[PATCH v17 01/10] landlock: Add object management

[Mickaël Salaün]

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 can't 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 object.  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.

Signed-off-by: Mickaël Salaün <mic at digikod.net>
Cc: James Morris <jmorris at namei.org>
Cc: Jann Horn <jannh at google.com>
Cc: Kees Cook <keescook at chromium.org>
Cc: Serge E. Hallyn <serge at hallyn.com>
---

Changes since v14:
* Simplify the object, rule and ruleset management at the expense of a
  less aggressive memory freeing (contributed by Jann Horn, with
  additional modifications):
  - Remove object->list aggregating the rules tied to an object.
  - Remove landlock_get_object(), landlock_drop_object(),
    {get,put}_object_cleaner() and landlock_rule_is_disabled().
  - Rewrite landlock_put_object() to use a more simple mechanism
    (no tricky RCU).
  - Replace enum landlock_object_type and landlock_release_object() with
    landlock_object_underops->release()
  - Adjust unions and Sparse annotations.
  Cf. https://lore.kernel.org/lkml/CAG48ez21bEn0wL1bbmTiiu8j9jP5iEWtHOwz4tURUJ+ki0ydYw@mail.gmail.com/
* Merge struct landlock_rule into landlock_ruleset_elem to simplify the
  rule management.
* Constify variables.
* Improve kernel documentation.
* Cosmetic variable renames.
* Remove the "default" in the Kconfig (suggested by Jann Horn).
* Only use refcount_inc() through getter helpers.
* Update Kconfig description.

Changes since v13:
* New dedicated implementation, removing the need for eBPF.

Previous changes:
https://lore.kernel.org/lkml/20190721213116.23476-6-mic@digikod.net/
---
 MAINTAINERS                | 10 +++++
 security/Kconfig           |  1 +
 security/Makefile          |  2 +
 security/landlock/Kconfig  | 18 ++++++++
 security/landlock/Makefile |  3 ++
 security/landlock/object.c | 66 +++++++++++++++++++++++++++
 security/landlock/object.h | 91 ++++++++++++++++++++++++++++++++++++++
 7 files changed, 191 insertions(+)
 create mode 100644 security/landlock/Kconfig
 create mode 100644 security/landlock/Makefile
 create mode 100644 security/landlock/object.c
 create mode 100644 security/landlock/object.h

diff --git a/MAINTAINERS b/MAINTAINERS
index 091ec22c1a23..011349c9c6c6 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -9487,6 +9487,16 @@ F:	net/core/sock_map.c
 F:	net/ipv4/tcp_bpf.c
 F:	net/ipv4/udp_bpf.c
 
+LANDLOCK SECURITY MODULE
+M:	Mickaël Salaün <mic at digikod.net>
+L:	linux-security-module at vger.kernel.org
+S:	Supported
+W:	https://landlock.io
+T:	git https://github.com/landlock-lsm/linux.git
+F:	security/landlock/
+K:	landlock
+K:	LANDLOCK
+
 LANTIQ / INTEL Ethernet drivers
 M:	Hauke Mehrtens <hauke at hauke-m.de>
 L:	netdev at vger.kernel.org
diff --git a/security/Kconfig b/security/Kconfig
index cd3cc7da3a55..582fd777a757 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -238,6 +238,7 @@ source "security/loadpin/Kconfig"
 source "security/yama/Kconfig"
 source "security/safesetid/Kconfig"
 source "security/lockdown/Kconfig"
+source "security/landlock/Kconfig"
 
 source "security/integrity/Kconfig"
 
diff --git a/security/Makefile b/security/Makefile
index 22e73a3482bd..b8380650bfb1 100644
--- a/security/Makefile
+++ b/security/Makefile
@@ -13,6 +13,7 @@ subdir-$(CONFIG_SECURITY_LOADPIN)	+= loadpin
 subdir-$(CONFIG_SECURITY_SAFESETID)    += safesetid
 subdir-$(CONFIG_SECURITY_LOCKDOWN_LSM)	+= lockdown
 subdir-$(CONFIG_BPF_LSM)		+= bpf
+subdir-$(CONFIG_SECURITY_LANDLOCK)		+= landlock
 
