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Message-Id: <20200227042002.3032-1-hdanton@sina.com>
Date: Thu, 27 Feb 2020 12:20:02 +0800
From: Hillf Danton <hdanton@...a.com>
To: Mickaël Salaün <mic@...ikod.net>
Cc: linux-kernel@...r.kernel.org,
Al Viro <viro@...iv.linux.org.uk>,
Andy Lutomirski <luto@...capital.net>,
Arnd Bergmann <arnd@...db.de>,
Casey Schaufler <casey@...aufler-ca.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
James Morris <jmorris@...ei.org>,
Jann Horn <jann@...jh.net>,
Jonathan Corbet <corbet@....net>,
Kees Cook <keescook@...omium.org>,
Michael Kerrisk <mtk.manpages@...il.com>,
Mickaël Salaün <mickael.salaun@....gouv.fr>,
"Serge E. Hallyn" <serge@...lyn.com>,
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-kselftest@...r.kernel.org,
linux-security-module@...r.kernel.org,
x86@...nel.org
Subject: Re: [RFC PATCH v14 01/10] landlock: Add object and rule management
On Mon, 24 Feb 2020 17:02:06 +0100 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 this 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@...ikod.net>
> Cc: Andy Lutomirski <luto@...capital.net>
> Cc: James Morris <jmorris@...ei.org>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Serge E. Hallyn <serge@...lyn.com>
> ---
>
> Changes since v13:
> * New dedicated implementation, removing the need for eBPF.
>
> Previous version:
> https://lore.kernel.org/lkml/20190721213116.23476-6-mic@digikod.net/
> ---
> MAINTAINERS | 10 ++
> security/Kconfig | 1 +
> security/Makefile | 2 +
> security/landlock/Kconfig | 15 ++
> security/landlock/Makefile | 3 +
> security/landlock/object.c | 339 +++++++++++++++++++++++++++++++++++++
> security/landlock/object.h | 134 +++++++++++++++
> 7 files changed, 504 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 fcd79fc38928..206f85768cd9 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -9360,6 +9360,16 @@ F: net/core/skmsg.c
> F: net/core/sock_map.c
> F: net/ipv4/tcp_bpf.c
>
> +LANDLOCK SECURITY MODULE
> +M: Mickaël Salaün <mic@...ikod.net>
> +L: linux-security-module@...r.kernel.org
> +W: https://landlock.io
> +T: git https://github.com/landlock-lsm/linux.git
> +S: Supported
> +F: security/landlock/
> +K: landlock
> +K: LANDLOCK
> +
> LANTIQ / INTEL Ethernet drivers
> M: Hauke Mehrtens <hauke@...ke-m.de>
> L: netdev@...r.kernel.org
> diff --git a/security/Kconfig b/security/Kconfig
> index 2a1a2d396228..9d9981394fb0 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 746438499029..2472ef96d40a 100644
> --- a/security/Makefile
> +++ b/security/Makefile
> @@ -12,6 +12,7 @@ subdir-$(CONFIG_SECURITY_YAMA) += yama
> subdir-$(CONFIG_SECURITY_LOADPIN) += loadpin
> subdir-$(CONFIG_SECURITY_SAFESETID) += safesetid
> subdir-$(CONFIG_SECURITY_LOCKDOWN_LSM) += lockdown
> +subdir-$(CONFIG_SECURITY_LANDLOCK) += landlock
>
> # always enable default capabilities
> obj-y += commoncap.o
> @@ -29,6 +30,7 @@ obj-$(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-$(CONFIG_CGROUP_DEVICE) += device_cgroup.o
>
> # Object integrity file lists
> diff --git a/security/landlock/Kconfig b/security/landlock/Kconfig
> new file mode 100644
> index 000000000000..4a321d5b3f67
> --- /dev/null
> +++ b/security/landlock/Kconfig
> @@ -0,0 +1,15 @@
> +# SPDX-License-Identifier: GPL-2.0-only
> +
> +config SECURITY_LANDLOCK
> + bool "Landlock support"
> + depends on SECURITY
> + default n
> + help
> + This selects Landlock, a safe sandboxing mechanism. It enables to
> + restrict processes on the fly (i.e. enforce an access control policy),
> + which can complement seccomp-bpf. The security policy is a set of access
> + rights tied to an object, which could be a file, a socket or a process.
> +
> + See Documentation/security/landlock/ for further information.
> +
> + If you are unsure how to answer this question, answer N.
> diff --git a/security/landlock/Makefile b/security/landlock/Makefile
> new file mode 100644
> index 000000000000..cb6deefbf4c0
> --- /dev/null
> +++ b/security/landlock/Makefile
> @@ -0,0 +1,3 @@
> +obj-$(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..38fbbb108120
> --- /dev/null
> +++ b/security/landlock/object.c
> @@ -0,0 +1,339 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Landlock LSM - Object and rule management
> + *
> + * Copyright © 2016-2020 Mickaël Salaün <mic@...ikod.net>
> + * Copyright © 2018-2020 ANSSI
> + *
> + * Principles and constraints of the object and rule management:
> + * - Do not leak memory.
> + * - Try as much as possible to free a memory allocation as soon as it is
> + * unused.
> + * - Do not use global lock.
> + * - Do not charge processes other than the one requesting a Landlock
> + * operation.
