MDEV-28838 password_reuse_check plugin mixes username and password

To prevent the problem of mixing user name and password and
host name and user name we add length of the hostname and user name
to the hash.

mysql-test/suite/plugins/r/password_reuse_check.result

@@ -30,7 +30,7 @@ Error	1819	Your password does not satisfy the current policy requirements
 Error	1396	Operation ALTER USER failed for 'user_name'@'localhost'
 select hex(hash) from mysql.password_reuse_check_history;
 hex(hash)
-6276C87127F2B65FC6B24E94E324A02FF0D393D7FB7DEAF6F5F49F0A8AB006711D5C6EF67E36A251AB6337E7E20D312F9ED66D70EB699A6EC85B1E0BC7F376C0
+B9F970DE4DA0145F842526C1BC9DBBBDB3EF80FDD7BE98061DAE3D18F492AA37668C07322DD21650C66B48FC78F0EAF6CB08245CC895BFDC43BE6921B07E5240
 # emulate old password
 update mysql.password_reuse_check_history set time= date_sub(now(), interval
 11 day);
@@ -71,4 +71,24 @@ Level	Code	Message
 Warning	1105	password_reuse_check:[1054] Unknown column 'time' in 'where clause'
 Error	1819	Your password does not satisfy the current policy requirements
 drop table mysql.password_reuse_check_history;
+#
+# MDEV-28838: password_reuse_check plugin mixes username and password
+#
+grant select on *.* to user_name@localhost identified by 'test_pwd';
+grant select on *.* to user_nam@localhost identified by 'etest_pwd';
+show warnings;
+Level	Code	Message
+drop user user_name@localhost;
+drop user user_nam@localhost;
+drop table mysql.password_reuse_check_history;
+grant select on *.* to user_name@localhost identified by 'test_pwd';
+grant select on *.* to tuser_name@localhos identified by 'test_pwd';
+show warnings;
+Level	Code	Message
+drop user user_name@localhost;
+drop user tuser_name@localhos;
+#
+# End of 10.7 tests
+#
+drop table mysql.password_reuse_check_history;
 uninstall plugin password_reuse_check;

mysql-test/suite/plugins/t/password_reuse_check.test

@@ -69,5 +69,32 @@ set global password_reuse_check_interval= 10;
 grant select on *.* to user_name@localhost identified by 'test_pwd';
 show warnings;
 
+drop table mysql.password_reuse_check_history;
+
+--echo #
+--echo # MDEV-28838: password_reuse_check plugin mixes username and password
+--echo #
+
+grant select on *.* to user_name@localhost identified by 'test_pwd';
+
+grant select on *.* to user_nam@localhost identified by 'etest_pwd';
+show warnings;
+
+drop user user_name@localhost;
+drop user user_nam@localhost;
+drop table mysql.password_reuse_check_history;
+
+grant select on *.* to user_name@localhost identified by 'test_pwd';
+
+grant select on *.* to tuser_name@localhos identified by 'test_pwd';
+show warnings;
+
+drop user user_name@localhost;
+drop user tuser_name@localhos;
+
+--echo #
+--echo # End of 10.7 tests
+--echo #
+
 drop table mysql.password_reuse_check_history;
 uninstall plugin password_reuse_check;

plugin/password_reuse_check/password_reuse_check.c

@@ -13,6 +13,7 @@
    along with this program; if not, write to the Free Software
    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1335  USA */
 
+#include <my_global.h> // for int2store
 #include <stdio.h>     // for snprintf
 #include <string.h>    // for memset
 #include <mysql/plugin_password_validation.h>
@@ -30,6 +31,22 @@ static unsigned interval= 0;
 // helping string for bin_to_hex512
 static char digits[]= "0123456789ABCDEF";
 
+/**
+  Store string with length
+
+  @param to              buffer where to put the length and string
+  @param from            the string to store
+
+  @return reference on the byte after copied string
+*/
+
+static char *store_str(char *to, const MYSQL_CONST_LEX_STRING *from)
+{
+  int2store(to, from->length);
+  memcpy(to + 2, from->str, from->length);
+  return to + 2 + from->length;
+}
+
 
 /**
   Convert string of 512 bits (64 bytes) to hex representation
@@ -149,7 +166,8 @@ static int validate(const MYSQL_CONST_LEX_STRING *username,
                     const MYSQL_CONST_LEX_STRING *hostname)
 {
   MYSQL *mysql= NULL;
-  size_t key_len= username->length + password->length + hostname->length;
+  size_t key_len= username->length + password->length + hostname->length +
+                  (3 * 2 /* space for storing length of the strings */);
   size_t buff_len= (key_len > SQL_BUFF_LEN ? key_len : SQL_BUFF_LEN);
   size_t len;
   char *buff= malloc(buff_len);
@@ -165,13 +183,16 @@ static int validate(const MYSQL_CONST_LEX_STRING *username,
     return 1;
   }
 
-  memcpy(buff, hostname->str, hostname->length);
-  memcpy(buff + hostname->length, username->str, username->length);
-  memcpy(buff + hostname->length + username->length, password->str,
-          password->length);
-  buff[key_len]= 0;
+  /*
+    Store: username, hostname, password
+    (password first to make its rewriting password in memory simplier)
+  */
+  store_str(store_str(store_str(buff, password), username), hostname);
+  buff[key_len]= 0; // safety
   memset(hash, 0, sizeof(hash));
   my_sha512(hash, buff, key_len);
+  // safety: rewrite password with zerows
+  memset(buff, 0, password->length);
   if (mysql_real_connect_local(mysql) == NULL)
     goto sql_error;
 
@@ -232,10 +253,10 @@ maria_declare_plugin(password_reuse_check)
   PLUGIN_LICENSE_GPL,
   NULL,
   NULL,
-  0x0100,
+  0x0200,
   NULL,
   sysvars,
-  "1.0",
+  "2.0",
   MariaDB_PLUGIN_MATURITY_GAMMA
 }
 maria_declare_plugin_end;