forward compatibility).
+ * @options: Must be 0.
+ *
+ * This system call enables to create a new Landlock ruleset, and returns the
+ * related file descriptor on success.
+ *
+ * Possible returned errors are:
+ *
+ * - EOPNOTSUPP: Landlock is supported by the kernel but disabled at boot time;
+ * - EINVAL: @options is not 0, or unknown access, or too small @ruleset_size;
+ * - ENODATA, E2BIG or EFAULT: @ruleset_ptr or @ruleset_size inconsistencies;
+ * - ENOMSG: empty &landlock_attr_ruleset.handled_access_fs.
+ */
+SYSCALL_DEFINE3(landlock_create_ruleset,
+		const struct landlock_attr_ruleset __user *const, ruleset_ptr,
+		const size_t, ruleset_size, const __u32, options)
+{
+	struct landlock_attr_ruleset attr_ruleset;
+	struct landlock_ruleset *ruleset;
+	int err, ruleset_fd;
+
+	if (!landlock_initialized)
+		return -EOPNOTSUPP;
+
+	/* No option for now. */
+	if (options)
+		return -EINVAL;
+
+	/* Copies raw user space buffer. */
+	err = copy_struct_if_any_from_user(&attr_ruleset, sizeof(attr_ruleset),
+			offsetofend(typeof(attr_ruleset), handled_access_fs),
+			ruleset_ptr, ruleset_size);
+	if (err)
+		return err;
+
+	/* Checks content (and 32-bits cast). */
+	if ((attr_ruleset.handled_access_fs | _LANDLOCK_ACCESS_FS_MASK) !=
+			_LANDLOCK_ACCESS_FS_MASK)
+		return -EINVAL;
+
+	/* Checks arguments and transforms to kernel struct. */
+	ruleset = landlock_create_ruleset(attr_ruleset.handled_access_fs);
+	if (IS_ERR(ruleset))
+		return PTR_ERR(ruleset);
+
+	/* Creates anonymous FD referring to the ruleset. */
+	ruleset_fd = anon_inode_getfd("landlock-ruleset", &ruleset_fops,
+			ruleset, O_RDWR | O_CLOEXEC);
+	if (ruleset_fd < 0)
+		landlock_put_ruleset(ruleset);
+	return ruleset_fd;
+}
+
+/*
+ * Returns an owned ruleset from a FD. It is thus needed to call
+ * landlock_put_ruleset() on the return value.
+ */
+static struct landlock_ruleset *get_ruleset_from_fd(const int fd,
+		const fmode_t mode)
+{
+	struct fd ruleset_f;
+	struct landlock_ruleset *ruleset;
+	int err;
+
+	ruleset_f = fdget(fd);
+	if (!ruleset_f.file)
+		return ERR_PTR(-EBADF);
+
+	/* Checks FD type and access right. */
+	err = 0;
+	if (ruleset_f.file->f_op != &ruleset_fops)
+		err = -EBADFD;
+	else if (!(ruleset_f.file->f_mode & mode))
+		err = -EPERM;
+	if (!err) {
+		ruleset = ruleset_f.file->private_data;
+		landlock_get_ruleset(ruleset);
+	}
+	fdput(ruleset_f);
+	return err ? ERR_PTR(err) : ruleset;
+}
+
+/* Path handling */
+
+/*
+ * @path: Must call put_path(@path) after the call if it succeeded.
+ */
+static int get_path_from_fd(const s32 fd, struct path *const path)
+{
+	struct fd f;
+	int err = 0;
+
+	BUILD_BUG_ON(!__same_type(fd,
+		((struct landlock_attr_path_beneath *)NULL)->parent_fd));
+
+	/* Handles O_PATH. */
+	f = fdget_raw(fd);
+	if (!f.file)
+		return -EBADF;
+	/*
+	 * Only allows O_PATH file descriptor: enables to restrict ambient
+	 * filesystem access without requiring to open and risk leaking or
+	 * misusing a file descriptor.  Forbid internal filesystems (e.g.
