[PATCH v15 06/10] landlock: Add syscall implementation

[MickaÃl SalaÃn]

This system call, inspired from seccomp(2) and bpf(2), is designed to be
used by unprivileged processes to sandbox themselves.  It 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.

Here are the motivations for this new syscall:
* 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.
* It is quite easy to whitelist this syscall with seccomp-bpf to enable
  all processes to use it.  It is also easy to filter specific commands
  or options to restrict a process to a subset of Landlock features.

There is currently four commands:
* LANDLOCK_CMD_GET_FEATURES: Gets the supported features (required for
  backward compatibility and best-effort security).
* LANDLOCK_CMD_CREATE_RULESET: Creates a ruleset and returns its file
  descriptor.
* LANDLOCK_CMD_ADD_RULE: Adds a rule (e.g. file hierarchy access) to a
  ruleset, identified by the dedicated file descriptor.
* LANDLOCK_CMD_ENFORCE_RULESET: Enforces a ruleset on the current thread
  and its future children (similar to seccomp).

See the user and code documentation for more details (provided by a
following commit): Documentation/security/landlock/

Signed-off-by: MickaÃl SalaÃn <mic@xxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: James Morris <jmorris@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Serge E. Hallyn <serge@xxxxxxxxxx>
---

Changes since v14:
* Remove the security_file_open() check in get_path_from_fd(): an
  opened FD should not be restricted here, and even less with this hook.
  As a result, it is now allowed to add a path to a ruleset even if the
  access to this path is not allowed (without O_PATH). This doesn't
  change the fact that enforcing a ruleset can't grant any right, only
  remove some rights.  The new layer levels add more consistent
  restrictions.
* Check minimal landlock_attr_* size/content. This fix the case when
  no data was provided and e.g., FD 0 was interpreted as ruleset_fd.
  Now this leads to a returned -EINVAL.
* Fix credential double-free error case.
* Complete struct landlock_attr_size with size_attr_enforce.
* Fix undefined reference to syscall when Landlock is not selected.
* Remove f.file->f_path.mnt check (suggested by Al Viro).
* Add build-time checks.
* Move ABI checks from fs.c .
* Constify variables.
* Fix spelling.
* Add comments.

Changes since v13:
* New implementation, replacing the dependency on seccomp(2) and bpf(2).
---
 include/linux/syscalls.h      |   3 +
 include/uapi/linux/landlock.h | 218 ++++++++++++++
 kernel/sys_ni.c               |   3 +
 security/landlock/Makefile    |   2 +-
 security/landlock/syscall.c   | 521 ++++++++++++++++++++++++++++++++++
 5 files changed, 746 insertions(+), 1 deletion(-)
 create mode 100644 security/landlock/syscall.c

diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 1815065d52f3..beaadcf4ef77 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -1003,6 +1003,9 @@ 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(unsigned int command, unsigned int options,
+			     size_t attr1_size, void __user *attr1_ptr,
+			     size_t attr2_size, void __user *attr2_ptr);
 
 /*
  * Architecture-specific system calls
diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h
index 32003e8b845a..3f60d0e148b8 100644
--- a/include/uapi/linux/landlock.h
+++ b/include/uapi/linux/landlock.h
@@ -9,6 +9,224 @@
 #ifndef _UAPI__LINUX_LANDLOCK_H__
 #define _UAPI__LINUX_LANDLOCK_H__
 