 # always enable default capabilities
 obj-y					+= commoncap.o
@@ -30,6 +31,7 @@ obj-$(CONFIG_SECURITY_YAMA)		+= yama/
 obj-$(CONFIG_SECURITY_LOADPIN)		+= loadpin/
 obj-$(CONFIG_SECURITY_SAFESETID)       += safesetid/
 obj-$(CONFIG_SECURITY_LOCKDOWN_LSM)	+= lockdown/
+obj-$(CONFIG_SECURITY_LANDLOCK)	+= landlock/
 obj-$(CONFIG_CGROUP_DEVICE)		+= device_cgroup.o
 obj-$(CONFIG_BPF_LSM)			+= bpf/
 
diff --git a/security/landlock/Kconfig b/security/landlock/Kconfig
new file mode 100644
index 000000000000..9ec7593a534a
--- /dev/null
+++ b/security/landlock/Kconfig
@@ -0,0 +1,18 @@
+# SPDX-License-Identifier: GPL-2.0-only
+
+config SECURITY_LANDLOCK
+	bool "Landlock support"
+	depends on SECURITY
+	select SECURITY_PATH
+	help
+	  Landlock is a safe sandboxing mechanism which enables processes to
+	  restrict themselves (and their future children) by gradually
+	  enforcing tailored access control policies.  A security policy is a
+	  set of access rights (e.g. open a file in read-only, make a
+	  directory) tied to a file hierarchy.  The configuration can be set by
+	  any processes, including unprivileged ones, thanks to the landlock()
+	  system call.
+
+	  See Documentation/security/landlock/ for further information.
+
+	  If you are unsure how to answer this question, answer N.
diff --git a/security/landlock/Makefile b/security/landlock/Makefile
new file mode 100644
index 000000000000..cb6deefbf4c0
--- /dev/null
+++ b/security/landlock/Makefile
@@ -0,0 +1,3 @@
+obj-$(CONFIG_SECURITY_LANDLOCK) := landlock.o
+
+landlock-y := object.o
diff --git a/security/landlock/object.c b/security/landlock/object.c
new file mode 100644
index 000000000000..27da1ed45eaf
--- /dev/null
+++ b/security/landlock/object.c
@@ -0,0 +1,66 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Landlock LSM - Object management
+ *
+ * Copyright © 2016-2020 Mickaël Salaün <mic at digikod.net>
+ * Copyright © 2018-2020 ANSSI
+ */
+
+#include <linux/bug.h>
+#include <linux/compiler_types.h>
+#include <linux/kernel.h>
+#include <linux/rcupdate.h>
+#include <linux/refcount.h>
+#include <linux/slab.h>
+#include <linux/spinlock.h>
+
+#include "object.h"
+
+struct landlock_object *landlock_create_object(
+		const struct landlock_object_underops *underops,
+		void *const underobj)
+{
+	struct landlock_object *new_object;
+
+	if (WARN_ON_ONCE(!underops || !underobj))
+		return NULL;
+	new_object = kzalloc(sizeof(*new_object), GFP_KERNEL);
+	if (!new_object)
+		return NULL;
+	refcount_set(&new_object->usage, 1);
+	spin_lock_init(&new_object->lock);
+	new_object->underops = underops;
+	new_object->underobj = underobj;
+	return new_object;
+}
+
+/*
+ * The caller must own the object (i.e. thanks to object->usage) to safely put
+ * it.
+ */
+void landlock_put_object(struct landlock_object *const object)
+{
+	/*
+	 * The call to @object->underops->release(object) might sleep e.g.,
+	 * because of iput().
+	 */
+	might_sleep();
+	if (!object)
+		return;
+
+	/*
+	 * If the @object's refcount can't drop to zero, we can just decrement
+	 * the refcount without holding a lock. Otherwise, the decrement must
+	 * happen under @object->lock for synchronization with things like
+	 * get_inode_object().
+	 */
+	if (refcount_dec_and_lock(&object->usage, &object->lock)) {
+		__acquire(&object->lock);
+		/*
