[PATCH v18 02/12] landlock: Add ruleset and domain management

Mickaël Salaün
2020-05-26T20:54:07Z

A Landlock ruleset is mainly a red-black tree with Landlock rules as
nodes. This enables quick update and lookup to match a requested access
e.g., to a file. A ruleset is usable through a dedicated file
descriptor (cf. following commit implementing the syscall) which enables
a process to create and populate a ruleset with new rules.

A domain is a ruleset tied to a set of processes. This group of rules
define the security policy enforced on these processes and their future
children. A domain can transition to a new domain which is the
intersection of all its constraints and those of a ruleset provided by
the current process. This modification only impact the current process.
This means that a process can only gain more constraints (i.e. lose
accesses) over time.

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

Changes since v17:
* Move include/uapi/linux/landlock.h and _LANDLOCK_ACCESS_FS_* to a
following patch.

Changes since v16:
* Allow enforcement of empty ruleset, which enables deny-all policies.

Changes since v15:
* Replace layer_levels and layer_depth with a bitfield of layers, cf.
filesystem commit.
* Rename the LANDLOCK_ACCESS_FS_{UNLINK,RMDIR} with
LANDLOCK_ACCESS_FS_REMOVE_{FILE,DIR} because it makes sense to use
them for the action of renaming a file or a directory, which may lead
to the removal of the source file or directory. Removes the
LANDLOCK_ACCESS_FS_{LINK_TO,RENAME_FROM,RENAME_TO} which are now
replaced with LANDLOCK_ACCESS_FS_REMOVE_{FILE,DIR} and
LANDLOCK_ACCESS_FS_MAKE_* .
* Update the documentation accordingly and highlight how the access
rights are taken into account.
* Change nb_rules from atomic_t to u32 because it is not use anymore by
show_fdinfo().
* Add safeguard for level variables types.
* Check max number of rules.
* Replace struct landlock_access (self and beneath bitfields) with one
bitfield.
* Remove useless variable.
* Add comments.

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):
- Make a domain immutable (remove the opportunistic cleaning).
- Remove RCU pointers.
- Merge struct landlock_ref and struct landlock_ruleset_elem into
landlock_rule: get ride of rule's RCU.
- Adjust union.
- Remove the landlock_insert_rule() check about a new object with the
same address as a previously disabled one, because it is not
possible to disable a rule anymore.
Cf. https://lore.kernel.org/lkml/CAG48ez21bEn0wL1bbmTiiu8j9jP5iEWtHOwz4tURUJ+ki0ydYw@mail.gmail.com/
* Fix nested domains by implementing a notion of layer level and depth:
- Update landlock_insert_rule() to manage such layers.
- Add an inherit_ruleset() helper to properly create a new domain.
- Rename landlock_find_access() to landlock_find_rule() and return a
full rule reference.
- Add a layer_level and a layer_depth fields to struct landlock_rule.
- Add a top_layer_level field to struct landlock_ruleset.
* Remove access rights that may be required for FD-only requests:
truncate, getattr, lock, chmod, chown, chgrp, ioctl. This will be
handle in a future evolution of Landlock, but right now the goal is to
lighten the code to ease review.
* Remove LANDLOCK_ACCESS_FS_OPEN and rename
LANDLOCK_ACCESS_FS_{READ,WRITE} with a FILE suffix.
* Rename LANDLOCK_ACCESS_FS_READDIR to match the *_FILE pattern.
* Remove LANDLOCK_ACCESS_FS_MAP which was useless.
* Fix memory leak in put_hierarchy() (reported by Jann Horn).
* Fix user-after-free and rename free_ruleset() (reported by Jann Horn).
* Replace the for loops with rbtree_postorder_for_each_entry_safe().
* Constify variables.
* Only use refcount_inc() through getter helpers.
* Change Landlock_insert_ruleset_access() to
Landlock_insert_ruleset_rule().
* Rename landlock_put_ruleset_enqueue() to landlock_put_ruleset_deferred().
* Improve kernel documentation and add a warning about the unhandled
access/syscall families.
* Move ABI check to syscall.c .

Changes since v13:
* New implementation, inspired by the previous inode eBPF map, but
agnostic to the underlying kernel object.