+#include <linux/types.h>
+
+/**
+ * enum landlock_cmd - Landlock commands
+ *
+ * First argument of sys_landlock().
+ */
+enum landlock_cmd {
+	/**
+	 * @LANDLOCK_CMD_GET_FEATURES: Asks the kernel for supported Landlock
+	 * features.  The option argument must contains
+	 * %LANDLOCK_OPT_GET_FEATURES.  This commands fills the &struct
+	 * landlock_attr_features provided as first attribute.
+	 */
+	LANDLOCK_CMD_GET_FEATURES = 1,
+	/**
+	 * @LANDLOCK_CMD_CREATE_RULESET: Creates a new ruleset and return its
+	 * file descriptor on success.  The option argument must contains
+	 * %LANDLOCK_OPT_CREATE_RULESET.  The ruleset is defined by the &struct
+	 * landlock_attr_ruleset provided as first attribute.
+	 */
+	LANDLOCK_CMD_CREATE_RULESET,
+	/**
+	 * @LANDLOCK_CMD_ADD_RULE: Adds a rule to a ruleset.  The option
+	 * argument must contains %LANDLOCK_OPT_ADD_RULE_PATH_BENEATH.  The
+	 * ruleset and the rule are both defined by the &struct
+	 * landlock_attr_path_beneath provided as first attribute.
+	 */
+	LANDLOCK_CMD_ADD_RULE,
+	/**
+	 * @LANDLOCK_CMD_ENFORCE_RULESET: Enforces a ruleset on the current
+	 * process.  The option argument must contains
+	 * %LANDLOCK_OPT_ENFORCE_RULESET.  The ruleset is defined by the
+	 * &struct landlock_attr_enforce provided as first attribute.
+	 */
+	LANDLOCK_CMD_ENFORCE_RULESET,
+};
+
+/**
+ * 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).
+ */
+
+/**
+ * DOC: options_get_features
+ *
+ * Options for ``LANDLOCK_CMD_GET_FEATURES``
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ *
+ * - %LANDLOCK_OPT_GET_FEATURES: the attr type is `struct
+ *   landlock_attr_features`.
+ */
+#define LANDLOCK_OPT_GET_FEATURES			(1ULL << 0)
+
+/**
+ * DOC: options_create_ruleset
+ *
+ * Options for ``LANDLOCK_CMD_CREATE_RULESET``
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ *
+ * - %LANDLOCK_OPT_CREATE_RULESET: the attr type is `struct
+ *   landlock_attr_ruleset`.
+ */
+#define LANDLOCK_OPT_CREATE_RULESET			(1ULL << 0)
+
+/**
+ * DOC: options_add_rule
+ *
+ * Options for ``LANDLOCK_CMD_ADD_RULE``
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ *
+ * - %LANDLOCK_OPT_ADD_RULE_PATH_BENEATH: the attr type is `struct
+ *   landlock_attr_path_beneath`.
+ */
+#define LANDLOCK_OPT_ADD_RULE_PATH_BENEATH		(1ULL << 0)
+
+/**
+ * DOC: options_enforce_ruleset
+ *
+ * Options for ``LANDLOCK_CMD_ENFORCE_RULESET``
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ *
+ * - %LANDLOCK_OPT_ENFORCE_RULESET: the attr type is `struct
+ *   landlock_attr_enforce`.
+ */
+#define LANDLOCK_OPT_ENFORCE_RULESET			(1ULL << 0)
+
+/**
+ * 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 the
+	 * %LANDLOCK_CMD_GET_FEATURES command. Cf. `Options`_.
+	 */
+	__aligned_u64 options_get_features;
+	/**
+	 * @options_create_ruleset: Options supported by the
+	 * %LANDLOCK_CMD_CREATE_RULESET command. Cf. `Options`_.
+	 */
+	__aligned_u64 options_create_ruleset;
+	/**
+	 * @options_add_rule: Options supported by the %LANDLOCK_CMD_ADD_RULE
+	 * command. Cf. `Options`_.
+	 */
+	__aligned_u64 options_add_rule;
+	/**
+	 * @options_enforce_ruleset: Options supported by the
+	 * %LANDLOCK_CMD_ENFORCE_RULESET command. Cf. `Options`_.
+	 */
+	__aligned_u64 options_enforce_ruleset;
+	/**
+	 * @access_fs: Subset of file system access supported by the running
+	 * kernel, used in &struct landlock_attr_ruleset and &struct
+	 * landlock_attr_path_beneath.  Cf. `Filesystem flags`_.
+	 */
+	__aligned_u64 access_fs;
+	/**
+	 * @size_attr_ruleset: Size of the &struct landlock_attr_ruleset as
+	 * known by the kernel (i.e.  ``sizeof(struct
+	 * landlock_attr_ruleset)``).
+	 */
+	__aligned_u64 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_path_beneath)``).
+	 */
+	__aligned_u64 size_attr_path_beneath;
+	/**
+	 * @size_attr_enforce: Size of the &struct landlock_attr_enforce as
+	 * known by the kernel (i.e.  ``sizeof(struct landlock_enforce)``).
+	 */
+	__aligned_u64 size_attr_enforce;
+};
+
+/**
+ * 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_SUPPORTED
+	 * and &struct landlock_attr_features, and then adjust the arguments of
+	 * the next calls to sys_landlock() accordingly.
+	 */
+	__aligned_u64 handled_access_fs;
+};
+
+/**
+ * struct landlock_attr_path_beneath - Defines a path hierarchy
+ */
+struct landlock_attr_path_beneath {
+	/**
+	 * @ruleset_fd: File descriptor tied to the ruleset which should be
+	 * extended with this new access.
+	 */
+	__aligned_u64 ruleset_fd;
+	/**
+	 * @parent_fd: File descriptor, open with ``O_PATH``, which identify
+	 * the parent directory of a file hierarchy, or just a file.
+	 */
+	__aligned_u64 parent_fd;
+	/**
+	 * @allowed_access: Bitmask of allowed actions for this file hierarchy
+	 * (cf. `Filesystem flags`_).
+	 */
+	__aligned_u64 allowed_access;
+};
+
+/**
+ * struct landlock_attr_enforce - Describes the enforcement
+ */
+struct landlock_attr_enforce {
+	/**
+	 * @ruleset_fd: File descriptor tied to the ruleset to merge with the
+	 * current domain.
+	 */
+	__aligned_u64 ruleset_fd;
+};
+
 /**
  * DOC: fs_access
  *
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index 3b69a560a7ac..70cea36eb7ce 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -264,6 +264,9 @@ COND_SYSCALL(request_key);
 COND_SYSCALL(keyctl);
 COND_SYSCALL_COMPAT(keyctl);
 
