BUG#55322: SHOW BINLOG EVENTS increases @@SESSION.MAX_ALLOWED_PACKET

Problem: when SHOW BINLOG EVENTS was issued, it increased the value of
@@session.max_allowed_packet. This allowed a non-root user to increase
the amount of memory used by her thread arbitrarily. Thus, it removes
the bound on the amount of system resources used by a client, so it
presents a security risk (DoS attack).

Fix: it is correct to increase the value of @@session.max_allowed_packet
while executing SHOW BINLOG EVENTS (see BUG 30435). However, the
increase should only be temporary. Thus, the fix is to restore the value
when SHOW BINLOG EVENTS ends.
The value of @@session.max_allowed_packet is also increased in
mysql_binlog_send (i.e., the binlog dump thread). It is not clear if this
can cause any trouble, since normally the client that issues
COM_BINLOG_DUMP will not issue any other commands that would be affected
by the increased value of @@session.max_allowed_packet. However, we
restore the value just in case.

1 files changed, 5 insertions, 0 deletions

diff --git a/sql/sql_repl.cc b/sql/sql_repl.cc
index c220f609c09..dcbc982b4aa 100644
--- a/sql/sql_repl.cc
+++ b/sql/sql_repl.cc
@@ -357,6 +357,7 @@ void mysql_binlog_send(THD* thd, char* log_ident, my_off_t pos,
 #ifndef DBUG_OFF
   int left_events = max_binlog_dump_events;
 #endif
+  int old_max_allowed_packet= thd->variables.max_allowed_packet;
   DBUG_ENTER("mysql_binlog_send");
   DBUG_PRINT("enter",("log_ident: '%s'  pos: %ld", log_ident, (long) pos));
 
@@ -762,6 +763,7 @@ end:
   pthread_mutex_lock(&LOCK_thread_count);
   thd->current_linfo = 0;
   pthread_mutex_unlock(&LOCK_thread_count);
+  thd->variables.max_allowed_packet= old_max_allowed_packet;
   DBUG_VOID_RETURN;
 
 err:
@@ -779,6 +781,7 @@ err:
   pthread_mutex_unlock(&LOCK_thread_count);
   if (file >= 0)
     (void) my_close(file, MYF(MY_WME));
+  thd->variables.max_allowed_packet= old_max_allowed_packet;
 
   my_message(my_errno, errmsg, MYF(0));
   DBUG_VOID_RETURN;
@@ -1422,6 +1425,7 @@ bool mysql_show_binlog_events(THD* thd)
   bool ret = TRUE;
   IO_CACHE log;
   File file = -1;
+  int old_max_allowed_packet= thd->variables.max_allowed_packet;
   DBUG_ENTER("mysql_show_binlog_events");
 
   Log_event::init_show_field_list(&field_list);
@@ -1560,6 +1564,7 @@ err:
   pthread_mutex_lock(&LOCK_thread_count);
   thd->current_linfo = 0;
   pthread_mutex_unlock(&LOCK_thread_count);
+  thd->variables.max_allowed_packet= old_max_allowed_packet;
   DBUG_RETURN(ret);
 }