Re: [PATCH] mbcache: convert mb_cache_entry.e_refcnt from atomic_t to refcount_t

[Jan Kara <jack@suse.cz>]

On Wed 29-11-17 13:22:20, Elena Reshetova wrote:
> atomic_t variables are currently used to implement reference
> counters with the following properties:
>  - counter is initialized to 1 using atomic_set()
>  - a resource is freed upon counter reaching zero
>  - once counter reaches zero, its further
>    increments aren't allowed
>  - counter schema uses basic atomic operations
>    (set, inc, inc_not_zero, dec_and_test, etc.)
> 
> Such atomic variables should be converted to a newly provided
> refcount_t type and API that prevents accidental counter overflows
> and underflows. This is important since overflows and underflows
> can lead to use-after-free situation and be exploitable.
> 
> The variable mb_cache_entry.e_refcnt is used as pure reference counter.
> Convert it to refcount_t and fix up the operations.
> 
> **Important note for maintainers:
> 
> Some functions from refcount_t API defined in lib/refcount.c
> have different memory ordering guarantees than their atomic
> counterparts.
> The full comparison can be seen in
> https://lkml.org/lkml/2017/11/15/57 and it is hopefully soon
> in state to be merged to the documentation tree.
> Normally the differences should not matter since refcount_t provides
> enough guarantees to satisfy the refcounting use cases, but in
> some rare cases it might matter.
> Please double check that you don't have some undocumented
> memory guarantees for this variable usage.
> 
> For the mb_cache_entry.e_refcnt it might make a difference
> in following places:
>  - mb_cache_destroy(), mb_cache_shrink() and mb_cache_entry_delete():
>    decrement in refcount_dec() provides RELEASE ordering vs. fully
>    unordered atomic counterpart. Since the change is for better, it
>    should not matter for these cases.
>  - mb_cache_entry_put(): decrement in refcount_dec_and_test() only
>    provides RELEASE ordering and control dependency on success
>    vs. fully ordered atomic counterpart
> 
> Suggested-by: Kees Cook <keescook@chromium.org>
> Reviewed-by: David Windsor <dwindsor@gmail.com>
> Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
> Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>