+/* security/landlock/syscall.c */
+COND_SYSCALL(landlock);
+
 /* 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..2afa28b5f587
--- /dev/null
+++ b/security/landlock/syscall.c
@@ -0,0 +1,521 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Landlock LSM - System call and user space interfaces
+ *
+ * Copyright  2016-2020 MickaÃl SalaÃn <mic@xxxxxxxxxxx>
+ * 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/landlock.h>
+#include <linux/limits.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 */
+
+#define _LANDLOCK_OPT_GET_FEATURES_LAST		LANDLOCK_OPT_GET_FEATURES
+#define _LANDLOCK_OPT_GET_FEATURES_MASK		((_LANDLOCK_OPT_GET_FEATURES_LAST << 1) - 1)
+
+#define _LANDLOCK_OPT_CREATE_RULESET_LAST	LANDLOCK_OPT_CREATE_RULESET
+#define _LANDLOCK_OPT_CREATE_RULESET_MASK	((_LANDLOCK_OPT_CREATE_RULESET_LAST << 1) - 1)
+
+#define _LANDLOCK_OPT_ADD_RULE_LAST		LANDLOCK_OPT_ADD_RULE_PATH_BENEATH
+#define _LANDLOCK_OPT_ADD_RULE_MASK		((_LANDLOCK_OPT_ADD_RULE_LAST << 1) - 1)
+
+#define _LANDLOCK_OPT_ENFORCE_RULESET_LAST	LANDLOCK_OPT_ENFORCE_RULESET
+#define _LANDLOCK_OPT_ENFORCE_RULESET_MASK	((_LANDLOCK_OPT_ENFORCE_RULESET_LAST << 1) - 1)
+
+static int syscall_get_features(const size_t attr_size,
+		void __user *const attr_ptr)
+{
+	size_t data_size, fill_size;
+	const struct landlock_attr_features supported = {
+		.options_get_features = _LANDLOCK_OPT_GET_FEATURES_MASK,
+		.options_create_ruleset = _LANDLOCK_OPT_CREATE_RULESET_MASK,
+		.options_add_rule = _LANDLOCK_OPT_ADD_RULE_MASK,
+		.options_enforce_ruleset = _LANDLOCK_OPT_ENFORCE_RULESET_MASK,
+		.access_fs = _LANDLOCK_ACCESS_FS_MASK,
+		.size_attr_ruleset = sizeof(struct landlock_attr_ruleset),
+		.size_attr_path_beneath = sizeof(struct
+				landlock_attr_path_beneath),
+		.size_attr_enforce = sizeof(struct landlock_attr_enforce),
+	};
+
+	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));
+
+	/* Checks attribute consistency. */
+	if (attr_size == 0)
+		return -ENODATA;
+	if (attr_size > PAGE_SIZE)
+		return -E2BIG;
+
+	/* Copy features to user space. */
+	data_size = min(sizeof(supported), attr_size);
+	if (copy_to_user(attr_ptr, &supported, data_size))
+		return -EFAULT;
+
+	/* Fills with zeros. */
+	fill_size = attr_size - data_size;
+	if (fill_size > 0 && clear_user(attr_ptr + data_size, fill_size))
+		return -EFAULT;
+	return 0;
+}
+
+/* Ruleset handling */
+
+#ifdef CONFIG_PROC_FS
+static void fop_ruleset_show_fdinfo(struct seq_file *const m,
+		struct file *const filp)
+{
+	const struct landlock_ruleset *ruleset = filp->private_data;
+
+	seq_printf(m, "handled_access_fs:\t%x\n", ruleset->fs_access_mask);
+	seq_printf(m, "nb_rules:\t%d\n", atomic_read(&ruleset->nb_rules));
+}
+#endif
+
+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 = {
+#ifdef CONFIG_PROC_FS
+	.show_fdinfo = fop_ruleset_show_fdinfo,
+#endif
+	.release = fop_ruleset_release,
+	.read = fop_dummy_read,
+	.write = fop_dummy_write,
+};
+
+static int syscall_create_ruleset(const size_t attr_size,
+		const void __user *const attr_ptr)
+{
+	struct landlock_attr_ruleset attr_ruleset;
