On Tue, Mar 16, 2021 at 09:42:48PM +0100, Mickaël Salaün wrote:

From: Mickaël Salaün <redacted>

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.

For the new-style extensible syscalls, you want only a "size" argument;
"flags" should be within the argument structure.

(And to this end, why 3 syscalls instead of 1, if you're going to use
extensibility anyway?)

+/**
+ * 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);
+}

I still wish this was built into copy_struct_from_user(). :) But yes,
this appears to be the way to protect one's self when using
copy_struct_from_user().

+/**
+ * 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),

The use of offsetofend() here appears to be kind of the "V1", "V2", ...
sizes used in other extensible syscall implementations?

+			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;
+	if (WARN_ON_ONCE(ruleset->num_layers != 1)) {
+		ruleset = ERR_PTR(-EINVAL);
+		goto out_fdput;
+	}
+	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.  Forbids ruleset FDs, 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_op == &ruleset_fops) ||
+			(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 that 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);
+ * - ENOMSG: Empty accesses (e.g. &landlock_path_beneath_attr.allowed_access);
+ * - EBADF: @ruleset_fd is not a file descriptor for the current thread, or a
+ *   member of @rule_attr is not a file descriptor as expected;
+ * - EBADFD: @ruleset_fd is not a ruleset file descriptor, or a member of
+ *   @rule_attr is not the expected file descriptor type (e.g. file open
+ *   without O_PATH);
+ * - 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)
+{

If this is an extensible syscall, I'd expect a structure holding
rule_type, rule_attr, and 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);
+
+	/*
+	 * Informs about useless rule: empty allowed_access (i.e. deny rules)
+	 * are ignored in path walks.
+	 */
+	if (!path_beneath_attr.allowed_access) {
+		err = -ENOMSG;
+		goto out_put_ruleset;
+	}
+	/*
+	 * Checks that allowed_access matches the @ruleset constraints
+	 * (ruleset->fs_access_masks[0] is automatically upgraded to 64-bits).
+	 */
+	if ((path_beneath_attr.allowed_access | ruleset->fs_access_masks[0]) !=
+			ruleset->fs_access_masks[0]) {
+		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_restrict_self - Enforce a ruleset on the calling thread
+ *
+ * @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
+ *   thread.
+ */
+SYSCALL_DEFINE2(landlock_restrict_self,
+		const int, ruleset_fd, const __u32, flags)
+{

Same observation about new style syscalls.

+	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;
+}
-- 
2.30.2

-- 
Kees Cook