Re: [RFC PATCH v3 3/4] X86/sgx: Introduce EMA as a new LSM module

[Casey Schaufler <casey@schaufler-ca.com>]

On 7/7/2019 4:41 PM, Cedric Xing wrote:
> As enclave pages have different lifespan than the existing MAP_PRIVATE and
> MAP_SHARED pages, a new data structure is required outside of VMA to track
> their protections and/or origins. Enclave Memory Area (or EMA for short) has
> been introduced to address the need. EMAs are maintained by a new LSM module
> named “ema”, which is similar to the idea of the “capability” LSM module.

First off, I'll say that this is an improvement over
the LSM integrated version that preceded it. I still
have some issues with the naming, but I'll address that
inline. I do have a suggestion that I think will make
this more conventional.

In this scheme you use an ema LSM to manage your ema data.
A quick sketch looks like:

	sgx_something_in() calls
		security_enclave_load() calls
			ema_enclave_load()
			selinux_enclave_load()
			otherlsm_enclave_load()

Why is this better than:

	sgx_something_in() calls
		ema_enclave_load()
		security_enclave_load() calls
			selinux_enclave_load()
			otherlsm_enclave_load()


If you did really want ema to behave like an LSM
you would put the file data that SELinux is managing
into the ema portion of the blob and provide interfaces
for the SELinux (or whoever) to use that. Also, it's
an abomination (as I've stated before) for ema to
rely on SELinux to provide a file_free() hook for
ema's data. If you continue down the LSM route, you
need to provide an ema_file_free() hook. You can't
count on SELinux to do it for you. If there are multiple
LSMs (coming soon!) that use the ema data, they'll all
try to free it, and then Bad Things can happen.

>
> This new “ema” module has LSM_ORDER_FIRST so will always be dispatched before
> other LSM_ORDER_MUTABLE modules (e.g. selinux, apparmor, etc.). It is
> responsible for initializing EMA maps, and inserting and freeing EMA nodes, and
> offers APIs for other LSM modules to query/update EMAs. Details could be found
> in include/linux/lsm_ema.h and security/commonema.c.
>
> Signed-off-by: Cedric Xing <cedric.xing@intel.com>
> ---
>  include/linux/lsm_ema.h |  97 ++++++++++++++

I still think this should be enclave.h (or commonema.h)
as it is not LSM code.

>  security/Makefile       |   1 +
>  security/commonema.c    | 277 ++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 375 insertions(+)
>  create mode 100644 include/linux/lsm_ema.h
>  create mode 100644 security/commonema.c
>
> diff --git a/include/linux/lsm_ema.h b/include/linux/lsm_ema.h
> new file mode 100644
> index 000000000000..59fc4ea6fa78
> --- /dev/null
> +++ b/include/linux/lsm_ema.h
> @@ -0,0 +1,97 @@
> +/* SPDX-License-Identifier: (GPL-2.0 OR BSD-3-Clause) */
> +/**
> + * Enclave Memory Area interface for LSM modules
> + *
> + * Copyright(c) 2016-19 Intel Corporation.
> + */
> +
> +#ifndef _LSM_EMA_H_
> +#define _LSM_EMA_H_
> +
> +#include <linux/list.h>
> +#include <linux/mutex.h>
> +#include <linux/fs.h>
> +#include <linux/file.h>
> +
> +/**
> + * ema - Enclave Memory Area structure for LSM modules

LSM modules is redundant. "LSM" or "LSMs" would be better.

> + *
> + * Data structure to track origins of enclave pages
> + *
> + * @link:
> + *	Link to adjacent EMAs. EMAs are sorted by their addresses in ascending
> + *	order
> + * @start:
> + *	Starting address
> + * @end:
> + *	Ending address
> + * @source:
> + *	File from which this range was loaded from, or NULL if not loaded from
> + *	any files
> + */
> +struct ema {
> +	struct list_head	link;
> +	size_t			start;
> +	size_t			end;
> +	struct file		*source;
> +};
> +
> +#define ema_data(ema, offset)	\
> +	((void *)((char *)((struct ema *)(ema) + 1) + offset))
> +
> +/**
> + * ema_map - LSM Enclave Memory Map structure for LSM modules

As above.

