Difference between revisions of "Forensic Live CD issues"

From ForensicsWiki
Jump to: navigation, search
m
m (Incorrect automount policy for removable media)
(7 intermediate revisions by the same user not shown)
Line 36: Line 36:
 
  |-
 
  |-
 
  |  ReiserFS
 
  |  ReiserFS
  |  In most cases
+
  |  File system has unfinished transactions
 
  |  "nolog" flag does not work (see ''man mount''). To disable journal updates: use "ro,loop" flags
 
  |  "nolog" flag does not work (see ''man mount''). To disable journal updates: use "ro,loop" flags
 
  |-
 
  |-
Line 44: Line 44:
 
  |}
 
  |}
  
Incorrect mount flags can be used to mount a file system on evidentiary media during the boot process or during the file system preview process. As described above, this may result in modification of a file system's data. For example, several Ubuntu-based forensic Linux Live CD distributions mount Ext3/4 file systems on fixed media (e.g. hard drives) during execution of ''initrd'' scripts (these scripts mount every supported file system type on every supported media type using only "-o ro" flag in order to find a root file system image).
+
Incorrect mount flags can be used to mount a file system on evidentiary media during the boot process or during the file system preview process. As described above, this may result in data writes to evidentiary media. For example, several Ubuntu-based forensic Linux Live CD distributions mount Ext3/4 file systems on fixed media (e.g. hard drives) during execution of [http://en.wikipedia.org/wiki/Initrd ''initrd''] scripts (these scripts mount every supported file system type on every supported media type using only "-o ro" flag in order to find a root file system image).
 +
 
 +
[[Image:ext3 recovery.png|thumb|right|[[Helix3]]: damaged Ext3 recovery during the boot]]
  
 
List of distributions that recover Ext3 (and sometimes Ext4) file systems during the boot:
 
List of distributions that recover Ext3 (and sometimes Ext4) file systems during the boot:
Line 68: Line 70:
 
=== Root file system spoofing ===
 
=== Root file system spoofing ===
  
Lets look at Casper scripts (that are used by initrd). Here is a function (see ''scripts/casper'' in the initrd image of Ubuntu-based forensic Linux Live CD distributions) that is used to search for a root file system image during the early stage of boot process:
+
Most Ubuntu-based forensic Live CD distributions use Casper (set of scripts used to complete initialization process during early stage of boot). Casper is responsible for searching for a root file system (typically, an image of live environment) on all supported devices (because a bootloader does not pass any information about device used for booting to the kernel), mounting it and executing ''/sbin/init'' program on a mounted root file system that will continue the boot process. Unfortunately, Casper was not designed to meet computer forensics requirements and is responsible for damaged Ext3/4 file systems recovery during the boot (see above) and root file system spoofing.
  
find_livefs() {
+
[[Image:Grml.png|thumb|right|[[grml]] mounted root file system from the [[hard drive]]]]
    timeout="${1}"
+
 
    # first look at the one specified in the command line
+
Currently, Casper may select fake root file system image on evidentiary media (e.g. [[HDD]]), because there are no authenticity checks performed (except optional UUID check for a possible live file system), and this fake root file system image may be used to execute malicious code during the boot with root privileges. Knoppix-based forensic Live CD distributions are vulnerable to the same attack.
    if [ ! -z "${LIVEMEDIA}" ]; then
+
 
        if check_dev "null" "${LIVEMEDIA}" "skip_uuid_check"; then
+
List of Ubuntu-based distributions that allow root file system spoofing:
            return 0
+
 
        fi
+
{| class="wikitable" border="1"
    fi
+
|-
    # don't start autodetection before timeout has expired
+
!  Distribution
    if [ -n "${LIVEMEDIA_TIMEOUT}" ]; then
+
!  Version
        if [ "${timeout}" -lt "${LIVEMEDIA_TIMEOUT}" ]; then
+
!  Notes
            return 1
+
|-
        fi
+
|  Helix3
    fi
+
|  2009R1
    # or do the scan of block devices
+
|
    for sysblock in $(echo /sys/block/* | tr ' ' '\n' | grep -v loop | grep -v ram); do
+
|-
        devname=$(sys2dev "${sysblock}")
+
|  Helix3 Pro
        fstype=$(get_fstype "${devname}")
+
| 2009R3
        if /lib/udev/cdrom_id ${devname} > /dev/null; then
+
|
            if check_dev "null" "${devname}" ; then
+
|-
                return 0
+
|  CAINE
            fi
+
|  1.5
        elif is_nice_device "${sysblock}" ; then
+
|
            for dev in $(subdevices "${sysblock}"); do
+
|-
                if check_dev "${dev}" ; then
+
|  DEFT Linux
                    return 0
+
|  5
                fi
+
|
            done
+
|-
        elif [ "${fstype}" = "squashfs" -o \
+
|  Raptor
                "${fstype}" = "ext3" -o \
+
|  20091026
                "${fstype}" = "ext2" ]; then
+
|
            # This is an ugly hack situation, the block device has
+
|-
            # an image directly on it. It's hopefully
+
|  grml
            # casper, so take it and run with it.
+
|  2009.10
            ln -s "${devname}" "${devname}.${fstype}"
+
|  Actually, [[grml]] uses live-initramfs scripts (Casper fork)
            echo "${devname}.${fstype}"
+
|-
            return 0
+
|  BackTrack
        fi
+
|  4
    done
+
  |
    return 1
+
|-
  }
+
|  SMART Linux (Ubuntu)
 +
|  2010-01-20
 +
|
 +
|-
 +
|  FCCU GNU/Linux Forensic Boot CD
 +
|  12.1
 +
  |
 +
|}
 +
 
 +
Vulnerable Knoppix-based distributions include: SPADA, LinEn boot CD, BitFlare.
  
