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

Kees Cook
2021-03-19T18:14:18Z

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.

> + 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>

--
Kees Cook