+	 * nsfs), including pseudo filesystems that will never be mountable
+	 * (e.g. sockfs, pipefs).
+	 */
+	if (!(f.file->f_mode & FMODE_PATH) ||
+			(f.file->f_path.mnt->mnt_flags & MNT_INTERNAL) ||
+			(f.file->f_path.dentry->d_sb->s_flags & SB_NOUSER) ||
+			d_is_negative(f.file->f_path.dentry) ||
+			IS_PRIVATE(d_backing_inode(f.file->f_path.dentry))) {
+		err = -EBADFD;
+		goto out_fdput;
+	}
+	path->mnt = f.file->f_path.mnt;
+	path->dentry = f.file->f_path.dentry;
+	path_get(path);
+
+out_fdput:
+	fdput(f);
+	return err;
+}
+
+/**
+ * sys_landlock_add_rule - Add a new rule to a ruleset
+ *
+ * @ruleset_fd: File descriptor tied to the ruleset which should be extended
+ *		with the new rule.
+ * @rule_type: Identify the structure type pointed to by @rule_ptr.
+ * @rule_ptr: Pointer to a rule (the currently only supported rule is &struct
+ *	      landlock_attr_path_beneath).
+ * @rule_size: Size of the struct pointed to by @rule_ptr.
+ * @options: Must be 0.
+ *
+ * This system call enables to define a new rule and add it to an existing
+ * ruleset.
+ *
+ * Possible returned errors are:
+ *
+ * - EOPNOTSUPP: Landlock is supported by the kernel but disabled at boot time;
+ * - EINVAL: @options is not 0, or inconsistent access in the rule (i.e.
+ *   &landlock_attr_path_beneath.allowed_access is not a subset of the rule's
+ *   accesses), or too small @rule_size (according to the underlying rule
+ *   type);
+ * - EBADF: @ruleset_fd is not a file descriptor for the current thread;
+ * - EBADFD: @ruleset_fd is not a ruleset file descriptor;
+ * - EPERM: @ruleset_fd has no write access to the underlying ruleset;
+ * - ENODATA, E2BIG or EFAULT: @rule_ptr or @rule_size inconsistencies;
+ */
+SYSCALL_DEFINE5(landlock_add_rule,
+		const int, ruleset_fd, const enum landlock_rule_type, rule_type,
+		const void __user *const, rule_ptr, const size_t, rule_size,
+		const __u32, options)
+{
+	struct landlock_attr_path_beneath attr_path_beneath;
+	struct path path;
+	struct landlock_ruleset *ruleset;
+	int err;
+
+	if (!landlock_initialized)
+		return -EOPNOTSUPP;
+
+	/* No option for now. */
+	if (options)
+		return -EINVAL;
+
+	if (rule_type != LANDLOCK_RULE_PATH_BENEATH)
+		return -EINVAL;
+
+	/* Copies raw user space buffer. */
+	err = copy_struct_if_any_from_user(&attr_path_beneath,
+			sizeof(attr_path_beneath),
+			offsetofend(typeof(attr_path_beneath), allowed_access),
+			rule_ptr, rule_size);
+	if (err)
+		return err;
+
+	/* Gets and checks the ruleset. */
+	ruleset = get_ruleset_from_fd(ruleset_fd, FMODE_CAN_WRITE);
+	if (IS_ERR(ruleset))
+		return PTR_ERR(ruleset);
+
+	/*
+	 * Checks that allowed_access matches the @ruleset constraints
+	 * (ruleset->fs_access_mask is automatically upgraded to 64-bits).
+	 * Allows empty allowed_access i.e., deny @ruleset->fs_access_mask .