Previous changes:
https://lore.kernel.org/lkml/20190721213116.23476-7-mic@digikod.net/
---
MAINTAINERS | 1 +
security/landlock/Makefile | 2 +-
security/landlock/ruleset.c | 342 ++++++++++++++++++++++++++++++++++++
security/landlock/ruleset.h | 157 +++++++++++++++++
4 files changed, 501 insertions(+), 1 deletion(-)
create mode 100644 security/landlock/ruleset.c
create mode 100644 security/landlock/ruleset.h

diff --git a/MAINTAINERS b/MAINTAINERS
index 5bf93663aad6..c0e1593b2c2b 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -9498,6 +9498,7 @@ L: linux-security-module@vger.kernel.org
S: Supported
W: https://landlock.io
T: git https://github.com/landlock-lsm/linux.git
+F: include/uapi/linux/landlock.h
F: security/landlock/
K: landlock
K: LANDLOCK
diff --git a/security/landlock/Makefile b/security/landlock/Makefile
index cb6deefbf4c0..d846eba445bb 100644
--- a/security/landlock/Makefile
+++ b/security/landlock/Makefile
@@ -1,3 +1,3 @@
obj-$(CONFIG_SECURITY_LANDLOCK) := landlock.o

-landlock-y := object.o
+landlock-y := object.o ruleset.o
diff --git a/security/landlock/ruleset.c b/security/landlock/ruleset.c
new file mode 100644
index 000000000000..9bd474291d8f
--- /dev/null
+++ b/security/landlock/ruleset.c
@@ -0,0 +1,342 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Landlock LSM - Ruleset management
+ *
+ * Copyright © 2016-2020 Mickaël Salaün <mic@digikod.net>
+ * Copyright © 2018-2020 ANSSI
+ */
+
+#include <linux/bits.h>
+#include <linux/bug.h>
+#include <linux/compiler_types.h>
+#include <linux/err.h>
+#include <linux/errno.h>
+#include <linux/kernel.h>
+#include <linux/limits.h>
+#include <linux/rbtree.h>
+#include <linux/refcount.h>
+#include <linux/slab.h>
+#include <linux/spinlock.h>
+#include <linux/workqueue.h>
+
+#include "object.h"
+#include "ruleset.h"
+
+static struct landlock_ruleset *create_ruleset(void)
+{
+ struct landlock_ruleset *ruleset;
+
+ ruleset = kzalloc(sizeof(*ruleset), GFP_KERNEL);
+ if (!ruleset)
+ return ERR_PTR(-ENOMEM);
+ refcount_set(&ruleset->usage, 1);
+ mutex_init(&ruleset->lock);
+ /*
+ * root = RB_ROOT
+ * hierarchy = NULL
+ * nb_rules = 0
+ * nb_layers = 0
+ * fs_access_mask = 0
+ */
+ return ruleset;
+}
+
+struct landlock_ruleset *landlock_create_ruleset(const u32 fs_access_mask)
+{
+ struct landlock_ruleset *ruleset;
+
+ /* Informs about useless ruleset. */
+ if (!fs_access_mask)
+ return ERR_PTR(-ENOMSG);
+ ruleset = create_ruleset();
+ if (!IS_ERR(ruleset))
+ ruleset->fs_access_mask = fs_access_mask;
+ return ruleset;
+}
+
+static struct landlock_rule *duplicate_rule(struct landlock_rule *const src)
+{
+ struct landlock_rule *new_rule;
+
+ new_rule = kzalloc(sizeof(*new_rule), GFP_KERNEL);
+ if (!new_rule)
+ return ERR_PTR(-ENOMEM);
+ RB_CLEAR_NODE(&new_rule->node);
+ landlock_get_object(src->object);
+ new_rule->object = src->object;
+ new_rule->access = src->access;
+ new_rule->layers = src->layers;
+ return new_rule;
+}
+
+static void put_rule(struct landlock_rule *const rule)
+{
+ might_sleep();
+ if (!rule)
+ return;
+ landlock_put_object(rule->object);
+ kfree(rule);
+}
+
+/*
+ * Assumptions:
+ * - An inserted rule can not be removed.
+ * - The underlying kernel object must be held by the caller.
+ *
+ * @rule: Read-only payload to be inserted (not own by this function).
+ * @is_merge: If true, intersects access rights and updates the rule's layers
+ * (e.g. merge two rulesets), else do a union of access rights and keep the
+ * rule's layers (e.g. extend a ruleset)
+ */
+int landlock_insert_rule(struct landlock_ruleset *const ruleset,
+ struct landlock_rule *const rule, const bool is_merge)
+{
+ struct rb_node **walker_node;
+ struct rb_node *parent_node = NULL;
+ struct landlock_rule *new_rule;
