Re: [PATCH v26 08/12] landlock: Add syscall implementations

[Mickaël Salaün]

Arnd, could you please Acked-by for the syscall signatures (which
haven't changed since) we agreed on?


On 09/12/2020 20:28, Mickaël Salaün wrote:
> From: Mickaël Salaün <mic@xxxxxxxxxxxxxxxxxxx>
> 
> 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_enforce_ruleset_current(2): Enforces a ruleset on the current
>   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@xxxxxxxx>
> Cc: James Morris <jmorris@xxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Serge E. Hallyn <serge@xxxxxxxxxx>
> Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxxxxxxxxxx>
> Reviewed-by: Jann Horn <jannh@xxxxxxxxxx>
> ---
> 
> Changes since v25:
> * Revert build_check_abi() as non-inline to trigger a warning if it is
>   not called.
> * Use the new limit names.
> 
> Changes since v24:
> * Add Reviewed-by: Jann Horn <jannh@xxxxxxxxxx>
> * Set build_check_abi() as inline.
> 
> Changes since v23:
> * Rewrite get_ruleset_from_fd() to please the 0-DAY CI Kernel Test
>   Service that reported an uninitialized variable (false positive):
>   https://lore.kernel.org/linux-security-module/202011101854.zGbWwusK-lkp@xxxxxxxxx/
>   Anyway, it is cleaner like this.
> * Add a comment about E2BIG which can be returned by
>   landlock_enforce_ruleset_current(2) when there is no more room for
>   another stacked ruleset (i.e. domain).
> 
> Changes since v22:
> * Replace security_capable() with ns_capable_noaudit() (suggested by
>   Jann Horn) and explicitly return EPERM.
> * Fix landlock_enforce_ruleset_current(2)'s out_put_creds (spotted by
>   Jann Horn).
> * Add __always_inline to copy_min_struct_from_user() to make its
>   BUILD_BUG_ON() checks reliable (suggested by Jann Horn).
> * Simplify path assignation in get_path_from_fd() (suggested by Jann
>   Horn).
> * Fix spelling (spotted by Jann Horn).
> 
> Changes since v21:
> * Fix and improve comments.
> 
> Changes since v20:
> * Remove two arguments to landlock_enforce_ruleset(2) (requested by Arnd
>   Bergmann) and rename it to landlock_enforce_ruleset_current(2): remove
>   the enum landlock_target_type and the target file descriptor (not used
>   for now).  A ruleset can only be enforced on the current thread.
> * Remove the size argument in landlock_add_rule() (requested by Arnd
>   Bergmann).
> * Remove landlock_get_features(2) (suggested by Arnd Bergmann).
> * Simplify and rename copy_struct_if_any_from_user() to
>   copy_min_struct_from_user().
> * Rename "options" to "flags" to allign with current syscalls.
> * Rename some types and variables in a more consistent way.
> * Fix missing type declarations in syscalls.h .
> 
> Changes since v19:
> * Replace the landlock(2) syscall with 4 syscalls (one for each
>   command): landlock_get_features(2), landlock_create_ruleset(2),
>   landlock_add_rule(2) and landlock_enforce_ruleset(2) (suggested by
>   Arnd Bergmann).
>   https://lore.kernel.org/lkml/56d15841-e2c1-2d58-59b8-3a6a09b23b4a@xxxxxxxxxxx/
> * Return EOPNOTSUPP (instead of ENOPKG) when Landlock is disabled.
> * Add two new fields to landlock_attr_features to fit with the new
>   syscalls: last_rule_type and last_target_type.  This enable to easily
>   identify which types are supported.
> * Pack landlock_attr_path_beneath struct because of the removed
>   ruleset_fd.
> * Update documentation and fix spelling.
> 
> Changes since v18:
> * Remove useless include.
> * Remove LLATTR_SIZE() which was only used to shorten lines. Cf. commit
>   bdc48fa11e46 ("checkpatch/coding-style: deprecate 80-column warning").
> 
> Changes since v17:
> * Synchronize syscall declaration.
> * Fix comment.
> 
> Changes since v16:
> * Add a size_attr_features field to struct landlock_attr_features for
>   self-introspection, and move the access_fs field to be more
>   consistent.