> + *
> + * Container for EMAs of an enclave
> + *
> + * @list:
> + *	Head of a list of sorted EMAs
> + * @lock:
> + *	Acquire before querying/updateing the list EMAs
> + */
> +struct ema_map {
> +	struct list_head	list;
> +	struct mutex		lock;
> +};
> +
> +size_t __init ema_request_blob(size_t blob_size);
> +struct ema_map *ema_get_map(struct file *encl);
> +int ema_apply_to_range(struct ema_map *map, size_t start, size_t end,
> +		       int (*cb)(struct ema *ema, void *arg), void *arg);
> +void ema_remove_range(struct ema_map *map, size_t start, size_t end);
> +
> +static inline int __must_check ema_lock_map(struct ema_map *map)
> +{
> +	return mutex_lock_interruptible(&map->lock);
> +}
> +
> +static inline void ema_unlock_map(struct ema_map *map)
> +{
> +	mutex_unlock(&map->lock);
> +}
> +
> +static inline int ema_lock_apply_to_range(struct ema_map *map,
> +					  size_t start, size_t end,
> +					  int (*cb)(struct ema *, void *),
> +					  void *arg)
> +{
> +	int rc = ema_lock_map(map);
> +	if (!rc) {
> +		rc = ema_apply_to_range(map, start, end, cb, arg);
> +		ema_unlock_map(map);
> +	}
> +	return rc;
> +}
> +
> +static inline int ema_lock_remove_range(struct ema_map *map,
> +					size_t start, size_t end)
> +{
> +	int rc = ema_lock_map(map);
> +	if (!rc) {
> +		ema_remove_range(map, start, end);
> +		ema_unlock_map(map);
> +	}
> +	return rc;
> +}
> +
> +#endif /* _LSM_EMA_H_ */
> diff --git a/security/Makefile b/security/Makefile
> index c598b904938f..b66d03a94853 100644
> --- a/security/Makefile
> +++ b/security/Makefile
> @@ -28,6 +28,7 @@ obj-$(CONFIG_SECURITY_YAMA)		+= yama/
>  obj-$(CONFIG_SECURITY_LOADPIN)		+= loadpin/
>  obj-$(CONFIG_SECURITY_SAFESETID)       += safesetid/
>  obj-$(CONFIG_CGROUP_DEVICE)		+= device_cgroup.o
> +obj-$(CONFIG_INTEL_SGX)			+= commonema.o

The config option and the file name ought to match,
or at least be closer.

>  
>  # Object integrity file lists
>  subdir-$(CONFIG_INTEGRITY)		+= integrity
> diff --git a/security/commonema.c b/security/commonema.c

Put this in a subdirectory. Please.

> new file mode 100644
> index 000000000000..c5b0bdfdc013
> --- /dev/null
> +++ b/security/commonema.c
> @@ -0,0 +1,277 @@
> +// SPDX-License-Identifier: (GPL-2.0 OR BSD-3-Clause)
> +// Copyright(c) 2016-18 Intel Corporation.
> +
> +#include <linux/lsm_ema.h>
> +#include <linux/lsm_hooks.h>
> +#include <linux/slab.h>
> +
> +static struct kmem_cache *_map_cache;
> +static struct kmem_cache *_node_cache;
> +static size_t _data_size __lsm_ro_after_init;
> +
> +static struct lsm_blob_sizes ema_blob_sizes __lsm_ro_after_init = {
> +	.lbs_file = sizeof(atomic_long_t),
> +};

If this is ema's data ema must manage it. You *must* have
a file_free().

> +
> +static atomic_long_t *_map_file(struct file *encl)
> +{
> +	return (void *)((char *)(encl->f_security) + ema_blob_sizes.lbs_file);

I don't trust all the casting going on here, especially since
you don't end up with the type you should be returning.

> +}
> +
> +static struct ema_map *_alloc_map(void)

Function header comments, please.

