On 27/02/2020 05:20, Hillf Danton wrote: > > 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@xxxxxxxxxxx> >> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx> >> Cc: James Morris <jmorris@xxxxxxxxx> >> Cc: Kees Cook <keescook@xxxxxxxxxxxx> >> Cc: Serge E. Hallyn <serge@xxxxxxxxxx> >> --- >> >> Changes since v13: >> * New dedicated implementation, removing the need for eBPF. >> >> Previous version: >> https://lore.kernel.org/lkml/20190721213116.23476-6-mic@xxxxxxxxxxx/ >> --- >> 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@xxxxxxxxxxx> >> +L: linux-security-module@xxxxxxxxxxxxxxx >> +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@xxxxxxxxxx> >> L: netdev@xxxxxxxxxxxxxxx >> 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@xxxxxxxxxxx> >> + * 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. Ok. This is to have a guarantee that the underlying_object (e.g. the inode pointer) is only used once. I'll add a comment. > >> + spin_unlock(&object->lock); >> + might_sleep(); > > Have trouble working out what might_sleep is put for. Patch 5 adds a call to landlock_release_inode(underlying_object, object) (LANDLOCK_OBJECT_INODE case), which can sleep e.g., with a call to iput(). > >> + 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. Well, it may not look nice but don't you think it is better than a WARN_ON_ONCE(1) in the if block? >> + __acquire(object->cleaners); >> + put_object_free(object); > > Why put object more than once? I just replied to Jann about this subject. This is to "unbalance" the counter to potentially free it (if there is no more user). I explain it here: https://lore.kernel.org/lkml/67465638-e22c-5d1a-df37-862b31d999a1@xxxxxxxxxxx/ > >> + } >> + } else { >> + spin_unlock(&object->lock); >> + } >> + } >> + put_object_free(object); >> +} >> + >
