btrfs: convert extent_state.refs from atomic_t to refcount_t

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -68,7 +68,7 @@ void btrfs_leak_debug_check(void)
 		pr_err("BTRFS: state leak: start %llu end %llu state %u in tree %d refs %d\n",
 		       state->start, state->end, state->state,
 		       extent_state_in_tree(state),
-		       atomic_read(&state->refs));
+		       refcount_read(&state->refs));
 		list_del(&state->leak_list);
 		kmem_cache_free(extent_state_cache, state);
 	}
@@ -238,7 +238,7 @@ static struct extent_state *alloc_extent_state(gfp_t mask)
 	state->failrec = NULL;
 	RB_CLEAR_NODE(&state->rb_node);
 	btrfs_leak_debug_add(&state->leak_list, &states);
-	atomic_set(&state->refs, 1);
+	refcount_set(&state->refs, 1);
 	init_waitqueue_head(&state->wq);
 	trace_alloc_extent_state(state, mask, _RET_IP_);
 	return state;
@@ -248,7 +248,7 @@ void free_extent_state(struct extent_state *state)
 {
 	if (!state)
 		return;
-	if (atomic_dec_and_test(&state->refs)) {
+	if (refcount_dec_and_test(&state->refs)) {
 		WARN_ON(extent_state_in_tree(state));
 		btrfs_leak_debug_del(&state->leak_list);
 		trace_free_extent_state(state, _RET_IP_);
@@ -641,7 +641,7 @@ again:
 		if (cached && extent_state_in_tree(cached) &&
 		    cached->start <= start && cached->end > start) {
 			if (clear)
-				atomic_dec(&cached->refs);
+				refcount_dec(&cached->refs);
 			state = cached;
 			goto hit_next;
 		}
@@ -793,7 +793,7 @@ process_node:
 
 		if (state->state & bits) {
 			start = state->start;
-			atomic_inc(&state->refs);
+			refcount_inc(&state->refs);
 			wait_on_state(tree, state);
 			free_extent_state(state);
 			goto again;
@@ -834,7 +834,7 @@ static void cache_state_if_flags(struct extent_state *state,
 	if (cached_ptr && !(*cached_ptr)) {
 		if (!flags || (state->state & flags)) {
 			*cached_ptr = state;
-			atomic_inc(&state->refs);
+			refcount_inc(&state->refs);
 		}
 	}
 }
@@ -1538,7 +1538,7 @@ static noinline u64 find_delalloc_range(struct extent_io_tree *tree,
 		if (!found) {
 			*start = state->start;
 			*cached_state = state;
-			atomic_inc(&state->refs);
+			refcount_inc(&state->refs);
 		}
 		found++;
 		*end = state->end;

fs/btrfs/extent_io.h

@@ -2,6 +2,7 @@
 #define __EXTENTIO__
 
 #include <linux/rbtree.h>
+#include <linux/refcount.h>
 #include "ulist.h"
 
 /* bits for the extent state */
@@ -143,7 +144,7 @@ struct extent_state {
 
 	/* ADD NEW ELEMENTS AFTER THIS */
 	wait_queue_head_t wq;
-	atomic_t refs;
+	refcount_t refs;
 	unsigned state;
 
 	struct io_failure_record *failrec;