bpf: Teach stack depth check about async callbacks.

Teach max stack depth checking algorithm about async callbacks
that don't increase bpf program stack size.
Also add sanity check that bpf_tail_call didn't sneak into async cb.
It's impossible, since PTR_TO_CTX is not available in async cb,
hence the program cannot contain bpf_tail_call(ctx,...);

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20210715005417.78572-10-alexei.starovoitov@gmail.com

include/linux/bpf_verifier.h

@@ -406,6 +406,7 @@ struct bpf_subprog_info {
 	bool has_tail_call;
 	bool tail_call_reachable;
 	bool has_ld_abs;
+	bool is_async_cb;
 };
 
 /* single container for all structs

kernel/bpf/verifier.c

@@ -3709,6 +3709,8 @@ process_func:
 continue_func:
 	subprog_end = subprog[idx + 1].start;
 	for (; i < subprog_end; i++) {
+		int next_insn;
+
 		if (!bpf_pseudo_call(insn + i) && !bpf_pseudo_func(insn + i))
 			continue;
 		/* remember insn and function to return to */
@@ -3716,13 +3718,22 @@ continue_func:
 		ret_prog[frame] = idx;
 
 		/* find the callee */
-		i = i + insn[i].imm + 1;
-		idx = find_subprog(env, i);
+		next_insn = i + insn[i].imm + 1;
+		idx = find_subprog(env, next_insn);
 		if (idx < 0) {
 			WARN_ONCE(1, "verifier bug. No program starts at insn %d\n",
-				  i);
+				  next_insn);
 			return -EFAULT;
 		}
+		if (subprog[idx].is_async_cb) {
+			if (subprog[idx].has_tail_call) {
+				verbose(env, "verifier bug. subprog has tail_call and async cb\n");
+				return -EFAULT;
+			}
+			 /* async callbacks don't increase bpf prog stack size */
+			continue;
+		}
+		i = next_insn;
 
 		if (subprog[idx].has_tail_call)
 			tail_call_reachable = true;
@@ -5761,6 +5772,7 @@ static int __check_func_call(struct bpf_verifier_env *env, struct bpf_insn *insn
 		struct bpf_verifier_state *async_cb;
 
 		/* there is no real recursion here. timer callbacks are async */
+		env->subprog_info[subprog].is_async_cb = true;
 		async_cb = push_async_cb(env, env->subprog_info[subprog].start,
 					 *insn_idx, subprog);
 		if (!async_cb)