libbpf: Add basic BTF sanity validation

Implement a simple and straightforward BTF sanity check when parsing BTF
data. Right now it's very basic and just validates that all the string
offsets and type IDs are within valid range. For FUNC we also check that
it points to FUNC_PROTO kinds.

Even with such simple checks it fixes a bunch of crashes found by OSS
fuzzer ([0]-[5]) and will allow fuzzer to make further progress.

Some other invariants will be checked in follow up patches (like
ensuring there is no infinite type loops), but this seems like a good
start already.

Adding FUNC -> FUNC_PROTO check revealed that one of selftests has
a problem with FUNC pointing to VAR instead, so fix it up in the same
commit.

  [0] https://github.com/libbpf/libbpf/issues/482
  [1] https://github.com/libbpf/libbpf/issues/483
  [2] https://github.com/libbpf/libbpf/issues/485
  [3] https://github.com/libbpf/libbpf/issues/613
  [4] https://github.com/libbpf/libbpf/issues/618
  [5] https://github.com/libbpf/libbpf/issues/619Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Reviewed-by: Song Liu <song@kernel.org>
Closes: https://github.com/libbpf/libbpf/issues/617
Link: https://lore.kernel.org/bpf/20230825202152.1813394-1-andrii@kernel.org

tools/lib/bpf/btf.c

@@ -448,6 +448,165 @@ static int btf_parse_type_sec(struct btf *btf)
 	return 0;
 }
 
+static int btf_validate_str(const struct btf *btf, __u32 str_off, const char *what, __u32 type_id)
+{
+	const char *s;
+
+	s = btf__str_by_offset(btf, str_off);
+	if (!s) {
+		pr_warn("btf: type [%u]: invalid %s (string offset %u)\n", type_id, what, str_off);
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
+static int btf_validate_id(const struct btf *btf, __u32 id, __u32 ctx_id)
+{
+	const struct btf_type *t;
+
+	t = btf__type_by_id(btf, id);
+	if (!t) {
+		pr_warn("btf: type [%u]: invalid referenced type ID %u\n", ctx_id, id);
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
+static int btf_validate_type(const struct btf *btf, const struct btf_type *t, __u32 id)
+{
+	__u32 kind = btf_kind(t);
+	int err, i, n;
+
+	err = btf_validate_str(btf, t->name_off, "type name", id);
+	if (err)
+		return err;
+
+	switch (kind) {
+	case BTF_KIND_UNKN:
+	case BTF_KIND_INT:
+	case BTF_KIND_FWD:
+	case BTF_KIND_FLOAT:
+		break;
+	case BTF_KIND_PTR:
+	case BTF_KIND_TYPEDEF:
+	case BTF_KIND_VOLATILE:
+	case BTF_KIND_CONST:
+	case BTF_KIND_RESTRICT:
+	case BTF_KIND_VAR:
+	case BTF_KIND_DECL_TAG:
+	case BTF_KIND_TYPE_TAG:
+		err = btf_validate_id(btf, t->type, id);
+		if (err)
+			return err;
+		break;
+	case BTF_KIND_ARRAY: {
+		const struct btf_array *a = btf_array(t);
+
+		err = btf_validate_id(btf, a->type, id);
+		err = err ?: btf_validate_id(btf, a->index_type, id);
+		if (err)
+			return err;
+		break;
+	}
+	case BTF_KIND_STRUCT:
+	case BTF_KIND_UNION: {
+		const struct btf_member *m = btf_members(t);
+
+		n = btf_vlen(t);
+		for (i = 0; i < n; i++, m++) {
+			err = btf_validate_str(btf, m->name_off, "field name", id);
+			err = err ?: btf_validate_id(btf, m->type, id);
+			if (err)
+				return err;
+		}
+		break;
+	}
+	case BTF_KIND_ENUM: {
+		const struct btf_enum *m = btf_enum(t);
+
+		n = btf_vlen(t);
+		for (i = 0; i < n; i++, m++) {
+			err = btf_validate_str(btf, m->name_off, "enum name", id);
+			if (err)
+				return err;
+		}
+		break;
+	}
+	case BTF_KIND_ENUM64: {
+		const struct btf_enum64 *m = btf_enum64(t);
+
+		n = btf_vlen(t);
+		for (i = 0; i < n; i++, m++) {
+			err = btf_validate_str(btf, m->name_off, "enum name", id);
+			if (err)
+				return err;
+		}
+		break;
+	}
+	case BTF_KIND_FUNC: {
+		const struct btf_type *ft;
+
+		err = btf_validate_id(btf, t->type, id);
+		if (err)
+			return err;
+		ft = btf__type_by_id(btf, t->type);
+		if (btf_kind(ft) != BTF_KIND_FUNC_PROTO) {
+			pr_warn("btf: type [%u]: referenced type [%u] is not FUNC_PROTO\n", id, t->type);
+			return -EINVAL;
+		}
+		break;
+	}
+	case BTF_KIND_FUNC_PROTO: {
+		const struct btf_param *m = btf_params(t);
+
+		n = btf_vlen(t);
+		for (i = 0; i < n; i++, m++) {
+			err = btf_validate_str(btf, m->name_off, "param name", id);
+			err = err ?: btf_validate_id(btf, m->type, id);
+			if (err)
+				return err;
+		}
+		break;
+	}
+	case BTF_KIND_DATASEC: {
+		const struct btf_var_secinfo *m = btf_var_secinfos(t);
+
+		n = btf_vlen(t);
+		for (i = 0; i < n; i++, m++) {
+			err = btf_validate_id(btf, m->type, id);
+			if (err)
+				return err;
+		}
+		break;
+	}
+	default:
+		pr_warn("btf: type [%u]: unrecognized kind %u\n", id, kind);
+		return -EINVAL;
+	}
+	return 0;
+}
+
+/* Validate basic sanity of BTF. It's intentionally less thorough than
+ * kernel's validation and validates only properties of BTF that libbpf relies
+ * on to be correct (e.g., valid type IDs, valid string offsets, etc)
+ */
+static int btf_sanity_check(const struct btf *btf)
+{
+	const struct btf_type *t;
+	__u32 i, n = btf__type_cnt(btf);
+	int err;
+
+	for (i = 1; i < n; i++) {
+		t = btf_type_by_id(btf, i);
+		err = btf_validate_type(btf, t, i);
+		if (err)
+			return err;
+	}
+	return 0;
+}
+
 __u32 btf__type_cnt(const struct btf *btf)
 {
 	return btf->start_id + btf->nr_types;
@@ -902,6 +1061,7 @@ static struct btf *btf_new(const void *data, __u32 size, struct btf *base_btf)
 