+	 */
+	if ((attr_path_beneath.allowed_access | ruleset->fs_access_mask) !=
+			ruleset->fs_access_mask) {
+		err = -EINVAL;
+		goto out_put_ruleset;
+	}
+
+	/* Gets and checks the new rule. */
+	err = get_path_from_fd(attr_path_beneath.parent_fd, &path);
+	if (err)
+		goto out_put_ruleset;
+
+	/* Imports the new rule. */
+	err = landlock_append_fs_rule(ruleset, &path,
+			attr_path_beneath.allowed_access);
+	path_put(&path);
+
+out_put_ruleset:
+	landlock_put_ruleset(ruleset);
+	return err;
+}
+
+/* Enforcement */
+
+/**
+ * sys_landlock_enforce_ruleset - Enforce a ruleset
+ *
+ * @ruleset_fd: File descriptor tied to the ruleset to merge with the target.
+ * @target_type: Identify which type of target to enforce the ruleset on,
+ *		 currently only the current thread is supported (i.e.
+ *		 seccomp-like).
+ * @target_fd: Must be -1.
+ * @options: Must be 0.
+ *
+ * This system call enables to enforce a Landlock ruleset on the current
+ * thread.  Enforcing a ruleset requires that the task has CAP_SYS_ADMIN in its
+ * namespace or be running with no_new_privs.  This avoids scenarios where
+ * unprivileged tasks can affect the behavior of privileged children.
+ *
+ * Possible returned errors are:
+ *
+ * - EOPNOTSUPP: Landlock is supported by the kernel but disabled at boot time;
+ * - EINVAL: @options is not 0, or @target_type is not
+ *   %LANDLOCK_TARGET_CURRENT_THREAD, or @target_fd is not -1;
+ * - EBADF: @ruleset_fd is not a file descriptor for the current thread;
+ * - EBADFD: @ruleset_fd is not a ruleset file descriptor;
+ * - EPERM: @ruleset_fd has no read access to the underlying ruleset, or the
+ *   current thread is not running with no_new_privs (or doesn't have
+ *   CAP_SYS_ADMIN in its namespace).
+ */
+SYSCALL_DEFINE4(landlock_enforce_ruleset,
+		const int, ruleset_fd, const enum landlock_target_type, target_type,
+		const int, target_fd, const __u32, options)
+{
+	struct landlock_ruleset *new_dom, *ruleset;
+	struct cred *new_cred;
+	struct landlock_cred_security *new_llcred;
+	int err;
+
+	if (!landlock_initialized)
+		return -EOPNOTSUPP;
+
+	/* No option for now. */
+	if (options)
+		return -EINVAL;
+
+	/* Only target the current thread for now. */
+	if (target_type != LANDLOCK_TARGET_CURRENT_THREAD)
+		return -EINVAL;
+	if (target_fd != -1)
+		return -EINVAL;
+
+	/*
+	 * Similar checks as for seccomp(2), except that an -EPERM may be
+	 * returned.
+	 */
+	if (!task_no_new_privs(current)) {
+		err = security_capable(current_cred(), current_user_ns(),
+				CAP_SYS_ADMIN, CAP_OPT_NOAUDIT);
+		if (err)
+			return err;
+	}
+
+	/* Gets and checks the ruleset. */
+	ruleset = get_ruleset_from_fd(ruleset_fd, FMODE_CAN_READ);
+	if (IS_ERR(ruleset))
+		return PTR_ERR(ruleset);
+
+	/* Prepares new credentials. */
+	new_cred = prepare_creds();
+	if (!new_cred) {
+		err = -ENOMEM;
+		goto out_put_ruleset;
+	}
+	new_llcred = landlock_cred(new_cred);
+
+	/*
+	 * There is no possible race condition while copying and manipulating
+	 * the current credentials because they are dedicated per thread.
+	 */
+	new_dom = landlock_merge_ruleset(new_llcred->domain, ruleset);
+	if (IS_ERR(new_dom)) {
+		err = PTR_ERR(new_dom);
+		goto out_put_creds;
+	}
+
+	/* Replaces the old (prepared) domain. */
+	landlock_put_ruleset(new_llcred->domain);
+	new_llcred->domain = new_dom;
+
+	landlock_put_ruleset(ruleset);
+	return commit_creds(new_cred);
+
+out_put_creds:
+	abort_creds(new_cred);
+	return err;
+
+out_put_ruleset:
+	landlock_put_ruleset(ruleset);
+	return err;
+}