+	struct landlock_ruleset *ruleset;
+	int err, ruleset_fd;
+
+	/* Copies raw user space buffer. */
+	err = copy_struct_if_any_from_user(&attr_ruleset, sizeof(attr_ruleset),
+			offsetofend(typeof(attr_ruleset), handled_access_fs),
+			attr_ptr, attr_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 u64 fd,
+		const fmode_t mode)
+{
+	struct fd ruleset_f;
+	struct landlock_ruleset *ruleset;
+	int err;
+
+	BUILD_BUG_ON(!__same_type(fd,
+		((struct landlock_attr_path_beneath *)NULL)->ruleset_fd));
+	BUILD_BUG_ON(!__same_type(fd,
+		((struct landlock_attr_enforce *)NULL)->ruleset_fd));
+
+	/* Checks 32-bits overflow. fdget() checks for INT_MAX/FD. */
+	if (fd > U32_MAX)
+		return ERR_PTR(-EINVAL);
+	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 = -EBADR;
+	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 */
+
+static inline bool is_user_mountable(const struct dentry *const dentry)
+{
+	/*
+	 * Checks pseudo-filesystems that will never be mountable (e.g. sockfs,
+	 * pipefs, bdev), cf. fs/libfs.c:init_pseudo().
+	 */
+	return d_is_positive(dentry) &&
+		!IS_PRIVATE(dentry->d_inode) &&
+		!(dentry->d_sb->s_flags & SB_NOUSER);
+}
+
+/*
+ * @path: Must call put_path(@path) after the call if it succeeded.
+ */
+static int get_path_from_fd(const u64 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));
+
+	/* Checks 32-bits overflow.  fdget_raw() checks for INT_MAX/FD. */
+	if (fd > U32_MAX)
+		return -EINVAL;
+
+	/* Handles O_PATH. */
+	f = fdget_raw(fd);
+	if (!f.file)
+		return -EBADF;
+
+	/*
+	 * Only allows O_PATH FD: enables to restrict ambient (FS) accesses
+	 * without requiring to open and risk leaking or misusing a FD.  Accept
+	 * removed, but still open directory (S_DEAD).
+	 */
+	if (!(f.file->f_mode & FMODE_PATH) ||
+			!is_user_mountable(f.file->f_path.dentry)) {
+		err = -EBADR;
+		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;
+}
+
+static int syscall_add_rule_path_beneath(const size_t attr_size,
+		const void __user *const attr_ptr)
+{
+	struct landlock_attr_path_beneath attr_path_beneath;
+	struct path path;
+	struct landlock_ruleset *ruleset;
+	int err;
+
+	/* 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),
+			attr_ptr, attr_size);
+	if (err)
+		return err;
+
+	/* Gets and checks the ruleset. */
+	ruleset = get_ruleset_from_fd(attr_path_beneath.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 */
+
+static int syscall_enforce_ruleset(const size_t attr_size,
+		const void __user *const attr_ptr)
+{
+	struct landlock_ruleset *new_dom, *ruleset;
+	struct cred *new_cred;
+	struct landlock_cred_security *new_llcred;
+	struct landlock_attr_enforce attr_enforce;
+	int err;
+
+	/*
+	 * Enforcing a Landlock 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.  These are 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;
+	}
+
+	/* Copies raw user space buffer. */
+	err = copy_struct_if_any_from_user(&attr_enforce, sizeof(attr_enforce),
+			offsetofend(typeof(attr_enforce), ruleset_fd),
+			attr_ptr, attr_size);
+	if (err)
+		return err;
+
+	/* Gets and checks the ruleset. */