 	err = btf_parse_str_sec(btf);
 	err = err ?: btf_parse_type_sec(btf);
+	err = err ?: btf_sanity_check(btf);
 	if (err)
 		goto done;
 

tools/testing/selftests/bpf/prog_tests/btf.c

@@ -7296,7 +7296,7 @@ static struct btf_dedup_test dedup_tests[] = {
 			BTF_FUNC_PROTO_ENC(0, 2),			/* [3] */
 				BTF_FUNC_PROTO_ARG_ENC(NAME_NTH(2), 1),
 				BTF_FUNC_PROTO_ARG_ENC(NAME_NTH(3), 1),
-			BTF_FUNC_ENC(NAME_NTH(4), 2),			/* [4] */
+			BTF_FUNC_ENC(NAME_NTH(4), 3),			/* [4] */
 			/* tag -> t */
 			BTF_DECL_TAG_ENC(NAME_NTH(5), 2, -1),		/* [5] */
 			BTF_DECL_TAG_ENC(NAME_NTH(5), 2, -1),		/* [6] */
@@ -7317,7 +7317,7 @@ static struct btf_dedup_test dedup_tests[] = {
 			BTF_FUNC_PROTO_ENC(0, 2),			/* [3] */
 				BTF_FUNC_PROTO_ARG_ENC(NAME_NTH(2), 1),
 				BTF_FUNC_PROTO_ARG_ENC(NAME_NTH(3), 1),
-			BTF_FUNC_ENC(NAME_NTH(4), 2),			/* [4] */
+			BTF_FUNC_ENC(NAME_NTH(4), 3),			/* [4] */
 			BTF_DECL_TAG_ENC(NAME_NTH(5), 2, -1),		/* [5] */
 			BTF_DECL_TAG_ENC(NAME_NTH(5), 4, -1),		/* [6] */
 			BTF_DECL_TAG_ENC(NAME_NTH(5), 4, 1),		/* [7] */