Portcullis Security Advisory

Vulnerable System: 

This vulnerability affects any workstation running the ACPI 1.6 BIOS
implementation. 

Vulnerability Title: 

BIOS code logic error 

Vulnerability discovery and development: 

The Portcullis R&D team discovered this vulnerability. Whilst assessing
a secured (software access control) laptop environment the team
discovered it was possible to cause any system running the vulnerable
version of the BIOS to become unstable and no longer boot. Once the
coding error was known it could be exploited to cause a near permanent
Denial of Service attack. 

Affected systems: 

The fault was initially found on a Toshiba Satellite Pro A60 running the
ACPI BIOS version 1.60. This is a fundamental error that could be
exploited in any workstation based on the 1.60 version of the ACPI BIOS.
All hard disks, including those encrypted by software or hardware, will
be affected. Note that later versions of the ACPI (v1.7 and v1.8), also
appear to contain the same coding error however, Portcullis have not
tested these versions. 

Details:

There is a programming error in the BIOS when the Master Boot Record
(MBR) is searched for the bootable disk partition. The error in the BIOS
code results in only the first slot in the MBR partition table being
tested for the active partition. Where the active partition is not
described in the first slot of the MBR table, the BIOS ignores the
remaining slots and searches other devices for a boot mechanism, and in
consequence fails to start the Operating System. 

The BIOS seeks to validate the contents of the MBR and the boot sector
in the active partition to achieve confidence in the integrity of the
boot mechanism. At various points on the start-up sequence the BIOS
reads the MBR from disk and tests the data therein. In at least one
case, the BIOS sets out to test all 4 entries in the MBR partition table
but due to treating an absolute pointer as one relative to the start of
the MBR, it drops out of the loop after the first iteration. Slots 2, 3
and 4 are never examined and the active partition not found.
Specifically, the 512 byte MBR end-of-table is tested by comparing the
pointer to see if it has reached end-of-table at 510 bytes.
Unfortunately, in the ACPI version the pointer is set to MBR-start plus
start-of-table within MBR. Therefore, the pointer begins with a value
larger than the size of the MBR. The result is that the first slot
fails, when testing for the active partition byte, the test will show an
end-of-table result: the subsequent slots are ignored.

An example of the problem is shown when the MBR is read into boot
address 0000:7C00h and then tested for an active partition.

    xor bx, bx             ;zero BX

    mov es, bx

    mov bx, 7C00h ;buffer address ES:BX set to 0000:7C00h

    mov cx, 1 ;set sector and cylinder value

    xor dh, dh ;and head value to read cylinder 0, head 0, sector 1

    mov dl, 80h ;set first hard disk drive

    mov ax, 0201h ;set read 1 sector

    int 13h ;call BIOS routine to read MBR and assume success

    add bx, 446 ;ES:BX point to start of partition table in MBR (BX =
7DBEh)

NextSlot:

    cmp es:[bx], 80h ;is this slot bootable?

    je ActiveOk ;yes, drop out of loop

    add bx, 16 ;increment by length of slot in table 
(BX becomes 7C00h+446+16)

    cmp bx, 510 ;end of slot table reached? error! 
should be cmp bx, 7C00h + 510

    jb NextSlot ;no, look at next entry - branch never taken

    stc ;set no active partition found status

    ret

ActiveOk:

    clc ;set active partition found status

ret

Impact:

The system will not boot. Standard analysis tools will not identify the
contents of the MBR partition as invalid. Unless the help-desk engineer
is aware of this BIOS error, he/she will be unable to diagnose the
fault. Therefore, the impact is denial of service: the system will not
boot and yet there is nothing wrong with the contents of the hard disk.

Exploit:

The error in the BIOS code means that any workstation using this version
of the BIOS, can be configured such that the bootable partition is
defined below the first slot in the MBR partition table and will not
boot. An attack at any time during an operating session can leave the
workstation subsequently unable to reboot. The nature of the fault means
that it is very difficult to identify and may leave the workstation
inoperable for an extended period of time.

Vendor Notified:

Toshiba were notified initially by email, as no response was received
this was followed up by telephone on Friday 4-Feb-2005 and again on
Wednesday 23-Feb-2005.

Vendor Response:

The A60 laptop works with their supplied configuration. Fault not
recognised. Reference supplied by Toshiba is 1-585 92244. When the fault
was reported again on 23-Feb-2005 a new fault reference was supplied
1-165 162202 but no technical response was provided by Toshiba.

Workaround/Fix

Either, the BIOS code must be corrected by the manufacturer, or a
monitoring device installed to detect any re-configuration that would
exploit the vulnerability. Validating and correcting the contents of the
MBR at system shutdown is a minimum requirement in these circumstances. 

Copyright: 

Copyright (c) Portcullis Computer Security Limited 2005. All rights
reserved worldwide.



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