bpf: Do btf_record_free outside map_free callback

Since the commit being fixed, we now miss freeing btf_record for local
storage maps which will have a btf_record populated in case they have
bpf_spin_lock element.

This was missed because I made the choice of offloading the job to free
kptr_off_tab (now btf_record) to the map_free callback when adding
support for kptrs.

Revisiting the reason for this decision, there is the possibility that
the btf_record gets used inside map_free callback (e.g. in case of maps
embedding kptrs) to iterate over them and free them, hence doing it
before the map_free callback would be leaking special field memory, and
do invalid memory access. The btf_record keeps module references which
is critical to ensure the dtor call made for referenced kptr is safe to
do.

If doing it after map_free callback, the map area is already freed, so
we cannot access bpf_map structure anymore.

To fix this and prevent such lapses in future, move bpf_map_free_record
out of the map_free callback, and do it after map_free by remembering
the btf_record pointer. There is no need to access bpf_map structure in
that case, and we can avoid missing this case when support for new map
types is added for other special fields.

Since a btf_record and its btf_field_offs are used together, for
consistency delay freeing of field_offs as well. While not a problem
right now, a lot of code assumes that either both record and field_offs
are set or none at once.

Note that in case of map of maps (outer maps), inner_map_meta->record is
only used during verification, not to free fields in map value, hence we
simply keep the bpf_map_free_record call as is in bpf_map_meta_free and
never touch map->inner_map_meta in bpf_map_free_deferred.

Add a comment making note of these details.

Fixes: db559117828d ("bpf: Consolidate spin_lock, timer management into btf_record")
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20221118015614.2013203-3-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/arraymap.c

@@ -430,7 +430,6 @@ static void array_map_free(struct bpf_map *map)
 			for (i = 0; i < array->map.max_entries; i++)
 				bpf_obj_free_fields(map->record, array_map_elem_ptr(array, i));
 		}
-		bpf_map_free_record(map);
 	}
 
 	if (array->map.map_type == BPF_MAP_TYPE_PERCPU_ARRAY)

kernel/bpf/hashtab.c

@@ -1511,7 +1511,6 @@ static void htab_map_free(struct bpf_map *map)
 		prealloc_destroy(htab);
 	}
 
-	bpf_map_free_record(map);
 	free_percpu(htab->extra_elems);
 	bpf_map_area_free(htab->buckets);
 	bpf_mem_alloc_destroy(&htab->pcpu_ma);

kernel/bpf/syscall.c

@@ -659,14 +659,24 @@ void bpf_obj_free_fields(const struct btf_record *rec, void *obj)
 static void bpf_map_free_deferred(struct work_struct *work)
 {
 	struct bpf_map *map = container_of(work, struct bpf_map, work);
+	struct btf_field_offs *foffs = map->field_offs;
+	struct btf_record *rec = map->record;
 
 	security_bpf_map_free(map);
-	kfree(map->field_offs);
 	bpf_map_release_memcg(map);
-	/* implementation dependent freeing, map_free callback also does
+	/* implementation dependent freeing */
-	 * bpf_map_free_record, if needed.
-	 */
 	map->ops->map_free(map);
+	/* Delay freeing of field_offs and btf_record for maps, as map_free
+	 * callback usually needs access to them. It is better to do it here
+	 * than require each callback to do the free itself manually.
+	 *
+	 * Note that the btf_record stashed in map->inner_map_meta->record was
+	 * already freed using the map_free callback for map in map case which
+	 * eventually calls bpf_map_free_meta, since inner_map_meta is only a
+	 * template bpf_map struct used during verification.
+	 */
+	kfree(foffs);
+	btf_record_free(rec);
 }
 
 static void bpf_map_put_uref(struct bpf_map *map)