coreboot-kgpe-d16/util/kbc1126

KBC1126 firmware tools

Many HP laptops use 8051-based SMSC KBC1098/KBC1126 as embedded controller. Two blobs can be found in the HP firmware images. The kbc1126_ec_dump and kbc1126_ec_insert tools are used to dump the two blobs from the factory firmware and insert them to the firmware image.

Firmware format

We can easily find the BIOS region of the HP laptop firmware from the HP firmware update tool, which can be downloaded from the HP website. Now I take HP Elitebook 8470p as an example. This laptop has a 16MB flash chip, the last 5MB of which is the BIOS region.

I use radare2 to analyze the firmware. Open the firmware image, and we can see 8 bytes at $s-0x100 ($s means the image size).

[0x00000000]> x @ $s-0x100

• offset - 0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDEF
  0x00ffff00 fff7 0008 f700 08ff 0000 0000 0000 0000 ................

X86 machines map the firmware at the end of the memory address space. These 8 bytes tell the address of the two blobs, which we call FW1 (uses bytes 0-3) and FW2 (uses bytes 4-7).

Let's look at FW1. The first two bytes mean the address of FW1 is 0xfff700 (these two bytes use big endian), i.e. $s-0x900. Byte 2 and 3 are just complements of byte 1 and 2 (in this case, 0x0008=0xffff-0xfff7).

[0x00000000]> x @ $s-0x900

• offset - 0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDEF
  0x00fff700 fc07 c13e 02ff 1000 0000 0000 0000 0000 ...>............

Both FW1 and FW2 use the same format: the first two bytes is payload length, then a two-byte checksum, then the payload. The payload length and checksum are both in little endian. The checksum is SYSV checksum.

How to use the tools

kbc1126_ec_dump is used to dump FW1 and FW2. Run kbc1126_ec_dump bios.rom, then bios.rom.fw1 and bios.rom.fw2 are generated in the working directory.

kbc1126_ec_insert will overwrite a firmware image by inserting FW1 and FW2 in it. Please run it for its usage. You need to specify the offsets for FW1 and FW2. Using negative offset is recommended, which means the distance to the end of the image. For example, if we want to insert FW1 and FW2 at $s-0x900 and $s-0x90000 as the hp/8470p factory firmware to coreboot.rom, you can run kbc1126_ec_insert coreboot.rom bios.rom.fw1 bios.rom.fw2 -0x900 -0x90000.