bpf: Verify ownership relationships for user BTF types

Ensure that there can be no ownership cycles among different types by
way of having owning objects that can hold some other type as their
element. For instance, a map value can only hold allocated objects, but
these are allowed to have another bpf_list_head. To prevent unbounded
recursion while freeing resources, elements of bpf_list_head in local
kptrs can never have a bpf_list_head which are part of list in a map
value. Later patches will verify this by having dedicated BTF selftests.

Also, to make runtime destruction easier, once btf_struct_metas is fully
populated, we can stash the metadata of the value type directly in the
metadata of the list_head fields, as that allows easier access to the
value type's layout to destruct it at runtime from the btf_field entry
of the list head itself.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20221118015614.2013203-8-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/bpf.h

@@ -191,6 +191,7 @@ struct btf_field_list_head {
 	struct btf *btf;
 	u32 value_btf_id;
 	u32 node_offset;
+	struct btf_record *value_rec;
 };
 
 struct btf_field {

include/linux/btf.h

@@ -178,6 +178,7 @@ int btf_find_spin_lock(const struct btf *btf, const struct btf_type *t);
 int btf_find_timer(const struct btf *btf, const struct btf_type *t);
 struct btf_record *btf_parse_fields(const struct btf *btf, const struct btf_type *t,
 				    u32 field_mask, u32 value_size);
+int btf_check_and_fixup_fields(const struct btf *btf, struct btf_record *rec);
 struct btf_field_offs *btf_parse_field_offs(struct btf_record *rec);
 bool btf_type_is_void(const struct btf_type *t);
 s32 btf_find_by_name_kind(const struct btf *btf, const char *name, u8 kind);

kernel/bpf/btf.c

@@ -3723,6 +3723,67 @@ end:
 	return ERR_PTR(ret);
 }
 
+int btf_check_and_fixup_fields(const struct btf *btf, struct btf_record *rec)
+{
+	int i;
+
+	/* There are two owning types, kptr_ref and bpf_list_head. The former
+	 * only supports storing kernel types, which can never store references
+	 * to program allocated local types, atleast not yet. Hence we only need
+	 * to ensure that bpf_list_head ownership does not form cycles.
+	 */
+	if (IS_ERR_OR_NULL(rec) || !(rec->field_mask & BPF_LIST_HEAD))
+		return 0;
+	for (i = 0; i < rec->cnt; i++) {
+		struct btf_struct_meta *meta;
+		u32 btf_id;
+
+		if (!(rec->fields[i].type & BPF_LIST_HEAD))
+			continue;
+		btf_id = rec->fields[i].list_head.value_btf_id;
+		meta = btf_find_struct_meta(btf, btf_id);
+		if (!meta)
+			return -EFAULT;
+		rec->fields[i].list_head.value_rec = meta->record;
+
+		if (!(rec->field_mask & BPF_LIST_NODE))
+			continue;
+
+		/* We need to ensure ownership acyclicity among all types. The
+		 * proper way to do it would be to topologically sort all BTF
+		 * IDs based on the ownership edges, since there can be multiple
+		 * bpf_list_head in a type. Instead, we use the following
+		 * reasoning:
+		 *
+		 * - A type can only be owned by another type in user BTF if it
+		 *   has a bpf_list_node.
+		 * - A type can only _own_ another type in user BTF if it has a
+		 *   bpf_list_head.
+		 *
+		 * We ensure that if a type has both bpf_list_head and
+		 * bpf_list_node, its element types cannot be owning types.
+		 *
+		 * To ensure acyclicity:
+		 *
+		 * When A only has bpf_list_head, ownership chain can be:
+		 *	A -> B -> C
+		 * Where:
+		 * - B has both bpf_list_head and bpf_list_node.
+		 * - C only has bpf_list_node.
+		 *
+		 * When A has both bpf_list_head and bpf_list_node, some other
+		 * type already owns it in the BTF domain, hence it can not own
+		 * another owning type through any of the bpf_list_head edges.
+		 *	A -> B
+		 * Where:
+		 * - B only has bpf_list_node.
+		 */
+		if (meta->record->field_mask & BPF_LIST_HEAD)
+			return -ELOOP;
+	}
+	return 0;
+}
+
 static int btf_field_offs_cmp(const void *_a, const void *_b, const void *priv)
 {
 	const u32 a = *(const u32 *)_a;
@@ -5413,6 +5474,16 @@ static struct btf *btf_parse(bpfptr_t btf_data, u32 btf_data_size,
 	}
 	btf->struct_meta_tab = struct_meta_tab;
 
+	if (struct_meta_tab) {
+		int i;
+
+		for (i = 0; i < struct_meta_tab->cnt; i++) {
+			err = btf_check_and_fixup_fields(btf, struct_meta_tab->types[i].record);
+			if (err < 0)
+				goto errout_meta;
+		}
+	}
+
 	if (log->level && bpf_verifier_log_full(log)) {
 		err = -ENOSPC;
 		goto errout_meta;

kernel/bpf/syscall.c

@@ -1054,6 +1054,10 @@ static int map_check_btf(struct bpf_map *map, const struct btf *btf,
 		}
 	}
 
+	ret = btf_check_and_fixup_fields(btf, map->record);
+	if (ret < 0)
+		goto free_map_tab;
+
 	if (map->ops->map_check_btf) {
 		ret = map->ops->map_check_btf(map, btf, key_type, value_type);
 		if (ret < 0)