bpf: Limit maximum modifier chain length in btf_check_type_tags

On processing a module BTF of module built for an older kernel, we might
sometimes find that some type points to itself forming a loop. If such a
type is a modifier, btf_check_type_tags's while loop following modifier
chain will be caught in an infinite loop.

Fix this by defining a maximum chain length and bailing out if we spin
any longer than that.

Fixes: eb596b09 ("bpf: Ensure type tags precede modifiers in BTF")
Reported-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20220615042151.2266537-1-memxor@gmail.com

kernel/bpf/btf.c

@@ -4815,6 +4815,7 @@ static int btf_check_type_tags(struct btf_verifier_env *env,
 	n = btf_nr_types(btf);
 	for (i = start_id; i < n; i++) {
 		const struct btf_type *t;
+		int chain_limit = 32;
 		u32 cur_id = i;
 
 		t = btf_type_by_id(btf, i);
@@ -4827,6 +4828,10 @@ static int btf_check_type_tags(struct btf_verifier_env *env,
 
 		in_tags = btf_type_is_type_tag(t);
 		while (btf_type_is_modifier(t)) {
+			if (!chain_limit--) {
+				btf_verifier_log(env, "Max chain length or cycle detected");
+				return -ELOOP;
+			}
 			if (btf_type_is_type_tag(t)) {
 				if (!in_tags) {
 					btf_verifier_log(env, "Type tags don't precede modifiers");