 
=== Swap space activation ===
 
=== Swap space activation ===
  
=== Incorrect automount policy for removable media ===
+
=== Incorrect automount policy ===
  
 
=== Incorrect write-blocking approach ===
 
=== Incorrect write-blocking approach ===
  
=== Software RAID (Linux RAID) activation ===
+
== See also ==
 +
 
 +
* [http://www.computer-forensics-lab.org/pdf/Linux_for_computer_forensic_investigators_2.pdf Linux for computer forensic investigators: problems of booting trusted operating system]
 +
* [http://www.computer-forensics-lab.org/pdf/Linux_for_computer_forensic_investigators.pdf Linux for computer forensic investigators: «pitfalls» of mounting file systems]

Revision as of 16:24, 3 February 2010

The problem

Forensic Linux Live CD distributions are widely used during computer forensic investigations. Currently, many vendors of such Live CD distributions spread false claims that their distributions "do not touch anything", "write protect everything" and so on. Community-developed distributions are not exception here, unfortunately. Finally, it turns out that many forensic Linux Live CD distributions are not tested properly and there are no suitable test cases developed.

Another side of the problem

Another side of the problem of insufficient testing of forensic Live CD distributions is that many users do not know what happens "under the hood" of such distributions and cannot adequately test them.

Example

For example, Forensic Cop Journal (Volume 1(3), Oct 2009) describes a test case when an Ext3 file system was mounted using "-o ro" mount flag as a way to write protect the data. The article says that all tests were successful (i.e. no data modification was found after unmounting the file system), but it is known that damaged (i.e not properly unmounted) Ext3 file systems cannot be write protected using only "-o ro" mount flags (write access will be enabled during file system recovery).

And the question is: will many users test damaged Ext3 file system (together with testing the clean one) when validating their favourite forensic Live CD distribution? My answer is "no", because many users are unaware of such traits.

Problems

Here is a list of common problems of forensic Linux Live CD distributions that can be used by developers and users for testing purposes. Each problem is followed by an up to date list of distributions affected.

Journaling file systems updates

When mounting (and unmounting) several journaling file systems with only "-o ro" mount flag a different number of data writes may occur. Here is a list of such file systems:

File system When data writes happen Notes
Ext3 File system requires journal recovery To disable recovery: use "noload" flag, or use "ro,loop" flags, or use "ext2" file system type
Ext4 File system requires journal recovery To disable recovery: use "noload" flag, or use "ro,loop" flags, or use "ext2" file system type
ReiserFS File system has unfinished transactions "nolog" flag does not work (see man mount). To disable journal updates: use "ro,loop" flags
XFS Always "norecovery" flag does not help. To disable data writes: use "ro,loop" flags. The bug was fixed in recent 2.6 kernels.

Incorrect mount flags can be used to mount a file system on evidentiary media during the boot process or during the file system preview process. As described above, this may result in data writes to evidentiary media. For example, several Ubuntu-based forensic Linux Live CD distributions mount Ext3/4 file systems on fixed media (e.g. hard drives) during execution of initrd scripts (these scripts mount every supported file system type on every supported media type using only "-o ro" flag in order to find a root file system image).

Helix3: damaged Ext3 recovery during the boot

List of distributions that recover Ext3 (and sometimes Ext4) file systems during the boot:

Distribution Version
Helix3 2009R1
SMART Linux (Ubuntu) 2010-01-20
FCCU GNU/Linux Forensic Boot CD 12.1
SPADA 4

Root file system spoofing

Most Ubuntu-based forensic Live CD distributions use Casper (set of scripts used to complete initialization process during early stage of boot). Casper is responsible for searching for a root file system (typically, an image of live environment) on all supported devices (because a bootloader does not pass any information about device used for booting to the kernel), mounting it and executing /sbin/init program on a mounted root file system that will continue the boot process. Unfortunately, Casper was not designed to meet computer forensics requirements and is responsible for damaged Ext3/4 file systems recovery during the boot (see above) and root file system spoofing.

grml mounted root file system from the hard drive

Currently, Casper may select fake root file system image on evidentiary media (e.g. HDD), because there are no authenticity checks performed (except optional UUID check for a possible live file system), and this fake root file system image may be used to execute malicious code during the boot with root privileges. Knoppix-based forensic Live CD distributions are vulnerable to the same attack.

List of Ubuntu-based distributions that allow root file system spoofing:

Distribution Version Notes
Helix3 2009R1
Helix3 Pro 2009R3
CAINE 1.5
DEFT Linux 5
Raptor 20091026
grml 2009.10 Actually, grml uses live-initramfs scripts (Casper fork)
BackTrack 4
SMART Linux (Ubuntu) 2010-01-20
FCCU GNU/Linux Forensic Boot CD 12.1

Vulnerable Knoppix-based distributions include: SPADA, LinEn boot CD, BitFlare.

Swap space activation

Incorrect automount policy

Incorrect write-blocking approach

See also