The patch looks good to me. You can add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
>  fs/mbcache.c            | 16 ++++++++--------
>  include/linux/mbcache.h |  6 +++---
>  2 files changed, 11 insertions(+), 11 deletions(-)
> 
> diff --git a/fs/mbcache.c b/fs/mbcache.c
> index d818fd2..8bdb52b 100644
> --- a/fs/mbcache.c
> +++ b/fs/mbcache.c
> @@ -90,7 +90,7 @@ int mb_cache_entry_create(struct mb_cache *cache, gfp_t mask, u32 key,
>  
>  	INIT_LIST_HEAD(&entry->e_list);
>  	/* One ref for hash, one ref returned */
> -	atomic_set(&entry->e_refcnt, 1);
> +	refcount_set(&entry->e_refcnt, 1);
>  	entry->e_key = key;
>  	entry->e_value = value;
>  	entry->e_reusable = reusable;
> @@ -109,7 +109,7 @@ int mb_cache_entry_create(struct mb_cache *cache, gfp_t mask, u32 key,
>  	spin_lock(&cache->c_list_lock);
>  	list_add_tail(&entry->e_list, &cache->c_list);
>  	/* Grab ref for LRU list */
> -	atomic_inc(&entry->e_refcnt);
> +	refcount_inc(&entry->e_refcnt);
>  	cache->c_entry_count++;
>  	spin_unlock(&cache->c_list_lock);
>  
> @@ -141,7 +141,7 @@ static struct mb_cache_entry *__entry_find(struct mb_cache *cache,
>  		entry = hlist_bl_entry(node, struct mb_cache_entry,
>  				       e_hash_list);
>  		if (entry->e_key == key && entry->e_reusable) {
> -			atomic_inc(&entry->e_refcnt);
> +			refcount_inc(&entry->e_refcnt);
>  			goto out;
>  		}
>  		node = node->next;
> @@ -204,7 +204,7 @@ struct mb_cache_entry *mb_cache_entry_get(struct mb_cache *cache, u32 key,
>  	hlist_bl_lock(head);
>  	hlist_bl_for_each_entry(entry, node, head, e_hash_list) {
>  		if (entry->e_key == key && entry->e_value == value) {
> -			atomic_inc(&entry->e_refcnt);
> +			refcount_inc(&entry->e_refcnt);
>  			goto out;
>  		}
>  	}
> @@ -239,7 +239,7 @@ void mb_cache_entry_delete(struct mb_cache *cache, u32 key, u64 value)
>  			if (!list_empty(&entry->e_list)) {
>  				list_del_init(&entry->e_list);
>  				cache->c_entry_count--;
> -				atomic_dec(&entry->e_refcnt);
> +				refcount_dec(&entry->e_refcnt);
>  			}
>  			spin_unlock(&cache->c_list_lock);
>  			mb_cache_entry_put(cache, entry);
> @@ -300,7 +300,7 @@ static unsigned long mb_cache_shrink(struct mb_cache *cache,
>  		hlist_bl_lock(head);
>  		if (!hlist_bl_unhashed(&entry->e_hash_list)) {
>  			hlist_bl_del_init(&entry->e_hash_list);
> -			atomic_dec(&entry->e_refcnt);
> +			refcount_dec(&entry->e_refcnt);
>  		}
>  		hlist_bl_unlock(head);
>  		if (mb_cache_entry_put(cache, entry))
> @@ -397,11 +397,11 @@ void mb_cache_destroy(struct mb_cache *cache)
>  	list_for_each_entry_safe(entry, next, &cache->c_list, e_list) {
>  		if (!hlist_bl_unhashed(&entry->e_hash_list)) {
>  			hlist_bl_del_init(&entry->e_hash_list);
> -			atomic_dec(&entry->e_refcnt);
> +			refcount_dec(&entry->e_refcnt);
>  		} else
>  			WARN_ON(1);
>  		list_del(&entry->e_list);
> -		WARN_ON(atomic_read(&entry->e_refcnt) != 1);
> +		WARN_ON(refcount_read(&entry->e_refcnt) != 1);
>  		mb_cache_entry_put(cache, entry);
>  	}
>  	kfree(cache->c_hash);
> diff --git a/include/linux/mbcache.h b/include/linux/mbcache.h
> index 20f1e3f..0bf2af6 100644
> --- a/include/linux/mbcache.h
> +++ b/include/linux/mbcache.h
> @@ -5,7 +5,7 @@
>  #include <linux/hash.h>
>  #include <linux/list_bl.h>
>  #include <linux/list.h>
> -#include <linux/atomic.h>
> +#include <linux/refcount.h>
>  #include <linux/fs.h>
>  
>  struct mb_cache;
> @@ -15,7 +15,7 @@ struct mb_cache_entry {
>  	struct list_head	e_list;
>  	/* Hash table list - protected by hash chain bitlock */
>  	struct hlist_bl_node	e_hash_list;
> -	atomic_t		e_refcnt;
> +	refcount_t		e_refcnt;
>  	/* Key in hash - stable during lifetime of the entry */
>  	u32			e_key;
>  	u32			e_referenced:1;
> @@ -33,7 +33,7 @@ void __mb_cache_entry_free(struct mb_cache_entry *entry);
>  static inline int mb_cache_entry_put(struct mb_cache *cache,
>  				     struct mb_cache_entry *entry)
>  {
> -	if (!atomic_dec_and_test(&entry->e_refcnt))
> +	if (!refcount_dec_and_test(&entry->e_refcnt))
>  		return 0;
>  	__mb_cache_entry_free(entry);
>  	return 1;
> -- 
> 2.7.4
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR