Message-ID: <20210204033057.GB29022@mail.hallyn.com>
Date:   Wed, 3 Feb 2021 21:30:57 -0600
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     Mickaël Salaün <mic@...ikod.net>
Cc:     James Morris <jmorris@...ei.org>, Jann Horn <jannh@...gle.com>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Arnd Bergmann <arnd@...db.de>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Jeff Dike <jdike@...toit.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Richard Weinberger <richard@....at>,
        Shuah Khan <shuah@...nel.org>,
        Vincent Dagonneau <vincent.dagonneau@....gouv.fr>,
        kernel-hardening@...ts.openwall.com, linux-api@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        linux-security-module@...r.kernel.org, x86@...nel.org,
        Mickaël Salaün <mic@...ux.microsoft.com>
Subject: Re: [PATCH v28 02/12] landlock: Add ruleset and domain management

On Tue, Feb 02, 2021 at 05:27:00PM +0100, Mickaël Salaün wrote:
> From: Mickaël Salaün <mic@...ux.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@...ei.org>
> Cc: Jann Horn <jannh@...gle.com>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Serge E. Hallyn <serge@...lyn.com>

Acked-by: Serge Hallyn <serge@...lyn.com>

> Signed-off-by: Mickaël Salaün <mic@...ux.microsoft.com>
> ---
> 
> Changes since v27:
> * Fix domains with layers of non-overlapping access rights.
> * Add stricter limit checks (same semantic).
> * Change the grow direction of a rule layer stack to make it the same as
>   the new ruleset fs_access_masks stack (cosmetic change).
> * Cosmetic fix for a comment block.
> 
> Changes since v26:
> * Fix spelling.
> 
> Changes since v25:
> * Add build-time checks for the num_layers and num_rules variables
>   according to LANDLOCK_MAX_NUM_LAYERS and LANDLOCK_MAX_NUM_RULES, and
>   move these limits to a dedicated file.
> * Cosmetic variable renames.
> 
> Changes since v24:
> * Update struct landlock_rule with a layer stack.  This reverts "Always
>   intersect access rights" from v24 and also adds the ability to tie
>   access rights with their policy layer.  As noted by Jann Horn, always
>   intersecting access rights made some use cases uselessly more
>   difficult to handle in user space.  Thanks to this new stack, we still
>   have a deterministic policy behavior whatever their level in the stack
>   of policies, while using a "union" of accesses when building a
>   ruleset.  The implementation use a FAM to keep the access checks quick
>   and memory efficient (4 bytes per layer per inode).  Update
>   insert_rule() accordingly.
> 
> 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/limits.h  |  17 ++
>  security/landlock/ruleset.c | 469 ++++++++++++++++++++++++++++++++++++
>  security/landlock/ruleset.h | 165 +++++++++++++
>  4 files changed, 652 insertions(+), 1 deletion(-)
>  create mode 100644 security/landlock/limits.h
>  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/limits.h b/security/landlock/limits.h
> new file mode 100644
> index 000000000000..b734f597bb0e
> --- /dev/null
> +++ b/security/landlock/limits.h
> @@ -0,0 +1,17 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Landlock LSM - Limits for different components
> + *
> + * Copyright © 2016-2020 Mickaël Salaün <mic@...ikod.net>
> + * Copyright © 2018-2020 ANSSI
> + */
> +
> +#ifndef _SECURITY_LANDLOCK_LIMITS_H
> +#define _SECURITY_LANDLOCK_LIMITS_H
> +
> +#include <linux/limits.h>
> +
> +#define LANDLOCK_MAX_NUM_LAYERS		64
> +#define LANDLOCK_MAX_NUM_RULES		U32_MAX
> +
> +#endif /* _SECURITY_LANDLOCK_LIMITS_H */
> diff --git a/security/landlock/ruleset.c b/security/landlock/ruleset.c
> new file mode 100644
> index 000000000000..59c86126ea1c
> --- /dev/null
> +++ b/security/landlock/ruleset.c
> @@ -0,0 +1,469 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Landlock LSM - Ruleset management
> + *
> + * Copyright © 2016-2020 Mickaël Salaün <mic@...ikod.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/lockdep.h>
> +#include <linux/overflow.h>
> +#include <linux/rbtree.h>
> +#include <linux/refcount.h>
> +#include <linux/slab.h>
> +#include <linux/spinlock.h>
> +#include <linux/workqueue.h>
> +
> +#include "limits.h"
