bpf: Check attach_func_proto more carefully in check_helper_call

Syzkaller found a problem similar to d1a6edec ("bpf: Check
attach_func_proto more carefully in check_return_code") where
attach_func_proto might be NULL:

RIP: 0010:check_helper_call+0x3dcb/0x8d50 kernel/bpf/verifier.c:7330
 do_check kernel/bpf/verifier.c:12302 [inline]
 do_check_common+0x6e1e/0xb980 kernel/bpf/verifier.c:14610
 do_check_main kernel/bpf/verifier.c:14673 [inline]
 bpf_check+0x661e/0xc520 kernel/bpf/verifier.c:15243
 bpf_prog_load+0x11ae/0x1f80 kernel/bpf/syscall.c:2620

With the following reproducer:

  bpf$BPF_PROG_RAW_TRACEPOINT_LOAD(0x5, &(0x7f0000000780)={0xf, 0x4, &(0x7f0000000040)=@framed={{}, [@call={0x85, 0x0, 0x0, 0xbb}]}, &(0x7f0000000000)='GPL\x00', 0x0, 0x0, 0x0, 0x0, 0x0, '\x00', 0x0, 0x2b, 0xffffffffffffffff, 0x8, 0x0, 0x0, 0x10, 0x0}, 0x80)

Let's do the same here, only check attach_func_proto for the prog types
where we are certain that attach_func_proto is defined.

Fixes: 69fd337a ("bpf: per-cgroup lsm flavor")
Reported-by: syzbot+0f8d989b1fba1addc5e0@syzkaller.appspotmail.com
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20220720164729.147544-1-sdf@google.com

kernel/bpf/verifier.c

@@ -7170,6 +7170,7 @@ static void update_loop_inline_state(struct bpf_verifier_env *env, u32 subprogno
 static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 			     int *insn_idx_p)
 {
+	enum bpf_prog_type prog_type = resolve_prog_type(env->prog);
 	const struct bpf_func_proto *fn = NULL;
 	enum bpf_return_type ret_type;
 	enum bpf_type_flag ret_flag;
@@ -7331,7 +7332,8 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
 		}
 		break;
 	case BPF_FUNC_set_retval:
-		if (env->prog->expected_attach_type == BPF_LSM_CGROUP) {
+		if (prog_type == BPF_PROG_TYPE_LSM &&
+		    env->prog->expected_attach_type == BPF_LSM_CGROUP) {
 			if (!env->prog->aux->attach_func_proto->type) {
 				/* Make sure programs that attach to void
 				 * hooks don't try to modify return value.