Difference between revisions of "Forensic Live CD issues"

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== The problem ==
 
== The problem ==
  
[[Tools#Forensics_Live_CDs | 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.
+
[[Live CD|Forensic Live CDs]] 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. Unfortunately, community-developed distributions are no exception here. Finally, it turns out that many Linux-based forensic Live CDs are not tested properly and there are no suitable test cases published.
  
 
== Another side of the problem ==
 
== 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.
+
Another side of the problem of insufficient testing of forensic Live CDs is that many users do not know what happens "under the hood" of the provided operating system and cannot adequately test them.
  
 
=== Example ===
 
=== Example ===
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== Problems ==
 
== 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.
+
Each problem is followed by an up to date list of distributions affected.
  
=== Journaling file systems updates ===
+
=== Journaling file system 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:
 
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:
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
 
  |-
 
  |-
 
  |  XFS
 
  |  XFS
  |  Always
+
  |  Always (when unmounting)
  |  "norecovery" flag does not help. To disable data writes: use "ro,loop" flags. The bug was fixed in recent 2.6 kernels.
+
  |  "norecovery" flag does not help (fixed in recent 2.6 kernels). To disable data writes: use "ro,loop" flags.
 
  |}
 
  |}
  
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).
+
Incorrect mount flags can be used to mount file systems 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 Live CD distributions mount and recover damaged 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]]
 
[[Image:ext3 recovery.png|thumb|right|[[Helix3]]: damaged Ext3 recovery during the boot]]
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  |  SPADA
 
  |  SPADA
 
  |  4
 
  |  4
 +
|-
 +
|  DEFT Linux
 +
|  7
 
  |}
 
  |}
  
 
=== Root file system spoofing ===
 
=== 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.
+
Most Ubuntu-based forensic Live CD distributions use Casper (a 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.
  
 
[[Image:Grml.png|thumb|right|[[grml]] mounted root file system from the [[hard drive]]]]
 
[[Image:Grml.png|thumb|right|[[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.
+
Currently, Casper may select fake root file system image on evidentiary media (e.g. [[Hard Drive|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:
 
List of Ubuntu-based distributions that allow root file system spoofing:
Line 82: Line 85:
 
  !  Distribution
 
  !  Distribution
 
  !  Version
 
  !  Version
!  Notes
 
 
  |-
 
  |-
 
  |  Helix3
 
  |  Helix3
 
  |  2009R1
 
  |  2009R1
|
 
 
  |-
 
  |-
 
  |  Helix3 Pro
 
  |  Helix3 Pro
 
  |  2009R3
 
  |  2009R3
|
 
 
  |-
 
  |-
 
  |  CAINE
 
  |  CAINE
 
  |  1.5
 
  |  1.5
|
 
 
  |-
 
  |-
 
  |  DEFT Linux
 
  |  DEFT Linux
 
  |  5
 
  |  5
|
 
 
  |-
 
  |-
 
  |  Raptor
 
  |  Raptor
  |  20091026
+
  |  2.0
|
+
|-
+
|  grml
+
|  2009.10
+
|  Actually, [[grml]] uses live-initramfs scripts (Casper fork)
+
 
  |-
 
  |-
 
  |  BackTrack
 
  |  BackTrack
 
  |  4
 
  |  4
|
 
 
  |-
 
  |-
 
  |  SMART Linux (Ubuntu)
 
  |  SMART Linux (Ubuntu)
 
  |  2010-01-20
 
  |  2010-01-20
|
 
 
  |-
 
  |-
 
  |  FCCU GNU/Linux Forensic Boot CD
 
  |  FCCU GNU/Linux Forensic Boot CD
 
  |  12.1
 
  |  12.1
|
 
 
  |}
 
  |}
  
Vulnerable Knoppix-based distributions include: SPADA, LinEn boot CD, BitFlare.
+
Vulnerable Knoppix-based distributions include: SPADA, LinEn Boot CD, BitFlare.
 +
 
 +
[http://anti-forensics.ru/ Anti-Forensics.Ru project] [http://digitalcorpora.org/corp/images/aor/ released several ISO 9660 images] used to test various Linux Live CD distributions for root file system spoofing (description for all images is [http://anti-forensics.ru/casper/ here]).
  
