Guard against short server scramble in sha256_password auth

mysqlnd_sha256_auth_get_auth_data() XORs SCRAMBLE_LENGTH bytes of the
server-supplied scramble into the password without checking the scramble
is at least that long, unlike the native and caching_sha2 plugins which
reject a short scramble with CR_MALFORMED_PACKET. A server reporting a
scramble shorter than 20 bytes shrinks the heap buffer the scramble is
copied into, so the XOR reads past it. Add the same length guard the
sibling plugins use.

Author: iliaal

ext/mysqli/tests/fake_server.inc

@@ -818,6 +818,20 @@ function my_mysqli_test_query_response_row_read_two_fields(my_mysqli_fake_server
     }
 }
 
+function my_mysqli_test_sha256_password_short_scramble(my_mysqli_fake_server_conn $conn): void
+{
+    $conn->send_server_greetings();
+    $conn->read();
+    // AuthSwitchRequest (0xfe) to sha256_password carrying a scramble shorter
+    // than SCRAMBLE_LENGTH (20). mnd_emalloc sizes the heap copy to this
+    // length, so the sha256 plugin's 20-byte XOR over-reads it without the
+    // length guard.
+    $payload = "\xfe" . "sha256_password\x00" . str_repeat("\x41", 8);
+    $header = substr(pack('V', strlen($payload)), 0, 3) . "\x02";
+    $conn->send($header . $payload, "Auth Switch Request [short sha256 scramble]");
+    $conn->read();
+}
+
 function run_fake_server(string $test_function, int|string $port = 0): int
 {
     $host = '127.0.0.1';

ext/mysqli/tests/mysqli_sha256_password_short_scramble.phpt

@@ -0,0 +1,26 @@
+--TEST--
+mysqlnd sha256_password: reject server scramble shorter than 20 bytes
+--EXTENSIONS--
+mysqli
+mysqlnd
+openssl
+--FILE--
+<?php
+require_once 'fake_server.inc';
+
+mysqli_report(MYSQLI_REPORT_OFF);
+
+$process = run_fake_server_in_background('sha256_password_short_scramble');
+$process->wait();
+
+$link = @new mysqli("127.0.0.1", "root", "", "", $process->getPort());
+printf("[%d] %s\n", mysqli_connect_errno(), mysqli_connect_error());
+
+$process->terminate();
+print "done!\n";
+?>
+--EXPECTF--
+[*] Server started on 127.0.0.1:%d
+%A
+[2027] The server sent wrong length for scramble
+done!

ext/mysqlnd/mysqlnd_auth.c

@@ -908,6 +908,11 @@ mysqlnd_sha256_auth_get_auth_data(struct st_mysqlnd_authentication_plugin * self
 	DBG_ENTER("mysqlnd_sha256_auth_get_auth_data");
 	DBG_INF_FMT("salt(%zu)=[%.*s]", auth_plugin_data_len, (int) auth_plugin_data_len, auth_plugin_data);
 
+	if (auth_plugin_data_len < SCRAMBLE_LENGTH) {
+		SET_CLIENT_ERROR(conn->error_info, CR_MALFORMED_PACKET, UNKNOWN_SQLSTATE, "The server sent wrong length for scramble");
+		DBG_ERR_FMT("The server sent wrong length for scramble %zu. Expected %u", auth_plugin_data_len, SCRAMBLE_LENGTH);
+		DBG_RETURN(NULL);
+	}
 
 	if (conn->vio->data->ssl) {
 		DBG_INF("simple clear text under SSL");