auth-pam: Immediately report interactive instructions to clients

SSH keyboard-interactive authentication method supports instructions but
sshd didn't show them until an user prompt was requested.

This is quite inconvenient for various PAM modules that need to notify
an user without requiring for their explicit input.

So, properly implement RFC4256 making instructions to be shown to users
when they are requested from PAM.

Closes: https://bugzilla.mindrot.org/show_bug.cgi?id=2876

auth-pam.c

@@ -136,6 +136,7 @@ typedef int SshPamDone;
 #define SshPamError -1
 #define SshPamNone 0
 #define SshPamAuthenticated 1
+#define SshPamAgain 2
 
 struct pam_ctxt {
 	sp_pthread_t	 pam_thread;
@@ -868,6 +869,8 @@ sshpam_query(void *ctx, char **name, char **info,
 	**prompts = NULL;
 	plen = 0;
 	*echo_on = xmalloc(sizeof(u_int));
+	ctxt->pam_done = SshPamNone;
+
 	while (ssh_msg_recv(ctxt->pam_psock, buffer) == 0) {
 		if (++nmesg > PAM_MAX_NUM_MSG)
 			fatal_f("too many query messages");
@@ -888,15 +891,13 @@ sshpam_query(void *ctx, char **name, char **info,
 			return (0);
 		case PAM_ERROR_MSG:
 		case PAM_TEXT_INFO:
-			/* accumulate messages */
-			len = plen + mlen + 2;
-			**prompts = xreallocarray(**prompts, 1, len);
-			strlcpy(**prompts + plen, msg, len - plen);
-			plen += mlen;
-			strlcat(**prompts + plen, "\n", len - plen);
-			plen++;
-			free(msg);
-			break;
+			*num = 0;
+			free(*info);
+			*info = msg; /* Steal the message */
+			msg = NULL;
+			ctxt->pam_done = SshPamAgain;
+			sshbuf_free(buffer);
+			return (0);
 		case PAM_ACCT_EXPIRED:
 		case PAM_MAXTRIES:
 			if (type == PAM_ACCT_EXPIRED)
@@ -1000,6 +1001,8 @@ sshpam_respond(void *ctx, u_int num, char **resp)
 		return (0);
 	case SshPamNone:
 		break;
+	case SshPamAgain:
+		return 1;	/* KbdintResultAgain */
 	default:
 		return (-1);
 	}