> +{
> +	struct ema_map *m;
> +
> +	m = kmem_cache_zalloc(_map_cache, GFP_KERNEL);
> +	if (likely(m)) {
> +		INIT_LIST_HEAD(&m->list);
> +		mutex_init(&m->lock);
> +	}
> +	return m;
> +}
> +
> +static struct ema *_new_ema(size_t start, size_t end, struct file *src)
> +{
> +	struct ema *ema;
> +
> +	if (unlikely(!_node_cache)) {
> +		struct kmem_cache *c;
> +
> +		c = kmem_cache_create("lsm-ema", sizeof(*ema) + _data_size,
> +				      __alignof__(typeof(*ema)), SLAB_PANIC,
> +				      NULL);
> +		if (atomic_long_cmpxchg((atomic_long_t *)&_node_cache, 0,
> +					(long)c))
> +			kmem_cache_destroy(c);
> +	}
> +
> +	ema = kmem_cache_zalloc(_node_cache, GFP_KERNEL);
> +	if (likely(ema)) {
> +		INIT_LIST_HEAD(&ema->link);
> +		ema->start = start;
> +		ema->end = end;
> +		if (src)
> +			ema->source = get_file(src);
> +	}
> +	return ema;
> +}
> +
> +static void _free_ema(struct ema *ema)
> +{
> +	if (ema->source)
> +		fput(ema->source);
> +	kmem_cache_free(_node_cache, ema);
> +}
> +
> +static void _free_map(struct ema_map *map)
> +{
> +	struct ema *p, *n;
> +
> +	WARN_ON(mutex_is_locked(&map->lock));
> +	list_for_each_entry_safe(p, n, &map->list, link)
> +		_free_ema(p);
> +	kmem_cache_free(_map_cache, map);
> +}
> +
> +static struct ema_map *_init_map(struct file *encl)
> +{
> +	struct ema_map *m = ema_get_map(encl);
> +	if (!m) {
> +		m = _alloc_map();
> +		if (atomic_long_cmpxchg(_map_file(encl), 0, (long)m)) {
> +			_free_map(m);
> +			m = ema_get_map(encl);
> +		}
> +	}
> +	return m;
> +}
> +
> +static inline struct ema *_next_ema(struct ema *p, struct ema_map *map)
> +{
> +	p = list_next_entry(p, link);
> +	return &p->link == &map->list ? NULL : p;
> +}
> +
> +static inline struct ema *_find_ema(struct ema_map *map, size_t a)
> +{
> +	struct ema *p;
> +
> +	WARN_ON(!mutex_is_locked(&map->lock));
> +
> +	list_for_each_entry(p, &map->list, link)
> +		if (a < p->end)
> +			break;
> +	return &p->link == &map->list ? NULL : p;
> +}
> +
> +static struct ema *_split_ema(struct ema *p, size_t at)
> +{
> +	typeof(p) n;
> +
> +	if (at <= p->start || at >= p->end)
> +		return p;
> +
> +	n = _new_ema(p->start, at, p->source);
> +	if (likely(n)) {
> +		memcpy(n + 1, p + 1, _data_size);
> +		p->start = at;
> +		list_add_tail(&n->link, &p->link);
> +	}
> +	return n;
> +}
> +
> +static int _merge_ema(struct ema *p, struct ema_map *map)
> +{
> +	typeof(p) prev = list_prev_entry(p, link);
> +
> +	WARN_ON(!mutex_is_locked(&map->lock));
> +
> +	if (&prev->link == &map->list || prev->end != p->start ||
> +	    prev->source != p->source || memcmp(prev + 1, p + 1, _data_size))
> +		return 0;
> +
> +	p->start = prev->start;
> +	fput(prev->source);
> +	_free_ema(prev);
> +	return 1;
> +}
> +
> +static inline int _insert_ema(struct ema_map *map, struct ema *n)
> +{
> +	typeof(n) p = _find_ema(map, n->start);
> +
> +	if (!p)
> +		list_add_tail(&n->link, &map->list);
> +	else if (n->end <= p->start)
> +		list_add_tail(&n->link, &p->link);
> +	else
> +		return -EEXIST;
> +
> +	_merge_ema(n, map);
> +	if (p)