> +#include "object.h"
> +#include "ruleset.h"
> +
> +static struct landlock_ruleset *create_ruleset(const u32 num_layers)
> +{
> +	struct landlock_ruleset *new_ruleset;
> +
> +	new_ruleset = kzalloc(struct_size(new_ruleset, fs_access_masks,
> +				num_layers), 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;
> +	new_ruleset->num_layers = num_layers;
> +	/*
> +	 * hierarchy = NULL
> +	 * num_rules = 0
> +	 * fs_access_masks[] = 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(1);
> +	if (!IS_ERR(new_ruleset))
> +		new_ruleset->fs_access_masks[0] = fs_access_mask;
> +	return new_ruleset;
> +}
> +
> +static void build_check_rule(void)
> +{
> +	const struct landlock_rule rule = {
> +		.num_layers = ~0,
> +	};
> +
> +	BUILD_BUG_ON(rule.num_layers < LANDLOCK_MAX_NUM_LAYERS);
> +}
> +
> +static struct landlock_rule *create_rule(
> +		struct landlock_object *const object,
> +		const struct landlock_layer (*const layers)[],
> +		const u32 num_layers,
> +		const struct landlock_layer *const new_layer)
> +{
> +	struct landlock_rule *new_rule;
> +	u32 new_num_layers;
> +
> +	build_check_rule();
> +	if (new_layer) {
> +		/* Should already be checked by landlock_merge_ruleset(). */
> +		if (WARN_ON_ONCE(num_layers >= LANDLOCK_MAX_NUM_LAYERS))
> +			return ERR_PTR(-E2BIG);
> +		new_num_layers = num_layers + 1;
> +	} else {
> +		new_num_layers = num_layers;
> +	}
> +	new_rule = kzalloc(struct_size(new_rule, layers, new_num_layers),
> +			GFP_KERNEL_ACCOUNT);
> +	if (!new_rule)
> +		return ERR_PTR(-ENOMEM);
> +	RB_CLEAR_NODE(&new_rule->node);
> +	landlock_get_object(object);
> +	new_rule->object = object;
> +	new_rule->num_layers = new_num_layers;
> +	/* Copies the original layer stack. */
> +	memcpy(new_rule->layers, layers,
> +			flex_array_size(new_rule, layers, num_layers));
> +	if (new_layer)
> +		/* Adds a copy of @new_layer on the layer stack. */
> +		new_rule->layers[new_rule->num_layers - 1] = *new_layer;
> +	return new_rule;
> +}
> +
> +static void put_rule(struct landlock_rule *const rule)
> +{
> +	might_sleep();
> +	if (!rule)
> +		return;
> +	landlock_put_object(rule->object);
> +	kfree(rule);
> +}
> +
> +static void build_check_ruleset(void)
> +{
> +	const struct landlock_ruleset ruleset = {
> +		.num_rules = ~0,
> +		.num_layers = ~0,
> +	};
> +
> +	BUILD_BUG_ON(ruleset.num_rules < LANDLOCK_MAX_NUM_RULES);
> +	BUILD_BUG_ON(ruleset.num_layers < LANDLOCK_MAX_NUM_LAYERS);
> +}
> +
> +/**
> + * insert_rule - Create and insert a rule in a ruleset
> + *
> + * @ruleset: The ruleset to be updated.
> + * @object: The object to build the new rule with.  The underlying kernel
> + *          object must be held by the caller.
> + * @layers: One or multiple layers to be copied into the new rule.
> + * @num_layers: The number of @layers entries.
> + *
> + * When user space requests to add a new rule to a ruleset, @layers only
> + * contains one entry and this entry is not assigned to any level.  In this
> + * case, the new rule will extend @ruleset, similarly to a boolean OR between
> + * access rights.
> + *
> + * When merging a ruleset in a domain, or copying a domain, @layers will be
> + * added to @ruleset as new constraints, similarly to a boolean AND between
> + * access rights.
> + */
> +static int insert_rule(struct landlock_ruleset *const ruleset,
> +		struct landlock_object *const object,
> +		const struct landlock_layer (*const layers)[],
> +		size_t num_layers)
> +{
> +	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(!object || !layers))
> +		return -ENOENT;
> +	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 != object) {
> +			parent_node = *walker_node;
> +			if (this->object < object)
> +				walker_node = &((*walker_node)->rb_right);
> +			else
> +				walker_node = &((*walker_node)->rb_left);
> +			continue;
> +		}
> +
> +		/* Only a single-level layer should match an existing rule. */
> +		if (WARN_ON_ONCE(num_layers != 1))
> +			return -EINVAL;
> +
> +		/* If there is a matching rule, updates it. */
> +		if ((*layers)[0].level == 0) {
> +			/*
> +			 * Extends access rights when the request comes from
> +			 * landlock_add_rule(2), i.e. @ruleset is not a domain.
