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BIOS ACPI DoS
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SUMMARY
A vulnerability in ACPI (Advanced Configuration and Power Interface) BIOS
has been found. The vulnerability allows a local attacker on any system
running the vulnerable version of the BIOS to make the system no longer
able to boot until the BIOS's vulnerability is fixed.
DETAILS
Vulnerable Systems:
* Toshiba Satellite Pro A60 running the ACPI BIOS version 1.60
* Any BIOS with ACPI version 1.60 and above may have this vulnerability
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
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.
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.
Disclosure Timeline:
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 Status:
The A60 laptop works with their supplied configuration. Fault not
recognized. 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:
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.
ADDITIONAL INFORMATION
The information has been provided by <mailto:PJD@portcullis-security.com>
Paul J Docherty.
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