+
+ might_sleep();
+ lockdep_assert_held(&ruleset->lock);
+ walker_node = &(ruleset->root.rb_node);
+ while (*walker_node) {
+ struct landlock_rule *const this = rb_entry(*walker_node,
+ struct landlock_rule, node);
+
+ if (this->object != rule->object) {
+ parent_node = *walker_node;
+ if (this->object < rule->object)
+ walker_node = &((*walker_node)->rb_right);
+ else
+ walker_node = &((*walker_node)->rb_left);
+ continue;
+ }
+
+ /* If there is a matching rule, updates it. */
+ if (is_merge) {
+ /* Intersects access rights. */
+ this->access &= rule->access;
+
+ /* Updates the rule layers with the next one. */
+ this->layers |= BIT_ULL(ruleset->nb_layers);
+ } else {
+ /* Extends access rights. */
+ this->access |= rule->access;
+ }
+ return 0;
+ }
+
+ /* There is no match for @rule->object. */
+ if (ruleset->nb_rules == U32_MAX)
+ return -E2BIG;
+ new_rule = duplicate_rule(rule);
+ if (IS_ERR(new_rule))
+ return PTR_ERR(new_rule);
+ if (is_merge)
+ /* Sets the rule layer to the next one. */
+ new_rule->layers = BIT_ULL(ruleset->nb_layers);
+ rb_link_node(&new_rule->node, parent_node, walker_node);
+ rb_insert_color(&new_rule->node, &ruleset->root);
+ ruleset->nb_rules++;
+ return 0;
+}
+
+static inline void get_hierarchy(struct landlock_hierarchy *const hierarchy)
+{
+ if (hierarchy)
+ refcount_inc(&hierarchy->usage);
+}
+
+static void put_hierarchy(struct landlock_hierarchy *hierarchy)
+{
+ while (hierarchy && refcount_dec_and_test(&hierarchy->usage)) {
+ const struct landlock_hierarchy *const freeme = hierarchy;
+
+ hierarchy = hierarchy->parent;
+ kfree(freeme);
+ }
+}
+
+static int merge_ruleset(struct landlock_ruleset *const dst,
+ struct landlock_ruleset *const src)
+{
+ struct landlock_rule *walker_rule, *next_rule;
+ int err = 0;
+
+ might_sleep();
+ if (!src)
+ return 0;
+ /* Only merge into a domain. */
+ if (WARN_ON_ONCE(!dst || !dst->hierarchy))
+ return -EFAULT;
+
+ mutex_lock(&dst->lock);
+ mutex_lock_nested(&src->lock, 1);
+ /*
+ * Makes a new layer, but only increments the number of layers after
+ * the rules are inserted.
+ */
+ if (dst->nb_layers == sizeof(walker_rule->layers) * BITS_PER_BYTE) {
+ err = -E2BIG;
+ goto out_unlock;
+ }
+ dst->fs_access_mask |= src->fs_access_mask;
+
+ /* Merges the @src tree. */
+ rbtree_postorder_for_each_entry_safe(walker_rule, next_rule,
+ &src->root, node) {
+ err = landlock_insert_rule(dst, walker_rule, true);
+ if (err)
+ goto out_unlock;
+ }
+ dst->nb_layers++;
+
+out_unlock:
+ mutex_unlock(&src->lock);
+ mutex_unlock(&dst->lock);
+ return err;
+}
+
+static struct landlock_ruleset *inherit_ruleset(
+ struct landlock_ruleset *const parent)
+{
+ struct landlock_rule *walker_rule, *next_rule;
+ struct landlock_ruleset *new_ruleset;
+ int err = 0;
+
+ might_sleep();
+ new_ruleset = create_ruleset();
+ if (IS_ERR(new_ruleset))
+ return new_ruleset;
+
+ new_ruleset->hierarchy = kzalloc(sizeof(*new_ruleset->hierarchy),
+ GFP_KERNEL);
+ if (!new_ruleset->hierarchy) {
+ err = -ENOMEM;
+ goto out_put_ruleset;
+ }
+ refcount_set(&new_ruleset->hierarchy->usage, 1);
+ if (!parent)
+ return new_ruleset;
+
+ mutex_lock(&new_ruleset->lock);
+ mutex_lock_nested(&parent->lock, 1);
+ new_ruleset->nb_layers = parent->nb_layers;
+ new_ruleset->fs_access_mask = parent->fs_access_mask;
+ WARN_ON_ONCE(!parent->hierarchy);
+ get_hierarchy(parent->hierarchy);
+ new_ruleset->hierarchy->parent = parent->hierarchy;
+
+ /* Copies the @parent tree. */
+ rbtree_postorder_for_each_entry_safe(walker_rule, next_rule,
+ &parent->root, node) {
+ err = landlock_insert_rule(new_ruleset, walker_rule, false);
+ if (err)
+ goto out_unlock;
+ }
+ mutex_unlock(&parent->lock);
+ mutex_unlock(&new_ruleset->lock);
+ return new_ruleset;
