Bug#19145698: READ OUT OF BOUNDS ISSUE

Problem:
========
In a master slave replication if a slave receives a
Start_log_event_v3 the payload is expected to be of fixed
size. If a payload which is smaller than the fixed size is
received it causes a read out of bounds issue.

Analysis:
========
According to documentation the fixed data part of
Start_log_event_v3 looks as shown below.

2 bytes: The binary log format version
50 bytes: The MySQL server's version
4 bytes: Timestamp in seconds when this event was created

Since the payload is expected to be of fixed size, therefore
ST_SERVER_VER_LEN (50) bytes are memcpy'ed into
server_version. But if a malicious master sends a shorter
payload it causes a read out of bounds issue.

Fix:
===
In Start_log_event_v3 event's constructor a check has been
added which expects the minimum payload length to be of size
common_header_len + ST_COMMON_HEADER_LEN_OFFSET bytes. If a
malicious packet of lesser length is received it will be
considered as an invalid event.

sql/log_event.cc

@@ -1307,7 +1307,7 @@ Log_event* Log_event::read_log_event(const char* buf, uint event_len,
       ev = new Execute_load_log_event(buf, event_len, description_event);
       break;
     case START_EVENT_V3: /* this is sent only by MySQL <=4.x */
-      ev = new Start_log_event_v3(buf, description_event);
+      ev = new Start_log_event_v3(buf, event_len, description_event);
       break;
     case STOP_EVENT:
       ev = new Stop_log_event(buf, description_event);
@@ -3788,11 +3788,17 @@ void Start_log_event_v3::print(FILE* file, PRINT_EVENT_INFO* print_event_info)
   Start_log_event_v3::Start_log_event_v3()
 */
 
-Start_log_event_v3::Start_log_event_v3(const char* buf,
+Start_log_event_v3::Start_log_event_v3(const char* buf, uint event_len,
                                        const Format_description_log_event
                                        *description_event)
-  :Log_event(buf, description_event)
+  :Log_event(buf, description_event), binlog_version(BINLOG_VERSION)
 {
+  if (event_len < (uint)description_event->common_header_len +
+      ST_COMMON_HEADER_LEN_OFFSET)
+  {
+    server_version[0]= 0;
+    return;
+  }
   buf+= description_event->common_header_len;
   binlog_version= uint2korr(buf+ST_BINLOG_VER_OFFSET);
   memcpy(server_version, buf+ST_SERVER_VER_OFFSET,
@@ -4082,16 +4088,15 @@ Format_description_log_event(const char* buf,
                              const
                              Format_description_log_event*
                              description_event)
-  :Start_log_event_v3(buf, description_event), event_type_permutation(0)
+  :Start_log_event_v3(buf, event_len, description_event),
+   common_header_len(0), post_header_len(NULL), event_type_permutation(0)
 {
   DBUG_ENTER("Format_description_log_event::Format_description_log_event(char*,...)");
+  if (!Start_log_event_v3::is_valid())
+    DBUG_VOID_RETURN; /* sanity check */
   buf+= LOG_EVENT_MINIMAL_HEADER_LEN;
   if ((common_header_len=buf[ST_COMMON_HEADER_LEN_OFFSET]) < OLD_HEADER_LEN)
-  {
-    /* this makes is_valid() return false. */
-    post_header_len= NULL;
     DBUG_VOID_RETURN; /* sanity check */
-  }
   number_of_event_types=
     event_len-(LOG_EVENT_MINIMAL_HEADER_LEN+ST_COMMON_HEADER_LEN_OFFSET+1);
   DBUG_PRINT("info", ("common_header_len=%d number_of_event_types=%d",

sql/log_event.h

@@ -2276,14 +2276,14 @@ class Start_log_event_v3: public Log_event
   void print(FILE* file, PRINT_EVENT_INFO* print_event_info);
 #endif
 
-  Start_log_event_v3(const char* buf,
+  Start_log_event_v3(const char* buf, uint event_len,
                      const Format_description_log_event* description_event);
   ~Start_log_event_v3() {}
   Log_event_type get_type_code() { return START_EVENT_V3;}
 #ifdef MYSQL_SERVER
   bool write(IO_CACHE* file);
 #endif
-  bool is_valid() const { return 1; }
+  bool is_valid() const { return server_version[0] != 0; }
   int get_data_size()
   {
     return START_V3_HEADER_LEN; //no variable-sized part