+		 * With @object->lock initially held, remove the reference from
+		 * @object->underobj to @object (if it still exists).
+		 */
+		object->underops->release(object);
+		kfree_rcu(object, rcu_free);
+	}
+}
diff --git a/security/landlock/object.h b/security/landlock/object.h
new file mode 100644
index 000000000000..942bc0e18064
--- /dev/null
+++ b/security/landlock/object.h
@@ -0,0 +1,91 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Landlock LSM - Object management
+ *
+ * Copyright © 2016-2020 Mickaël Salaün <mic at digikod.net>
+ * Copyright © 2018-2020 ANSSI
+ */
+
+#ifndef _SECURITY_LANDLOCK_OBJECT_H
+#define _SECURITY_LANDLOCK_OBJECT_H
+
+#include <linux/compiler_types.h>
+#include <linux/refcount.h>
+#include <linux/spinlock.h>
+
+struct landlock_object;
+
+/**
+ * struct landlock_object_underops - Operations on an underlying object
+ */
+struct landlock_object_underops {
+	/**
+	 * @release: Releases the underlying object (e.g. iput() for an inode).
+	 */
+	void (*release)(struct landlock_object *const object)
+		__releases(object->lock);
+};
+
+/**
+ * struct landlock_object - Security blob tied to a kernel object
+ *
+ * The goal of this structure is to enable to tie a set of ephemeral access
+ * rights (pertaining to different domains) to a kernel object (e.g an inode)
+ * in a safe way.  This imply to handle concurrent use and modification.
+ *
+ * The lifetime of a &struct landlock_object depends of the rules referring to
+ * it.
+ */
+struct landlock_object {
+	/**
+	 * @usage: This counter is used to tie an object to the rules matching
+	 * it or to keep it alive while adding a new rule.  If this counter
+	 * reaches zero, this struct must not be modified, but this counter can
+	 * still be read from within an RCU read-side critical section.  When
+	 * adding a new rule to an object with a usage counter of zero, we must
+	 * wait until the pointer to this object is set to NULL (or recycled).
+	 */
+	refcount_t usage;
+	/**
+	 * @lock: Guards against concurrent modifications.  This lock must be
+	 * from the time @usage drops to zero until any weak references from
+	 * @underobj to this object have been cleaned up.
+	 *
+	 * Lock ordering: inode->i_lock nests inside this.
+	 */
+	spinlock_t lock;
+	/**
+	 * @underobj: Used when cleaning up an object and to mark an object as
+	 * tied to its underlying kernel structure.  This pointer is protected
+	 * by @lock.  Cf. landlock_release_inodes() and release_inode().
+	 */
+	void *underobj;
+	union {
+		/**
+		 * @rcu_free: Enables lockless use of @usage, @lock and
+		 * @underobj from within an RCU read-side critical section.
+		 * @rcu_free and @underops are only used by
+		 * landlock_put_object().
+		 */
+		struct rcu_head rcu_free;
+		/**
+		 * @underops: Enables landlock_put_object() to release the
+		 * underlying object (e.g. inode).
+		 */
+		const struct landlock_object_underops *underops;
+	};
+};
+
+struct landlock_object *landlock_create_object(
+		const struct landlock_object_underops *const underops,
+		void *const underojb);
+
+void landlock_put_object(struct landlock_object *const object);
+
+static inline void landlock_get_object(struct landlock_object *const object)
+{
+	if (object)
+		refcount_inc(&object->usage);
+}
+
+#endif /* _SECURITY_LANDLOCK_OBJECT_H */
-- 
2.26.2