BACKGROUND

-----------

Xinetd is a popular inetd replacement. Shortly after the 2.3.9 release in

September 2002, it was realized that xinetd was leaking file descriptors.

That problem turned out to be that file descriptors were not always being

closed whenever a connection was rejected. 2.3.10 was released with this

fixup among others in January.

Sometime in February, a machine that I admin was hit by an ftp worm. It

created > 5000 connections in 1 second. Xinetd promptly keeled over.

Xinetd had been running for over a month with no downtime. The machine has

next to no ftp traffic and only from 2 sources, so it was configured to be

run via xinetd rejecting connections via tcp_wrappers. The machine had

weathered worm attacks in the past, so this puzzled me.

TESTING

-------

Eventually, I started looking at xinetd with valgrind. I used the

following commandline:

valgrind --leak-check=yes --leak-resolution=med --num-callers=8 --logfile-fd=9 /usr/sbin/xinetd -d -pidfile /var/run/xinetd.pid -stayalive 9> out.txt

Depending on your setup, you may need to use something higher than 9.

Xinetd was tested on connections that succeed and connections that are

rejected due to configuration settings. The easiest way to test this is to

use the following setup for chargen:

service chargen

{

type = INTERNAL

user = root

protocol = tcp

wait = no

access_times = 2:00-3:00

# only_from = 192.168.1.3/24

# no_access = 192.168.1.3/24

}

The point is to set it up in a way that the connection is guaranteed to be

rejected. Then do a:

telnet localhost chargen

After a couple seconds "ctl-] quit"

Then, /etc/rc.d/init.d/xinetd stop

Valgrind reports the following:

==18939== 144 bytes in 1 blocks are definitely lost in loss record 36 of 45

==18939== at 0x40160DB8: malloc (vg_clientfuncs.c:103)

==18939== by 0x804FE22: (within /usr/sbin/xinetd)

==18939== by 0x805A496: (within /usr/sbin/xinetd)

==18939== by 0x8053611: (within /usr/sbin/xinetd)

==18939== by 0x805340D: (within /usr/sbin/xinetd)

==18939== by 0x40294A46: __libc_start_main (in /lib/libc-2.3.2.so)

==18939== by 0x804A310: (within /usr/sbin/xinetd)

==18939==

THE PROBLEM

-----------

Using objdump -S /usr/sbin/xinetd, the block of code in question comes

from service.c:

void svc_request( struct service *sp )

{

connection_s *cp ;

status_e ret_code;

cp = conn_new( sp ) ;

if ( cp == CONN_NULL )

return ;

if (sp->svc_not_generic)

ret_code = spec_service_handler(sp, cp);

else

ret_code = svc_generic_handler(sp, cp);

if ( ret_code != OK )

{

if ( SVC_LOGS_USERID_ON_FAILURE( sp ) )

if( spec_service_handler( LOG_SERVICE( ps ), cp ) == FAILED ) {

conn_free( cp, 1 );

return;

}

CONN_CLOSE(cp);

}

}

The above code has several problems. One background piece of information

is that the sigchld handler in xinetd (child_exit->server_end->

svc_postmortem) normally frees the connection's data. If the ret_code is

not OK, the connection was only closed. This is little more than close(cp-

>co_descriptor); This does not free cp since sigchld will not be called.

It was only if the log service call failed that the connection was freed.

The above code also did not take into account ret_code == OK if the

service was no_wait or special. In both of those cases, the sigchld

handler is not invoked so the memory pointed to by cp is lost when the

call returns.

CONSEQUENCES

------------

The memory area pointed to by cp is 144 bytes. Since the variable goes out

of scope, it is permanently lost with no way of finding it again. The

memory losses are cumulative, too. It would take little more than

while true; do telnet localhost chargen < /dev/null; done;

to DOS the services provided by xinetd if you could identify a machine

that uses xinetd to reject connections. Xinetd provides a rich set of

options for rejecting connections, this includes: tcp_wrappers, only_from,

no_access, sensors, access_times, cps, load_avg, etc.

It should also be noted that if you DO NOT have any statements in the

xinetd.conf file that would cause xinetd to reject a connection, then you

are free from this problem.

SOLUTION

--------

Xinetd 2.3.11 fixes the memory leaks as well as other problems discovered

since 2.3.10 was released. All users of xinetd 2.3.10 are strongly urged

to upgrade ASAP to avoid DOS conditions. Anyone running 2.3.9 is also

strongly urged to upgrade since they are leaking file descriptors.

Your xinetd version can be determined by typing "xinetd -version" (that's

version with 1 dash).

The new tarball is: www.xinetd.org/xinetd-2.3.11.tar.gz

This problem has been assigned CAN-2003-0211 to track the bug.

This bug was also reported here:

https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=88537

If you are affected, see if your vendor has an updated xinetd for you.

-Steve Grubb

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