> * Replace __aligned_u64 types of attribute fields with __u16, __s32,
>   __u32 and __u64, and check at build time that these structures does
>   not contain hole and that they are aligned the same way (8-bits) on
>   all architectures.  This shrinks the size of the userspace ABI, which
>   may be appreciated especially for struct landlock_attr_features which
>   could grow a lot in the future.  For instance, struct
>   landlock_attr_features shrinks from 72 bytes to 32 bytes.  This change
>   also enables to remove 64-bits to 32-bits conversion checks.
> * Switch syscall attribute pointer and size arguments to follow similar
>   syscall argument order (e.g. bpf, clone3, openat2).
> * Set LANDLOCK_OPT_* types to 32-bits.
> * Allow enforcement of empty ruleset, which enables deny-all policies.
> * Fix documentation inconsistency.
> 
> Changes since v15:
> * Do not add file descriptors referring to internal filesystems (e.g.
>   nsfs) in a ruleset.
> * Replace is_user_mountable() with in-place clean checks.
> * Replace EBADR with EBADFD in get_ruleset_from_fd() and
>   get_path_from_fd().
> * Remove ruleset's show_fdinfo() for now.
> 
> 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      |   7 +
>  include/uapi/linux/landlock.h |  53 +++++
>  kernel/sys_ni.c               |   5 +
>  security/landlock/Makefile    |   2 +-
>  security/landlock/syscall.c   | 427 ++++++++++++++++++++++++++++++++++
>  5 files changed, 493 insertions(+), 1 deletion(-)
>  create mode 100644 security/landlock/syscall.c
> 
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 37bea07c12f2..ee3e24095c5f 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -68,6 +68,8 @@ union bpf_attr;
>  struct io_uring_params;
>  struct clone_args;
>  struct open_how;
> +struct landlock_ruleset_attr;
> +enum landlock_rule_type;
>  
>  #include <linux/types.h>
>  #include <linux/aio_abi.h>
> @@ -1008,6 +1010,11 @@ 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_create_ruleset(const struct landlock_ruleset_attr __user *attr,
> +		size_t size, __u32 flags);
> +asmlinkage long sys_landlock_add_rule(int ruleset_fd, enum landlock_rule_type rule_type,
> +		const void __user *rule_attr, __u32 flags);
> +asmlinkage long sys_landlock_enforce_ruleset_current(int ruleset_fd, __u32 flags);
>  
>  /*
>   * Architecture-specific system calls
> diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h
> index d547bd49fe38..1a2299fa958a 100644
> --- a/include/uapi/linux/landlock.h
> +++ b/include/uapi/linux/landlock.h
> @@ -9,6 +9,59 @@
>  #ifndef _UAPI__LINUX_LANDLOCK_H__
>  #define _UAPI__LINUX_LANDLOCK_H__
>  
> +#include <linux/types.h>
> +
> +/**
> + * struct landlock_ruleset_attr - Ruleset definition
> + *
> + * Argument of sys_landlock_create_ruleset().  This structure can grow in
> + * future versions.
> + */
> +struct landlock_ruleset_attr {
> +	/**
> +	 * @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.
> +	 */
> +	__u64 handled_access_fs;
> +};
> +
> +/**
> + * 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_path_beneath_attr .
> +	 */
> +	LANDLOCK_RULE_PATH_BENEATH = 1,
> +};
> +
> +/**
> + * struct landlock_path_beneath_attr - Path hierarchy definition
> + *
> + * Argument of sys_landlock_add_rule().
> + */
> +struct landlock_path_beneath_attr {
> +	/**
> +	 * @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 identifies
> +	 * the parent directory of a file hierarchy, or just a file.