 
=== Swap space activation ===
 
=== Swap space activation ===
  
=== Incorrect automount policy for removable media ===
+
''Feel free to add information about swap space activation during the boot in some distributions''
 +
 
 +
=== Incorrect mount policy ===
 +
 
 +
==== rebuildfstab and scanpartitions scripts ====
 +
 
 +
Several forensic Linux Live CD distributions (Helix3 2009R1, Helix3 Pro 2009R3, old versions of CAINE, old versions of grml) use rebuildfstab and scanpartition scripts to create entries for attached devices in ''/etc/fstab''. Some versions of these scripts use wrong wildcards while searching for available block devices (''/dev/?d?'' instead of ''/dev/?d*''), this results in missing several "exotic" devices (like /dev/sdad, /dev/sdad1, etc) and in data writes when mounting them (because fstab lacks of read-only mount options for these devices).
  
 
=== Incorrect write-blocking approach ===
 
=== Incorrect write-blocking approach ===
  
== See also ==
+
Some forensic Linux Live CD distributions rely on [[hdparm]] and [[blockdev]] programs to mount file systems in read-only mode (by setting the underlying block device to read-only mode). Unfortunately, setting the block device to read-only mode does not guarantee that [http://archives.free.net.ph/message/20090721.105120.99250e3f.en.html no write commands will be passed to the drive].
 +
 
 +
== External links ==
  
 
* [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_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]
 
* [http://www.computer-forensics-lab.org/pdf/Linux_for_computer_forensic_investigators.pdf Linux for computer forensic investigators: «pitfalls» of mounting file systems]
 +
 +
[[Category:Live CD]]

Latest revision as of 04:28, 7 October 2012

Contents

The problem

Forensic Live CDs 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. Unfortunately, community-developed distributions are no exception here. Finally, it turns out that many Linux-based forensic Live CDs are not tested properly and there are no suitable test cases published.

Another side of the problem

Another side of the problem of insufficient testing of forensic Live CDs is that many users do not know what happens "under the hood" of the provided operating system 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

Each problem is followed by an up to date list of distributions affected.

Journaling file system 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 (when unmounting) "norecovery" flag does not help (fixed in recent 2.6 kernels). To disable data writes: use "ro,loop" flags.

Incorrect mount flags can be used to mount file systems 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 Live CD distributions mount and recover damaged 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
DEFT Linux 7

Root file system spoofing

Most Ubuntu-based forensic Live CD distributions use Casper (a 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
Helix3 2009R1
Helix3 Pro 2009R3
CAINE 1.5
DEFT Linux 5
Raptor 2.0
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.

Anti-Forensics.Ru project released several ISO 9660 images used to test various Linux Live CD distributions for root file system spoofing (description for all images is here).

Swap space activation

Feel free to add information about swap space activation during the boot in some distributions

Incorrect mount policy

rebuildfstab and scanpartitions scripts

Several forensic Linux Live CD distributions (Helix3 2009R1, Helix3 Pro 2009R3, old versions of CAINE, old versions of grml) use rebuildfstab and scanpartition scripts to create entries for attached devices in /etc/fstab. Some versions of these scripts use wrong wildcards while searching for available block devices (/dev/?d? instead of /dev/?d*), this results in missing several "exotic" devices (like /dev/sdad, /dev/sdad1, etc) and in data writes when mounting them (because fstab lacks of read-only mount options for these devices).

Incorrect write-blocking approach

Some forensic Linux Live CD distributions rely on hdparm and blockdev programs to mount file systems in read-only mode (by setting the underlying block device to read-only mode). Unfortunately, setting the block device to read-only mode does not guarantee that no write commands will be passed to the drive.

External links