Difference between revisions of "Forensic Live CD issues"

Revision as of 22:30, 1 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:

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).

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

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:

find_livefs() {
    timeout="${1}"
    # first look at the one specified in the command line
    if [ ! -z "${LIVEMEDIA}" ]; then
        if check_dev "null" "${LIVEMEDIA}" "skip_uuid_check"; then
            return 0
        fi
    fi
    # don't start autodetection before timeout has expired
    if [ -n "${LIVEMEDIA_TIMEOUT}" ]; then
        if [ "${timeout}" -lt "${LIVEMEDIA_TIMEOUT}" ]; then
            return 1
        fi
    fi
    # or do the scan of block devices
    for sysblock in $(echo /sys/block/* | tr ' ' '\n' | grep -v loop | grep -v ram); do
        devname=$(sys2dev "${sysblock}")
        fstype=$(get_fstype "${devname}")
        if /lib/udev/cdrom_id ${devname} > /dev/null; then
            if check_dev "null" "${devname}" ; then
                return 0
            fi
        elif is_nice_device "${sysblock}" ; then
            for dev in $(subdevices "${sysblock}"); do
                if check_dev "${dev}" ; then
                    return 0
                fi
            done
        elif [ "${fstype}" = "squashfs" -o \
                "${fstype}" = "ext3" -o \
                "${fstype}" = "ext2" ]; then
            # This is an ugly hack situation, the block device has
            # an image directly on it.  It's hopefully
            # casper, so take it and run with it.
            ln -s "${devname}" "${devname}.${fstype}"
            echo "${devname}.${fstype}"
            return 0
        fi
    done
    return 1
}

Swap space activation

Incorrect automount policy for removable media

Incorrect write-blocking approach

Software RAID (Linux RAID) activation