> +		_merge_ema(p, map);
> +	return 0;
> +}
> +
> +static void ema_file_free_security(struct file *encl)
> +{
> +	struct ema_map *m = ema_get_map(encl);
> +	if (m)
> +		_free_map(m);
> +}
> +
> +static int ema_enclave_load(struct file *encl, size_t start, size_t end,
> +			    size_t flags, struct vm_area_struct *vma)
> +{
> +	struct ema_map *m;
> +	struct ema *ema;
> +	int rc;
> +
> +	m = _init_map(encl);
> +	if (unlikely(!m))
> +		return -ENOMEM;
> +
> +	ema = _new_ema(start, end, vma ? vma->vm_file : NULL);
> +	if (unlikely(!ema))
> +		return -ENOMEM;
> +
> +	rc = ema_lock_map(m);
> +	if (!rc) {
> +		rc = _insert_ema(m, ema);
> +		ema_unlock_map(m);
> +	}
> +	if (rc)
> +		_free_ema(ema);
> +	return rc;
> +}
> +
> +static int ema_enclave_init(struct file *encl, struct sgx_sigstruct *sigstruct,
> +			    struct vm_area_struct *vma)
> +{
> +	if (unlikely(!_init_map(encl)))
> +		return -ENOMEM;
> +	return 0;
> +}
> +
> +static struct security_hook_list ema_hooks[] __lsm_ro_after_init = {
> +	LSM_HOOK_INIT(file_free_security, ema_file_free_security),
> +	LSM_HOOK_INIT(enclave_load, ema_enclave_load),
> +	LSM_HOOK_INIT(enclave_init, ema_enclave_init),
> +};
> +
> +static int __init ema_init(void)
> +{
> +	_map_cache = kmem_cache_create("lsm-ema_map", sizeof(struct ema_map),
> +				       __alignof__(struct ema_map), SLAB_PANIC,
> +				       NULL);
> +	security_add_hooks(ema_hooks, ARRAY_SIZE(ema_hooks), "ema");
> +	return 0;
> +}
> +
> +DEFINE_LSM(ema) = {
> +	.name = "ema",
> +	.order = LSM_ORDER_FIRST,
> +	.init = ema_init,
> +	.blobs = &ema_blob_sizes,
> +};
> +
> +/* ema_request_blob shall only be called from LSM module init function */
> +size_t __init ema_request_blob(size_t size)
> +{
> +	typeof(_data_size) offset = _data_size;
> +	_data_size += size;
> +	return offset;
> +}
> +
> +struct ema_map *ema_get_map(struct file *encl)
> +{
> +	return (struct ema_map *)atomic_long_read(_map_file(encl));
> +}
> +
> +/**
> + * Invoke a callback function on every EMA falls within range, split EMAs as
> + * needed
> + */
> +int ema_apply_to_range(struct ema_map *map, size_t start, size_t end,
> +		       int (*cb)(struct ema *, void *), void *arg)
> +{
> +	struct ema *ema;
> +	int rc;
> +
> +	ema = _find_ema(map, start);
> +	while (ema && end > ema->start) {
> +		if (start > ema->start)
> +			_split_ema(ema, start);
> +		if (end < ema->end)
> +			ema = _split_ema(ema, end);
> +
> +		rc = (*cb)(ema, arg);
> +		_merge_ema(ema, map);
> +		if (rc)
> +			return rc;
> +
> +		ema = _next_ema(ema, map);
> +	}
> +
> +	if (ema)
> +		_merge_ema(ema, map);
> +	return 0;
> +}
> +
> +/* Remove all EMAs falling within range, split EMAs as needed */
> +void ema_remove_range(struct ema_map *map, size_t start, size_t end)
> +{
> +	struct ema *ema, *n;
> +
> +	ema = _find_ema(map, start);
> +	while (ema && end > ema->start) {
> +		if (start > ema->start)
> +			_split_ema(ema, start);
> +		if (end < ema->end)
> +			ema = _split_ema(ema, end);
> +
> +		n = _next_ema(ema, map);
> +		_free_ema(ema);
> +		ema = n;
> +	}
> +}