Linux ISO9660 handling flaws

[Michal Zalewski <lcamtuf@xxxxxxxxxxxx>]

Good morning,

There appears to be a fair number of kernel-level range checking flaws in
ISO9660 filesystem handler (and Rock Ridge / Juliet extensions) in Linux
up to and including 2.6.11. These bugs range from DoS conditions to
potentially exploitable memory corruption - all this whenever a specially
crafted filesystem is mounted or directories are examined.

Most apparent flaws are expected to be fixed in Linux 2.6.12 (rc to show
up by tomorrow or so), although, as per Linus words, "that code is
horrid", and it may take some time to work out all the issues.

The impact is not dramatic, but there are two obvious ways such flaws can
be used to benefit remote attackers:

  1) Bugs in removable media filesystems may be used to automatically
     compromise any system whose owner decided to examine a newly acquired
     CD-ROM, even if extreme caution is observed (that is, autorun is
     disabled, and no files are executed).

  2) For all types of filesystems, such problems can be additionally used
     to subvert forensic analysis efforts. Disk images from compromised
     machine may infect forensic examiner's system and alter results,
     or simply render the machine unusable.

Attached is a trivial fuzz script that can be used to test fs drivers
against most obvious fault conditions. With little effort, it can be
further altered to test filesystems other than ISO9660, and OSes other
than Linux.

Regards,
Michal Zalewski

Obligatory plug: http://lcamtuf.coredump.cx/silence/

#!/bin/bash

cd /tmp || exit 1

echo '[*] Compiling mangler...'

cat >mangle.c <<_EOF_
char buf[10240];
main() {
  int i,x;
  srand(time(0) ^ getpid());
  while ( (i = read(0,buf,sizeof(buf))) > 0) {
    x = rand() % (i/20);
    while (x--) buf[rand() % i] = rand();
    write(1,buf,i);
  }
}
_EOF_

gcc -O3 mangle.c -o mangle || exit 1
rm -f mangle.c

echo '[*] Preparing ISO master (feel free to alter this code)...'

mkdir cd_dir || exit 1
cd cd_dir

CNT=0
while [ "$CNT" -lt "200" ]; do
  mkdir A; cd A
  CNT=$[CNT+1]
done

cd /tmp/cd_dir

A=`perl -e '{print "A"x255}' 2>/dev/null`
CNT=0
while [ "$CNT" -lt "3" ]; do
  mkdir "$A"; cd "$A"
  CNT=$[CNT+1]
done

cd /tmp

echo '[*] Creating image (alter filesystem or parameters as needed)...'

mkisofs -U -R -J -o cd.iso cd_dir 2>/dev/null || exit 1
rm -rf cd_dir

echo '[*] STRESS TEST PHASE...'

while :; do 
  DIR="/tmp/cdtest-$$-$RANDOM"
  mkdir "$DIR"
  dmesg -c 2>/dev/null
  cat cd.iso | ./mangle >cd_mod.iso
  mount -t iso9660 -o loop,ro /tmp/cd_mod.iso "$DIR" 2>/dev/null
  # ls -lAR "$DIR" - Uncomment if you like when it HURTS...
  umount "$DIR" 2>/dev/null
  rm -rf "$DIR" 2>/dev/null
  FAULT=`dmesg | grep -Ei 'oops|unable to handle'`
  test "$FAULT" = "" || break
done

dmesg | tail -30

echo '[+] Something found (/tmp/cd-mod.iso)...'

rm -f cd.iso mangle
exit 0