+
+out_unlock:
+ mutex_unlock(&parent->lock);
+ mutex_unlock(&new_ruleset->lock);
+
+out_put_ruleset:
+ landlock_put_ruleset(new_ruleset);
+ return ERR_PTR(err);
+}
+
+static void free_ruleset(struct landlock_ruleset *const ruleset)
+{
+ struct landlock_rule *freeme, *next;
+
+ might_sleep();
+ rbtree_postorder_for_each_entry_safe(freeme, next, &ruleset->root,
+ node)
+ put_rule(freeme);
+ put_hierarchy(ruleset->hierarchy);
+ kfree(ruleset);
+}
+
+void landlock_put_ruleset(struct landlock_ruleset *const ruleset)
+{
+ might_sleep();
+ if (ruleset && refcount_dec_and_test(&ruleset->usage))
+ free_ruleset(ruleset);
+}
+
+static void free_ruleset_work(struct work_struct *const work)
+{
+ struct landlock_ruleset *ruleset;
+
+ ruleset = container_of(work, struct landlock_ruleset, work_free);
+ free_ruleset(ruleset);
+}
+
+void landlock_put_ruleset_deferred(struct landlock_ruleset *const ruleset)
+{
+ if (ruleset && refcount_dec_and_test(&ruleset->usage)) {
+ INIT_WORK(&ruleset->work_free, free_ruleset_work);
+ schedule_work(&ruleset->work_free);
+ }
+}
+
+/*
+ * Creates a new transition domain, intersection of @parent and @ruleset, or
+ * return @parent if @ruleset is empty. If @parent is empty, returns a
+ * duplicate of @ruleset.
+ */
+struct landlock_ruleset *landlock_merge_ruleset(
+ struct landlock_ruleset *const parent,
+ struct landlock_ruleset *const ruleset)
+{
+ struct landlock_ruleset *new_dom;
+ int err;
+
+ might_sleep();
+ /*
+ * Merging duplicates a ruleset, so a new ruleset can't be
+ * the same as the parent, but they can have similar content.
+ */
+ if (WARN_ON_ONCE(!ruleset || parent == ruleset)) {
+ landlock_get_ruleset(parent);
+ return parent;
+ }
+
+ new_dom = inherit_ruleset(parent);
+ if (IS_ERR(new_dom))
+ return new_dom;
+
+ err = merge_ruleset(new_dom, ruleset);
+ if (err) {
+ landlock_put_ruleset(new_dom);
+ return ERR_PTR(err);
+ }
+ return new_dom;
+}
+
+/*
+ * The returned access has the same lifetime as @ruleset.
+ */
+const struct landlock_rule *landlock_find_rule(
+ const struct landlock_ruleset *const ruleset,
+ const struct landlock_object *const object)
+{
+ const struct rb_node *node;
+
+ if (!object)
+ return NULL;
+ node = ruleset->root.rb_node;
+ while (node) {
+ struct landlock_rule *this = rb_entry(node,
+ struct landlock_rule, node);
+
+ if (this->object == object)
+ return this;
+ if (this->object < object)
+ node = node->rb_right;
+ else
+ node = node->rb_left;
+ }
+ return NULL;
+}
diff --git a/security/landlock/ruleset.h b/security/landlock/ruleset.h
new file mode 100644
index 000000000000..fd7d2d810db9
--- /dev/null
+++ b/security/landlock/ruleset.h
@@ -0,0 +1,157 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Landlock LSM - Ruleset management
+ *
+ * Copyright © 2016-2020 Mickaël Salaün <mic@digikod.net>
+ * Copyright © 2018-2020 ANSSI
+ */
+
+#ifndef _SECURITY_LANDLOCK_RULESET_H
+#define _SECURITY_LANDLOCK_RULESET_H
+
+#include <linux/mutex.h>
+#include <linux/rbtree.h>
+#include <linux/refcount.h>
+#include <linux/workqueue.h>
+
+#include "object.h"
+
+/**
+ * struct landlock_rule - Access rights tied to an object
+ *
+ * When enforcing a ruleset (i.e. merging a ruleset into the current domain),
+ * the layer level of a new rule is the incremented top layer level (cf.
+ * &struct landlock_ruleset). If there is no rule (from this domain) tied to
+ * the same object, then the depth of the new rule is 1. However, if there is
+ * already a rule tied to the same object and if this rule's layer level is the
+ * previous top layer level, then the depth and the layer level are both
+ * incremented and the rule is updated with the new access rights (boolean
+ * AND).
+ */
+struct landlock_rule {
+ /**
