[PATCH v24 02/12] landlock: Add ruleset and domain management

Mickaël Salaün
2020-11-12T20:51:56Z

From: Mickaël Salaün <mic@linux.microsoft.com>

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 syscalls) 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
defines 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.

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>
Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
---

Changes since v23:
* Always intersect access rights. Following the filesystem change
logic, make ruleset updates more consistent by always intersecting
access rights (boolean AND) instead of combining them (boolean OR) for
the same layer. This defensive approach could also help avoid user
space to inadvertently allow multiple access rights for the same
object (e.g. write and execute access on a path hierarchy) instead of
dealing with such inconsistency. This can happen when there is no
deduplication of objects (e.g. paths and underlying inodes) whereas
they get different access rights with landlock_add_rule(2).
* Add extra checks to make sure that:
- there is always an (allocated) object in each used rules;
- when updating a ruleset with a new rule (i.e. not merging two
rulesets), the ruleset doesn't contain multiple layers.
* Hide merge parameter from the public landlock_insert_rule() API. This
helps avoid misuse of this function.
* Replace a remaining hardcoded 1 with SINGLE_DEPTH_NESTING.

Changes since v22:
* Explicitely use RB_ROOT and SINGLE_DEPTH_NESTING (suggested by Jann
Horn).
* Improve comments and fix spelling (suggested by Jann Horn).

Changes since v21:
* Add and clean up comments.

Changes since v18:
* Account rulesets to kmemcg.
* Remove struct holes.
* Cosmetic changes.

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/
---
security/landlock/Makefile | 2 +-
security/landlock/ruleset.c | 361 ++++++++++++++++++++++++++++++++++++
security/landlock/ruleset.h | 158 ++++++++++++++++
3 files changed, 520 insertions(+), 1 deletion(-)
create mode 100644 security/landlock/ruleset.c
create mode 100644 security/landlock/ruleset.h

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..afa5a6f20d1d
--- /dev/null
+++ b/security/landlock/ruleset.c
@@ -0,0 +1,361 @@
+// 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/lockdep.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 *new_ruleset;
+
+ new_ruleset = kzalloc(sizeof(*new_ruleset), GFP_KERNEL_ACCOUNT);
+ if (!new_ruleset)
+ return ERR_PTR(-ENOMEM);
+ refcount_set(&new_ruleset->usage, 1);
+ mutex_init(&new_ruleset->lock);
+ new_ruleset->root = RB_ROOT;
+ /*
+ * hierarchy = NULL
+ * nb_rules = 0
+ * nb_layers = 0
+ * fs_access_mask = 0
+ */
+ return new_ruleset;
+}
+
+struct landlock_ruleset *landlock_create_ruleset(const u32 fs_access_mask)
+{
+ struct landlock_ruleset *new_ruleset;
+
+ /* Informs about useless ruleset. */
+ if (!fs_access_mask)
+ return ERR_PTR(-ENOMSG);
+ new_ruleset = create_ruleset();
+ if (!IS_ERR(new_ruleset))
+ new_ruleset->fs_access_mask = fs_access_mask;
+ return new_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_ACCOUNT);
+ 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);
+}
+
+/**
+ * insert_rule - Insert a rule in a ruleset
+ *
+ * Intersects access rights of the rule with those of the ruleset.
+ *
+ * @ruleset: The ruleset to be updated.
+ * @rule: Read-only payload to be inserted (not owned by this function).
+ * @is_merge: If true, handle the rule layers.
+ *
+ * Assumptions:
+ *
+ * - An inserted rule cannot be removed.
+ * - The underlying kernel object must be held by the caller.
+ */
+static int 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);
+ if (WARN_ON_ONCE(!rule->object))
+ return -ENOENT;
+ if (!is_merge && WARN_ON_ONCE(ruleset->nb_layers != 0))
+ return -EINVAL;
+ 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)
+ /* Updates the rule layers with the next one. */
+ this->layers |= BIT_ULL(ruleset->nb_layers);
+ /* Intersects 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;
+}
+
+int landlock_insert_rule(struct landlock_ruleset *const ruleset,
+ struct landlock_rule *const rule)
+{
+ return insert_rule(ruleset, rule, false);
+}
+
+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;
+
+ /*
+ * The ruleset being modified (@dst) is locked first, then the ruleset
+ * being copied (@src).
+ */
+ mutex_lock(&dst->lock);
+ mutex_lock_nested(&src->lock, SINGLE_DEPTH_NESTING);
+ /*
+ * 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 = 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_ACCOUNT);
+ 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, SINGLE_DEPTH_NESTING);
+
+ /* 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);
+ if (err)
+ goto out_unlock;
+ }
+ 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;
+
+ 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);
+ }
+}
+
+/**
+ * landlock_merge_ruleset - Merge a ruleset with a domain
+ *
+ * @parent: Parent domain.
+ * @ruleset: New ruleset to be merged.
+ *
+ * Returns the intersection of @parent and @ruleset, or returns @parent if
+ * @ruleset is empty, or returns a duplicate of @ruleset if @parent is empty.
+ */
+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 cannot 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..04c2cc5e42cb
--- /dev/null
+++ b/security/landlock/ruleset.h
@@ -0,0 +1,158 @@
+/* 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 ruleset's 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 points to an allocated object because
+ * each rule increments the refcount of its object.
+ */
+ struct landlock_object *object;
+ /**
+ * @layers: Bitfield to identify the layers which resulted to @access
+ * from different consecutive intersections.
+ */
+ u64 layers;
+ /**
+ * @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;
+};
+
+/**
+ * struct landlock_hierarchy - Node in a ruleset hierarchy
+ */
+struct landlock_hierarchy {
+ /**
+ * @parent: Pointer to the parent node, or NULL if it is a root
+ * Landlock 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 contain 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. Once a ruleset is tied to a process (i.e. as a domain), this
+ * tree is immutable until @usage reaches zero.
+ */
+ 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 @lock, @usage, @nb_layers, @nb_rules and
+ * @fs_access_mask are then unused.
+ */
+ struct work_struct work_free;
+ struct {
+ /**
+ * @lock: Guards against concurrent modifications of
+ * @root, if @usage is greater than zero.
+ */
+ struct mutex lock;
+ /**
+ * @usage: Number of processes (i.e. domains) or file
+ * descriptors referencing this ruleset.
+ */
+ refcount_t usage;
+ /**
+ * @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 identifies 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);
+
+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 */
--
2.29.2