> +	 */
> +	__s32 parent_fd;
> +	/*
> +	 * This struct is packed to avoid trailing 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 f27ac94d5fa7..0906cb79e801 100644
> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -264,6 +264,11 @@ COND_SYSCALL(request_key);
>  COND_SYSCALL(keyctl);
>  COND_SYSCALL_COMPAT(keyctl);
>  
> +/* security/landlock/syscall.c */
> +COND_SYSCALL(landlock_create_ruleset);
> +COND_SYSCALL(landlock_add_rule);
> +COND_SYSCALL(landlock_enforce_ruleset_current);
> +
>  /* 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..a21508fbbc7a
> --- /dev/null
> +++ b/security/landlock/syscall.c
> @@ -0,0 +1,427 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Landlock LSM - System call implementations and user space interfaces
> + *
> + * Copyright © 2016-2020 Mickaël Salaün <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/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 "limits.h"
> +#include "ruleset.h"
> +#include "setup.h"
> +
> +/**
> + * copy_min_struct_from_user - Safe future-proof argument copying
> + *
> + * Extend copy_struct_from_user() to check for consistent user buffer.
> + *
> + * @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 __always_inline int copy_min_struct_from_user(void *const dst,
> +		const size_t ksize, const size_t ksize_min,
> +		const void __user *const src, const size_t usize)
> +{
> +	/* Checks buffer inconsistencies. */
> +	BUILD_BUG_ON(!dst);
> +	if (!src)
> +		return -EFAULT;
> +
> +	/* Checks size ranges. */
> +	BUILD_BUG_ON(ksize <= 0);
> +	BUILD_BUG_ON(ksize < ksize_min);
> +	if (usize < ksize_min)
> +		return -EINVAL;
> +	if (usize > PAGE_SIZE)
> +		return -E2BIG;
> +
> +	/* Copies user buffer and fills with zeros. */
> +	return copy_struct_from_user(dst, ksize, src, usize);
> +}
> +
> +/*
> + * 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 ruleset_size, path_beneath_size;
> +
> +	/*
> +	 * 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.
> +	 */
> +	ruleset_size = sizeof_field(struct landlock_ruleset_attr, handled_access_fs);
> +	BUILD_BUG_ON(sizeof(struct landlock_ruleset_attr) != ruleset_size);
> +	BUILD_BUG_ON(sizeof(struct landlock_ruleset_attr) != 8);
> +
> +	path_beneath_size = sizeof_field(struct landlock_path_beneath_attr, allowed_access);
> +	path_beneath_size += sizeof_field(struct landlock_path_beneath_attr, parent_fd);
> +	BUILD_BUG_ON(sizeof(struct landlock_path_beneath_attr) != path_beneath_size);
> +	BUILD_BUG_ON(sizeof(struct landlock_path_beneath_attr) != 12);
> +}
> +
> +/* 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
> + *
> + * @attr: Pointer to a &struct landlock_ruleset_attr identifying the scope of
> + *        the new ruleset.
> + * @size: Size of the pointed &struct landlock_ruleset_attr (needed for
> + *        backward and forward compatibility).
> + * @flags: 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: @flags is not 0, or unknown access, or too small @size;
> + * - E2BIG or EFAULT: @attr or @size inconsistencies;
> + * - ENOMSG: empty &landlock_ruleset_attr.handled_access_fs.
> + */
> +SYSCALL_DEFINE3(landlock_create_ruleset,
> +		const struct landlock_ruleset_attr __user *const, attr,
> +		const size_t, size, const __u32, flags)
> +{
> +	struct landlock_ruleset_attr ruleset_attr;
> +	struct landlock_ruleset *ruleset;
> +	int err, ruleset_fd;
> +
> +	/* Build-time checks. */
> +	build_check_abi();
> +
> +	if (!landlock_initialized)
> +		return -EOPNOTSUPP;
> +
> +	/* No flag for now. */
> +	if (flags)
> +		return -EINVAL;
> +
> +	/* Copies raw user space buffer. */
> +	err = copy_min_struct_from_user(&ruleset_attr, sizeof(ruleset_attr),
> +			offsetofend(typeof(ruleset_attr), handled_access_fs),
> +			attr, size);
> +	if (err)
> +		return err;
> +
> +	/* Checks content (and 32-bits cast). */
> +	if ((ruleset_attr.handled_access_fs | LANDLOCK_MASK_ACCESS_FS) !=
> +			LANDLOCK_MASK_ACCESS_FS)
> +		return -EINVAL;
> +
> +	/* Checks arguments and transforms to kernel struct. */
> +	ruleset = landlock_create_ruleset(ruleset_attr.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;
> +
> +	ruleset_f = fdget(fd);
> +	if (!ruleset_f.file)
> +		return ERR_PTR(-EBADF);
> +
> +	/* Checks FD type and access right. */
> +	if (ruleset_f.file->f_op != &ruleset_fops) {
> +		ruleset = ERR_PTR(-EBADFD);
> +		goto out_fdput;
> +	}
> +	if (!(ruleset_f.file->f_mode & mode)) {
> +		ruleset = ERR_PTR(-EPERM);
> +		goto out_fdput;
> +	}
> +	ruleset = ruleset_f.file->private_data;
> +	landlock_get_ruleset(ruleset);
> +
> +out_fdput:
> +	fdput(ruleset_f);
> +	return 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_path_beneath_attr *)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 = f.file->f_path;
> +	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_attr (only
> + *             LANDLOCK_RULE_PATH_BENEATH for now).
