Documentation: Introduce HP Sure Start and the method to bypass it

Change-Id: Id198afdaa13b4c361e1b77a56d5a2436ed1c4c86
Signed-off-by: Iru Cai <mytbk920423@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/45577
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diff --git a/Documentation/mainboard/hp/hp_sure_start.md b/Documentation/mainboard/hp/hp_sure_start.md
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+# HP Sure Start
+
+According to the [HP Sure Start Technical Whitepaper], HP Sure Start is a chipset
+and processor independent firmware intrusion detection and automatic repair system.
+It is implemented in HP notebooks since 2013, and desktops since 2015.
+
+This document talks about some mechanism of HP Sure Start on some machines, and
+the method to bypass it.
+
+## Laptops with SMSC MEC1322 embedded controller
+
+Haswell EliteBook, ZBook and ProBook 600 series use SMSC MEC1322 embedded controller.
+The EC firmware implements HP Sure Start.
+
+A Haswell EliteBook has two flash chips. According to the strings in the EC firmware,
+the 16MiB flash chip that stores the BIOS firmware is called the *system flash*, and
+the 2MiB flash chip that stores part of the system flash content is called the
+*private flash*. A Haswell ProBook 600 series laptop also uses MEC1322 and has similar
+EC firmware, but the HP Sure Start functions are not enabled.
+
+The private flash is connected to the EC, and is not accessible by the OS.
+It contains the following:
+
+- HP Sure Start policy header (starting with the string "POLI")
+- A copy of the Intel Flash Descriptor
+- A copy of the GbE firmware
+- