bpf: Fix map leak in HASH_OF_MAPS map

Fix HASH_OF_MAPS bug of not putting inner map pointer on bpf_map_elem_update()
operation. This is due to per-cpu extra_elems optimization, which bypassed
free_htab_elem() logic doing proper clean ups. Make sure that inner map is put
properly in optimized case as well.

Fixes: 8c290e60 ("bpf: fix hashmap extra_elems logic")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200729040913.2815687-1-andriin@fb.com

kernel/bpf/hashtab.c

@@ -779,15 +779,20 @@ static void htab_elem_free_rcu(struct rcu_head *head)
 	htab_elem_free(htab, l);
 }
 
-static void free_htab_elem(struct bpf_htab *htab, struct htab_elem *l)
+static void htab_put_fd_value(struct bpf_htab *htab, struct htab_elem *l)
 {
 	struct bpf_map *map = &htab->map;
+	void *ptr;
 
 	if (map->ops->map_fd_put_ptr) {
-		void *ptr = fd_htab_map_get_ptr(map, l);
-
+		ptr = fd_htab_map_get_ptr(map, l);
 		map->ops->map_fd_put_ptr(ptr);
 	}
+}
+
+static void free_htab_elem(struct bpf_htab *htab, struct htab_elem *l)
+{
+	htab_put_fd_value(htab, l);
 
 	if (htab_is_prealloc(htab)) {
 		__pcpu_freelist_push(&htab->freelist, &l->fnode);
@@ -839,6 +844,7 @@ static struct htab_elem *alloc_htab_elem(struct bpf_htab *htab, void *key,
 			 */
 			pl_new = this_cpu_ptr(htab->extra_elems);
 			l_new = *pl_new;
+			htab_put_fd_value(htab, old_elem);
 			*pl_new = old_elem;
 		} else {
 			struct pcpu_freelist_node *l;