> + */
> +
> +#include <linux/bug.h>
> +#include <linux/compiler.h>
> +#include <linux/compiler_types.h>
> +#include <linux/err.h>
> +#include <linux/errno.h>
> +#include <linux/fs.h>
> +#include <linux/kernel.h>
> +#include <linux/list.h>
> +#include <linux/rbtree.h>
> +#include <linux/rcupdate.h>
> +#include <linux/refcount.h>
> +#include <linux/slab.h>
> +#include <linux/spinlock.h>
> +#include <linux/workqueue.h>
> +
> +#include "object.h"
> +
> +struct landlock_object *landlock_create_object(
> + const enum landlock_object_type type, void *underlying_object)
> +{
> + struct landlock_object *object;
> +
> + if (WARN_ON_ONCE(!underlying_object))
> + return NULL;
> + object = kzalloc(sizeof(*object), GFP_KERNEL);
> + if (!object)
> + return NULL;
> + refcount_set(&object->usage, 1);
> + refcount_set(&object->cleaners, 1);
> + spin_lock_init(&object->lock);
> + INIT_LIST_HEAD(&object->rules);
> + object->type = type;
> + WRITE_ONCE(object->underlying_object, underlying_object);
> + return object;
> +}
> +
> +struct landlock_object *landlock_get_object(struct landlock_object *object)
> + __acquires(object->usage)
> +{
> + __acquire(object->usage);
> + /*
> + * If @object->usage equal 0, then it will be ignored by writers, and
> + * underlying_object->object may be replaced, but this is not an issue
> + * for release_object().
> + */
> + if (object && refcount_inc_not_zero(&object->usage)) {
> + /*
> + * It should not be possible to get a reference to an object if
> + * its underlying object is being terminated (e.g. with
> + * landlock_release_object()), because an object is only
> + * modifiable through such underlying object. This is not the
> + * case with landlock_get_object_cleaner().
> + */
> + WARN_ON_ONCE(!READ_ONCE(object->underlying_object));
> + return object;
> + }
> + return NULL;
> +}
> +
> +static struct landlock_object *get_object_cleaner(
> + struct landlock_object *object)
> + __acquires(object->cleaners)
> +{
> + __acquire(object->cleaners);
> + if (object && refcount_inc_not_zero(&object->cleaners))
> + return object;
> + return NULL;
> +}
> +
> +/*
> + * There is two cases when an object should be free and the reference to the
> + * underlying object should be put:
> + * - when the last rule tied to this object is removed, which is handled by
> + * landlock_put_rule() and then release_object();
> + * - when the object is being terminated (e.g. no more reference to an inode),
> + * which is handled by landlock_put_object().
> + */
> +static void put_object_free(struct landlock_object *object)
> + __releases(object->cleaners)
> +{
> + __release(object->cleaners);
> + if (!refcount_dec_and_test(&object->cleaners))
> + return;
> + WARN_ON_ONCE(refcount_read(&object->usage));
> + /*
> + * Ensures a safe use of @object in the RCU block from
> + * landlock_put_rule().
> + */
> + kfree_rcu(object, rcu_free);
> +}
> +
> +/*
> + * Destroys a newly created and useless object.
> + */
> +void landlock_drop_object(struct landlock_object *object)
> +{
> + if (WARN_ON_ONCE(!refcount_dec_and_test(&object->usage)))
> + return;
> + __acquire(object->cleaners);
> + put_object_free(object);
> +}
> +
> +/*
> + * Puts the underlying object (e.g. inode) if it is the first request to
> + * release @object, without calling landlock_put_object().
> + *
> + * Return true if this call effectively marks @object as released, false
> + * otherwise.
> + */
> +static bool release_object(struct landlock_object *object)
> + __releases(&object->lock)
> +{
> + void *underlying_object;
> +
> + lockdep_assert_held(&object->lock);
> +
> + underlying_object = xchg(&object->underlying_object, NULL);
A one-line comment looks needed for xchg.
> + spin_unlock(&object->lock);
> + might_sleep();
Have trouble working out what might_sleep is put for.
> + if (!underlying_object)
> + return false;
> +
> + switch (object->type) {
> + case LANDLOCK_OBJECT_INODE:
> + break;
> + default:
> + WARN_ON_ONCE(1);
> + }
> + return true;
> +}
> +
> +static void put_object_cleaner(struct landlock_object *object)
> + __releases(object->cleaners)
> +{
> + /* Let's try an early lockless check. */
> + if (list_empty(&object->rules) &&
> + READ_ONCE(object->underlying_object)) {
> + /*
> + * Puts @object if there is no rule tied to it and the
> + * remaining user is the underlying object. This check is
> + * atomic because @object->rules and @object->underlying_object
> + * are protected by @object->lock.
> + */
> + spin_lock(&object->lock);
> + if (list_empty(&object->rules) &&
> + READ_ONCE(object->underlying_object) &&
> + refcount_dec_if_one(&object->usage)) {
> + /*
> + * Releases @object, in place of
> + * landlock_release_object().
> + *
> + * @object is already empty, implying that all its
> + * previous rules are already disabled.
> + *
> + * Unbalance the @object->cleaners counter to reflect
> + * the underlying object release.
> + */
> + if (!WARN_ON_ONCE(!release_object(object))) {
Two ! hurt more than help.
> + __acquire(object->cleaners);
> + put_object_free(object);
Why put object more than once?
> + }
> + } else {
> + spin_unlock(&object->lock);
> + }
> + }
> + put_object_free(object);
> +}
> +
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