> +			 */
> +			if (WARN_ON_ONCE(this->num_layers != 1))
> +				return -EINVAL;
> +			if (WARN_ON_ONCE(this->layers[0].level != 0))
> +				return -EINVAL;
> +			this->layers[0].access |= (*layers)[0].access;
> +			return 0;
> +		}
> +
> +		if (WARN_ON_ONCE(this->layers[0].level == 0))
> +			return -EINVAL;
> +
> +		/*
> +		 * Intersects access rights when it is a merge between a
> +		 * ruleset and a domain.
> +		 */
> +		new_rule = create_rule(object, &this->layers, this->num_layers,
> +				&(*layers)[0]);
> +		if (IS_ERR(new_rule))
> +			return PTR_ERR(new_rule);
> +		rb_replace_node(&this->node, &new_rule->node, &ruleset->root);
> +		put_rule(this);
> +		return 0;
> +	}
> +
> +	/* There is no match for @object. */
> +	build_check_ruleset();
> +	if (ruleset->num_rules >= LANDLOCK_MAX_NUM_RULES)
> +		return -E2BIG;
> +	new_rule = create_rule(object, layers, num_layers, NULL);
> +	if (IS_ERR(new_rule))
> +		return PTR_ERR(new_rule);
> +	rb_link_node(&new_rule->node, parent_node, walker_node);
> +	rb_insert_color(&new_rule->node, &ruleset->root);
> +	ruleset->num_rules++;
> +	return 0;
> +}
> +
> +static void build_check_layer(void)
> +{
> +	const struct landlock_layer layer = {
> +		.level = ~0,
> +	};
> +
> +	BUILD_BUG_ON(layer.level < LANDLOCK_MAX_NUM_LAYERS);
> +}
> +
> +/* @ruleset must be locked by the caller. */
> +int landlock_insert_rule(struct landlock_ruleset *const ruleset,
> +		struct landlock_object *const object, const u32 access)
> +{
> +	struct landlock_layer layers[] = {{
> +		.access = access,
> +		/* When @level is zero, insert_rule() extends @ruleset. */
> +		.level = 0,
> +	}};
> +
> +	build_check_layer();
> +	return insert_rule(ruleset, object, &layers, ARRAY_SIZE(layers));
> +}
> +
> +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();
> +	/* Should already be checked by landlock_merge_ruleset() */
> +	if (WARN_ON_ONCE(!src))
> +		return 0;
> +	/* Only merge into a domain. */
> +	if (WARN_ON_ONCE(!dst || !dst->hierarchy))
> +		return -EINVAL;
> +
> +	/* Locks @dst first because we are its only owner. */
> +	mutex_lock(&dst->lock);
> +	mutex_lock_nested(&src->lock, SINGLE_DEPTH_NESTING);
> +
> +	/* Stacks the new layer. */
> +	if (WARN_ON_ONCE(src->num_layers != 1 || dst->num_layers < 1)) {
> +		err = -EINVAL;
> +		goto out_unlock;
> +	}
> +	dst->fs_access_masks[dst->num_layers - 1] = src->fs_access_masks[0];
> +
> +	/* Merges the @src tree. */
> +	rbtree_postorder_for_each_entry_safe(walker_rule, next_rule,
> +			&src->root, node) {
> +		struct landlock_layer layers[] = {{
> +			.level = dst->num_layers,
> +		}};
> +
> +		if (WARN_ON_ONCE(walker_rule->num_layers != 1)) {
> +			err = -EINVAL;
> +			goto out_unlock;
> +		}
> +		if (WARN_ON_ONCE(walker_rule->layers[0].level != 0)) {
> +			err = -EINVAL;
> +			goto out_unlock;
> +		}
> +		layers[0].access = walker_rule->layers[0].access;
> +		err = insert_rule(dst, walker_rule->object, &layers,
> +				ARRAY_SIZE(layers));
> +		if (err)
> +			goto out_unlock;
> +	}
> +
> +out_unlock:
> +	mutex_unlock(&src->lock);
> +	mutex_unlock(&dst->lock);
> +	return err;
> +}
> +
> +static int inherit_ruleset(struct landlock_ruleset *const parent,
> +		struct landlock_ruleset *const child)
> +{
> +	struct landlock_rule *walker_rule, *next_rule;
> +	int err = 0;
> +
> +	might_sleep();
> +	if (!parent)
> +		return 0;
> +
> +	/* Locks @child first because we are its only owner. */
> +	mutex_lock(&child->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 = insert_rule(child, walker_rule->object,
> +				&walker_rule->layers, walker_rule->num_layers);
> +		if (err)
> +			goto out_unlock;
> +	}
> +
> +	if (WARN_ON_ONCE(child->num_layers <= parent->num_layers)) {
> +		err = -EINVAL;
> +		goto out_unlock;
> +	}
> +	/* Copies the parent layer stack and leaves a space for the new layer. */
> +	memcpy(child->fs_access_masks, parent->fs_access_masks,