+ * @node: Node in the red-black tree.
+ */
+ struct rb_node node;
+ /**
+ * @object: Pointer to identify a kernel object (e.g. an inode). This
+ * is used as a key for this ruleset element. This pointer is set once
+ * and never modified. It always point to an allocated object because
+ * each rule increment the refcount of there object.
+ */
+ struct landlock_object *object;
+ /**
+ * @access: Bitfield of allowed actions on the kernel object. They are
+ * relative to the object type (e.g. %LANDLOCK_ACTION_FS_READ). This
+ * may be the result of the merged access rights (boolean AND) from
+ * multiple layers referring to the same object.
+ */
+ u32 access;
+ /**
+ * @layers: Bitfield to identify the layers which resulted to @access
+ * from different consecutive intersections.
+ */
+ u64 layers;
+};
+
+/**
+ * struct landlock_hierarchy - Node in a ruleset hierarchy
+ */
+struct landlock_hierarchy {
+ /**
+ * @parent: Pointer to the parent node, or NULL if it is a root Lanlock
+ * domain.
+ */
+ struct landlock_hierarchy *parent;
+ /**
+ * @usage: Number of potential children domains plus their parent
+ * domain.
+ */
+ refcount_t usage;
+};
+
+/**
+ * struct landlock_ruleset - Landlock ruleset
+ *
+ * This data structure must contains unique entries, be updatable, and quick to
+ * match an object.
+ */
+struct landlock_ruleset {
+ /**
+ * @root: Root of a red-black tree containing &struct landlock_rule
+ * nodes.
+ */
+ struct rb_root root;
+ /**
+ * @hierarchy: Enables hierarchy identification even when a parent
+ * domain vanishes. This is needed for the ptrace protection.
+ */
+ struct landlock_hierarchy *hierarchy;
+ union {
+ /**
+ * @work_free: Enables to free a ruleset within a lockless
+ * section. This is only used by
+ * landlock_put_ruleset_deferred() when @usage reaches zero.
+ * The fields @usage, @lock, @nb_layers, @nb_rules and
+ * @fs_access_mask are then unused.
+ */
+ struct work_struct work_free;
+ struct {
+ /**
+ * @usage: Number of processes (i.e. domains) or file
+ * descriptors referencing this ruleset.
+ */
+ refcount_t usage;
+ /**
+ * @lock: Guards against concurrent modifications of
+ * @root, if @usage is greater than zero.
+ */
+ struct mutex lock;
+ /**
+ * @nb_rules: Number of non-overlapping (i.e. not for
+ * the same object) rules in this ruleset.
+ */
+ u32 nb_rules;
+ /**
+ * @nb_layers: Number of layers which are used in this
+ * ruleset. This enables to check that all the layers
+ * allow an access request. A value of 0 identify a
+ * non-merged ruleset (i.e. not a domain).
+ */
+ u32 nb_layers;
+ /**
+ * @fs_access_mask: Contains the subset of filesystem
+ * actions which are restricted by a ruleset. This is
+ * used when merging rulesets and for user space
+ * backward compatibility (i.e. future-proof). Set
+ * once and never changed for the lifetime of the
+ * ruleset.
+ */
+ u32 fs_access_mask;
+ };
+ };
+};
+
+struct landlock_ruleset *landlock_create_ruleset(const u32 fs_access_mask);
+
+void landlock_put_ruleset(struct landlock_ruleset *const ruleset);
+void landlock_put_ruleset_deferred(struct landlock_ruleset *const ruleset);
+
+int landlock_insert_rule(struct landlock_ruleset *const ruleset,
+ struct landlock_rule *const rule, const bool is_merge);
+
+struct landlock_ruleset *landlock_merge_ruleset(
+ struct landlock_ruleset *const parent,
+ struct landlock_ruleset *const ruleset);
+
+const struct landlock_rule *landlock_find_rule(
+ const struct landlock_ruleset *const ruleset,
+ const struct landlock_object *const object);
+
+static inline void landlock_get_ruleset(struct landlock_ruleset *const ruleset)
+{
+ if (ruleset)
+ refcount_inc(&ruleset->usage);
+}
+
+#endif /* _SECURITY_LANDLOCK_RULESET_H */
--
2.26.2