bpf: Ensure type tags precede modifiers in BTF

It is guaranteed that for modifiers, clang always places type tags
before other modifiers, and then the base type. We would like to rely on
this guarantee inside the kernel to make it simple to parse type tags
from BTF.

However, a user would be allowed to construct a BTF without such
guarantees. Hence, add a pass to check that in modifier chains, type
tags only occur at the head of the chain, and then don't occur later in
the chain.

If we see a type tag, we can have one or more type tags preceding other
modifiers that then never have another type tag. If we see other
modifiers, all modifiers following them should never be a type tag.

Instead of having to walk chains we verified previously, we can remember
the last good modifier type ID which headed a good chain. At that point,
we must have verified all other chains headed by type IDs less than it.
This makes the verification process less costly, and it becomes a simple
O(n) pass.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20220419164608.1990559-2-memxor@gmail.com

kernel/bpf/btf.c

@@ -4541,6 +4541,48 @@ static int btf_parse_hdr(struct btf_verifier_env *env)
 	return 0;
 }
 
+static int btf_check_type_tags(struct btf_verifier_env *env,
+			       struct btf *btf, int start_id)
+{
+	int i, n, good_id = start_id - 1;
+	bool in_tags;
+
+	n = btf_nr_types(btf);
+	for (i = start_id; i < n; i++) {
+		const struct btf_type *t;
+		u32 cur_id = i;
+
+		t = btf_type_by_id(btf, i);
+		if (!t)
+			return -EINVAL;
+		if (!btf_type_is_modifier(t))
+			continue;
+
+		cond_resched();
+
+		in_tags = btf_type_is_type_tag(t);
+		while (btf_type_is_modifier(t)) {
+			if (btf_type_is_type_tag(t)) {
+				if (!in_tags) {
+					btf_verifier_log(env, "Type tags don't precede modifiers");
+					return -EINVAL;
+				}
+			} else if (in_tags) {
+				in_tags = false;
+			}
+			if (cur_id <= good_id)
+				break;
+			/* Move to next type */
+			cur_id = t->type;
+			t = btf_type_by_id(btf, cur_id);
+			if (!t)
+				return -EINVAL;
+		}
+		good_id = i;
+	}
+	return 0;
+}
+
 static struct btf *btf_parse(bpfptr_t btf_data, u32 btf_data_size,
 			     u32 log_level, char __user *log_ubuf, u32 log_size)
 {
@@ -4608,6 +4650,10 @@ static struct btf *btf_parse(bpfptr_t btf_data, u32 btf_data_size,
 	if (err)
 		goto errout;
 
+	err = btf_check_type_tags(env, btf, 1);
+	if (err)
+		goto errout;
+
 	if (log->level && bpf_verifier_log_full(log)) {
 		err = -ENOSPC;
 		goto errout;
@@ -4809,6 +4855,10 @@ struct btf *btf_parse_vmlinux(void)
 	if (err)
 		goto errout;
 
+	err = btf_check_type_tags(env, btf, 1);
+	if (err)
+		goto errout;
+
 	/* btf_parse_vmlinux() runs under bpf_verifier_lock */
 	bpf_ctx_convert.t = btf_type_by_id(btf, bpf_ctx_convert_btf_id[0]);
 
@@ -4894,6 +4944,10 @@ static struct btf *btf_parse_module(const char *module_name, const void *data, u
 	if (err)
 		goto errout;
 
+	err = btf_check_type_tags(env, btf, btf_nr_types(base_btf));
+	if (err)
+		goto errout;
+
 	btf_verifier_env_free(env);
 	refcount_set(&btf->refcnt, 1);
 	return btf;