+	ruleset = get_ruleset_from_fd(attr_enforce.ruleset_fd, FMODE_CAN_READ);
+	if (IS_ERR(ruleset))
+		return PTR_ERR(ruleset);
+
+	/* Informs about useless ruleset. */
+	if (!atomic_read(&ruleset->nb_rules)) {
+		err = -ENOMSG;
+		goto out_put_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;
+}
+
+/**
+ * landlock - System call to enable a process to safely sandbox itself
+ *
+ * @command: Landlock command to perform miscellaneous, but safe, actions. Cf.
+ *           `Commands`_.
+ * @options: Bitmask of options dedicated to one command. Cf. `Options`_.
+ * @attr1_size: First attribute size (i.e. size of the struct).
+ * @attr1_ptr: Pointer to the first attribute. Cf. `Attributes`_.
+ * @attr2_size: Unused for now.
+ * @attr2_ptr: Unused for now.
+ *
+ * The @command and @options arguments enable a seccomp-bpf policy to control
+ * the requested actions.  However, it should be noted that Landlock is
+ * designed from the ground to enable unprivileged process to drop privileges
+ * and accesses in a way that can not harm other processes.  This syscall and
+ * all its arguments should then be allowed for any process, which will then
+ * enable applications to strengthen the security of the whole system.
+ *
+ * @attr2_size and @attr2_ptr describe a second attribute which could be used
+ * in the future to compose with the first attribute (e.g. a
+ * landlock_attr_path_beneath with a landlock_attr_ioctl).
+ *
+ * The order of return errors begins with ENOPKG (disabled Landlock),
+ * EOPNOTSUPP (unknown command or option) and then EINVAL (invalid attribute).
+ * The other error codes may be specific to each command.
+ */
+SYSCALL_DEFINE6(landlock, const unsigned int, command,
+		const unsigned int, options,
+		const size_t, attr1_size, void __user *const, attr1_ptr,
+		const size_t, attr2_size, void __user *const, attr2_ptr)
+{
+	/*
+	 * Enables user space to identify if Landlock is disabled, thanks to a
+	 * specific error code.
+	 */
+	if (!landlock_initialized)
+		return -ENOPKG;
+
+	switch ((enum landlock_cmd)command) {
+	case LANDLOCK_CMD_GET_FEATURES:
+		if (options == LANDLOCK_OPT_GET_FEATURES) {
+			if (attr2_size || attr2_ptr)
+				return -EINVAL;
+			return syscall_get_features(attr1_size, attr1_ptr);
+		}
+		return -EOPNOTSUPP;
+
+	case LANDLOCK_CMD_CREATE_RULESET:
+		if (options == LANDLOCK_OPT_CREATE_RULESET) {
+			if (attr2_size || attr2_ptr)
+				return -EINVAL;
+			return syscall_create_ruleset(attr1_size, attr1_ptr);
+		}
+		return -EOPNOTSUPP;
+
+	case LANDLOCK_CMD_ADD_RULE:
+		/*
+		 * A future extension could add a
+		 * LANDLOCK_OPT_ADD_RULE_PATH_RANGE.
+		 */
+		if (options == LANDLOCK_OPT_ADD_RULE_PATH_BENEATH) {
+			if (attr2_size || attr2_ptr)
+				return -EINVAL;
+			return syscall_add_rule_path_beneath(attr1_size,
+					attr1_ptr);
+		}
+		return -EOPNOTSUPP;
+
+	case LANDLOCK_CMD_ENFORCE_RULESET:
+		if (options == LANDLOCK_OPT_ENFORCE_RULESET) {
+			if (attr2_size || attr2_ptr)
+				return -EINVAL;
+			return syscall_enforce_ruleset(attr1_size, attr1_ptr);
+		}
+		return -EOPNOTSUPP;
+	}
+	return -EOPNOTSUPP;
+}
-- 
2.26.0.rc2