> +			flex_array_size(parent, fs_access_masks, parent->num_layers));
> +
> +	if (WARN_ON_ONCE(!parent->hierarchy)) {
> +		err = -EINVAL;
> +		goto out_unlock;
> +	}
> +	get_hierarchy(parent->hierarchy);
> +	child->hierarchy->parent = parent->hierarchy;
> +
> +out_unlock:
> +	mutex_unlock(&parent->lock);
> +	mutex_unlock(&child->lock);
> +	return 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;
> +	u32 num_layers;
> +	int err;
> +
> +	might_sleep();
> +	if (WARN_ON_ONCE(!ruleset || parent == ruleset))
> +		return ERR_PTR(-EINVAL);
> +
> +	if (parent) {
> +		if (parent->num_layers >= LANDLOCK_MAX_NUM_LAYERS)
> +			return ERR_PTR(-E2BIG);
> +		num_layers = parent->num_layers + 1;
> +	} else {
> +		num_layers = 1;
> +	}
> +
> +	/* Creates a new domain... */
> +	new_dom = create_ruleset(num_layers);
> +	if (IS_ERR(new_dom))
> +		return new_dom;
> +	new_dom->hierarchy = kzalloc(sizeof(*new_dom->hierarchy),
> +			GFP_KERNEL_ACCOUNT);
> +	if (!new_dom->hierarchy) {
> +		err = -ENOMEM;
> +		goto out_put_dom;
> +	}
> +	refcount_set(&new_dom->hierarchy->usage, 1);
> +
> +	/* ...as a child of @parent... */
> +	err = inherit_ruleset(parent, new_dom);
> +	if (err)
> +		goto out_put_dom;
> +
> +	/* ...and including @ruleset. */
> +	err = merge_ruleset(new_dom, ruleset);
> +	if (err)
> +		goto out_put_dom;
> +
> +	return new_dom;
> +
> +out_put_dom:
> +	landlock_put_ruleset(new_dom);
> +	return ERR_PTR(err);
> +}
> +
> +/*
> + * 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..6b1198458b37
> --- /dev/null
> +++ b/security/landlock/ruleset.h
> @@ -0,0 +1,165 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Landlock LSM - Ruleset management
> + *
> + * Copyright © 2016-2020 Mickaël Salaün <mic@...ikod.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_layer - Access rights for a given layer
> + */
> +struct landlock_layer {
> +	/**
> +	 * @level: Position of this layer in the layer stack.
> +	 */
> +	u16 level;
> +	/**
> +	 * @access: Bitfield of allowed actions on the kernel object.  They are
> +	 * relative to the object type (e.g. %LANDLOCK_ACTION_FS_READ).
> +	 */
> +	u16 access;
> +};
> +
> +/**
> + * struct landlock_rule - Access rights tied to an object
> + */
> +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;
> +	/**
> +	 * @num_layers: Number of entries in @layers.
> +	 */
> +	u32 num_layers;
> +	/**
> +	 * @layers: Stack of layers, from the latest to the newest, implemented
> +	 * as a flexible array member (FAM).
> +	 */
> +	struct landlock_layer 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
> +	 * 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, @num_rules, @num_layers and
> +		 * @fs_access_masks 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;
> +			/**
> +			 * @num_rules: Number of non-overlapping (i.e. not for
> +			 * the same object) rules in this ruleset.
> +			 */
> +			u32 num_rules;
> +			/**
> +			 * @num_layers: Number of layers that 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 num_layers;
> +			/**
> +			 * @fs_access_masks: Contains the subset of filesystem
> +			 * actions that are restricted by a ruleset.  A domain
> +			 * saves all layers of merged rulesets in a stack
> +			 * (FAM), starting from the first layer to the last
> +			 * one.  These layers are used when merging rulesets,
> +			 * for user space backward compatibility (i.e.
> +			 * future-proof), and to properly handle merged
> +			 * rulesets without overlapping access rights.  These
> +			 * layers are set once and never changed for the
> +			 * lifetime of the ruleset.
> +			 */
> +			u16 fs_access_masks[];
> +		};
> +	};
> +};
> +
> +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_object *const object, const u32 access);
> +
> +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 */
> -- 
> 2.30.0

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