Message-ID: <ec5e6c0c-2979-7334-6f40-b46b8ec319ce@digikod.net>
Date:   Fri, 19 Mar 2021 19:57:41 +0100
From:   Mickaël Salaün <mic@...ikod.net>
To:     Kees Cook <keescook@...omium.org>
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>,
        David Howells <dhowells@...hat.com>,
        Jeff Dike <jdike@...toit.com>,
        Jonathan Corbet <corbet@....net>,
        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 v30 01/12] landlock: Add object management


On 19/03/2021 19:13, Kees Cook wrote:
> On Tue, Mar 16, 2021 at 09:42:41PM +0100, Mickaël Salaün wrote:
>> From: Mickaël Salaün <mic@...ux.microsoft.com>
>>
>> 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 cannot 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 objects.  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.
>>
>> Cc: James Morris <jmorris@...ei.org>
>> Cc: Kees Cook <keescook@...omium.org>
>> Signed-off-by: Mickaël Salaün <mic@...ux.microsoft.com>
>> Reviewed-by: Jann Horn <jannh@...gle.com>
>> Acked-by: Serge Hallyn <serge@...lyn.com>
>> Link: https://lore.kernel.org/r/20210316204252.427806-2-mic@digikod.net
>> ---
>>
>> Changes since v28:
>> * Improve Kconfig description (suggested by Serge Hallyn).
>> * Add Acked-by Serge Hallyn.
>> * Clean up comment.
>>
>> Changes since v27:
>> * Update Kconfig for landlock_restrict_self(2).
>> * Cosmetic fixes: use 80 columns in Kconfig and align Makefile
>>   declarations.
>>
>> Changes since v26:
>> * Update Kconfig for landlock_enforce_ruleset_self(2).
>> * Fix spelling.
>>
>> Changes since v24:
>> * Fix typo in comment (spotted by Jann Horn).
>> * Add Reviewed-by: Jann Horn <jannh@...gle.com>
>>
>> Changes since v23:
>> * Update landlock_create_object() to return error codes instead of NULL.
>>   This help error handling in callers.
>> * When using make oldconfig with a previous configuration already
>>   including the CONFIG_LSM variable, no question is asked to update its
>>   content.  Update the Kconfig help to warn about LSM stacking
>>   configuration.
>> * Constify variable (spotted by Vincent Dagonneau).
>>
>> Changes since v22:
>> * Fix spelling (spotted by Jann Horn).
>>
>> Changes since v21:
>> * Update Kconfig help.
>> * Clean up comments.
>>
>> Changes since v18:
>> * Account objects to kmemcg.
>>
>> 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  | 21 +++++++++
>>  security/landlock/Makefile |  3 ++
>>  security/landlock/object.c | 67 ++++++++++++++++++++++++++++
>>  security/landlock/object.h | 91 ++++++++++++++++++++++++++++++++++++++
>>  7 files changed, 195 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 aa84121c5611..87a2738dfdec 100644
>> --- a/MAINTAINERS
>> +++ b/MAINTAINERS
>> @@ -9997,6 +9997,16 @@ 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@...ikod.net>
>> +L:	linux-security-module@...r.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@...ke-m.de>
>>  L:	netdev@...r.kernel.org
>> diff --git a/security/Kconfig b/security/Kconfig
>> index 7561f6f99f1d..15a4342b5d01 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 3baf435de541..47e432900e24 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-$(CONFIG_SECURITY_LANDLOCK)	+= landlock
>>  
>>  # always enable default capabilities
>>  obj-y					+= commoncap.o
>> @@ -32,6 +33,7 @@ obj-$(CONFIG_SECURITY_SAFESETID)       += safesetid/
>>  obj-$(CONFIG_SECURITY_LOCKDOWN_LSM)	+= lockdown/
>>  obj-$(CONFIG_CGROUPS)			+= device_cgroup.o
>>  obj-$(CONFIG_BPF_LSM)			+= bpf/
>> +obj-$(CONFIG_SECURITY_LANDLOCK)		+= landlock/
>>  
>>  # Object integrity file lists
>>  subdir-$(CONFIG_INTEGRITY)		+= integrity
>> diff --git a/security/landlock/Kconfig b/security/landlock/Kconfig
>> new file mode 100644
>> index 000000000000..c1e862a38410
>> --- /dev/null
>> +++ b/security/landlock/Kconfig
>> @@ -0,0 +1,21 @@
>> +# SPDX-License-Identifier: GPL-2.0-only
>> +
>> +config SECURITY_LANDLOCK
>> +	bool "Landlock support"
>> +	depends on SECURITY
>> +	select SECURITY_PATH
>> +	help
>> +	  Landlock is a sandboxing mechanism that enables processes to restrict
>> +	  themselves (and their future children) by gradually enforcing
>> +	  tailored access control policies.  A Landlock security policy is a
>> +	  set of access rights (e.g. open a file in read-only, make a
>> +	  directory, etc.) tied to a file hierarchy.  Such policy can be
>> +	  configured and enforced by any processes for themselves using the
>> +	  dedicated system calls: landlock_create_ruleset(),
>> +	  landlock_add_rule(), and landlock_restrict_self().
>> +
>> +	  See Documentation/userspace-api/landlock.rst for further information.
>> +
>> +	  If you are unsure how to answer this question, answer N.  Otherwise,
>> +	  you should also prepend "landlock," to the content of CONFIG_LSM to
>> +	  enable Landlock at boot time.
>> diff --git a/security/landlock/Makefile b/security/landlock/Makefile
>> new file mode 100644
>> index 000000000000..cb6deefbf4c0
>> --- /dev/null
>> +++ b/security/landlock/Makefile
>> @@ -0,0 +1,3 @@
>> +obj-$(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..d674fdf9ff04
>> --- /dev/null
>> +++ b/security/landlock/object.c
>> @@ -0,0 +1,67 @@
>> +// SPDX-License-Identifier: GPL-2.0-only
>> +/*
>> + * Landlock LSM - Object management
>> + *
>> + * Copyright © 2016-2020 Mickaël Salaün <mic@...ikod.net>
>> + * Copyright © 2018-2020 ANSSI
>> + */
>> +
>> +#include <linux/bug.h>
>> +#include <linux/compiler_types.h>
>> +#include <linux/err.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 *const underops,
>> +		void *const underobj)
>> +{
>> +	struct landlock_object *new_object;
>> +
>> +	if (WARN_ON_ONCE(!underops || !underobj))
>> +		return ERR_PTR(-ENOENT);
>> +	new_object = kzalloc(sizeof(*new_object), GFP_KERNEL_ACCOUNT);
> 
> Is there any benefit to using a dedicated kmem_cache instead of kmalloc?
> I see later that you end up with variable-sized allocations, so this
> might be fine as-is.

I though about that, but for the sake of simplicity I choose to not use
that in this series. It may comes with a future update if there is a
visible performance improvement though.

> 
>> +	if (!new_object)
>> +		return ERR_PTR(-ENOMEM);
>> +	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 cannot 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..3e5d5b6941c3
>> --- /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@...ikod.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 implies 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: Protects against concurrent modifications.  This lock must be
>> +	 * held 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 underobj);
>> +
>> +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 */
>> -- 
>> 2.30.2
>>
> 
> Either way, this object lifetime management looks okay to me:
> 
> Reviewed-by: Kees Cook <keescook@...omium.org>
> 

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