> + * @rule_attr: Pointer to a rule (only of type &struct
> + *             landlock_path_beneath_attr for now).
> + * @flags: 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: @flags is not 0, or inconsistent access in the rule (i.e.
> + *   &landlock_path_beneath_attr.allowed_access is not a subset of the rule's
> + *   accesses);
> + * - 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;
> + * - EFAULT: @rule_attr inconsistency.
> + */
> +SYSCALL_DEFINE4(landlock_add_rule,
> +		const int, ruleset_fd, const enum landlock_rule_type, rule_type,
> +		const void __user *const, rule_attr, const __u32, flags)
> +{
> +	struct landlock_path_beneath_attr path_beneath_attr;
> +	struct path path;
> +	struct landlock_ruleset *ruleset;
> +	int res, err;
> +
> +	if (!landlock_initialized)
> +		return -EOPNOTSUPP;
> +
> +	/* No flag for now. */
> +	if (flags)
> +		return -EINVAL;
> +
> +	if (rule_type != LANDLOCK_RULE_PATH_BENEATH)
> +		return -EINVAL;
> +
> +	/* Copies raw user space buffer, only one type for now. */
> +	res = copy_from_user(&path_beneath_attr, rule_attr,
> +			sizeof(path_beneath_attr));
> +	if (res)
> +		return -EFAULT;
> +
> +	/* 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 ((path_beneath_attr.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(path_beneath_attr.parent_fd, &path);
> +	if (err)
> +		goto out_put_ruleset;
> +
> +	/* Imports the new rule. */
> +	err = landlock_append_fs_rule(ruleset, &path,
> +			path_beneath_attr.allowed_access);
> +	path_put(&path);
> +
> +out_put_ruleset:
> +	landlock_put_ruleset(ruleset);
> +	return err;
> +}
> +
> +/* Enforcement */
> +
> +/**
> + * sys_landlock_enforce_ruleset_current - Enforce a ruleset on the current task
> + *
> + * @ruleset_fd: File descriptor tied to the ruleset to merge with the target.
> + * @flags: 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 is 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: @flags is not 0.
> + * - 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 it doesn't have
> + *   CAP_SYS_ADMIN in its namespace.
> + * - E2BIG: The maximum number of stacked rulesets is reached for the current
> + *   task.
> + */
> +SYSCALL_DEFINE2(landlock_enforce_ruleset_current,
> +		const int, ruleset_fd, const __u32, flags)
> +{
> +	struct landlock_ruleset *new_dom, *ruleset;
> +	struct cred *new_cred;
> +	struct landlock_cred_security *new_llcred;
> +	int err;
> +
> +	if (!landlock_initialized)
> +		return -EOPNOTSUPP;
> +
> +	/* No flag for now. */
> +	if (flags)
> +		return -EINVAL;
> +
> +	/*
> +	 * Similar checks as for seccomp(2), except that an -EPERM may be
> +	 * returned.
> +	 */
> +	if (!task_no_new_privs(current) &&
> +			!ns_capable_noaudit(current_user_ns(), CAP_SYS_ADMIN))
> +		return -EPERM;
> +
> +	/* 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);
> +
> +out_put_ruleset:
> +	landlock_put_ruleset(ruleset);
> +	return err;
> +}
>