Re: [PATCH v30 01/12] landlock: Add object management

Mickaël Salaün
2021-03-19T18:58:23Z

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@linux.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@namei.org>
>> Cc: Kees Cook <keescook@chromium.org>
>> Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
>> Reviewed-by: Jann Horn <jannh@google.com>
>> Acked-by: Serge Hallyn <serge@hallyn.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@google.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@digikod.net>
>> +L: linux-security-module@vger.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@hauke-m.de>
>> L: netdev@vger.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@digikod.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@digikod.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@chromium.org>
>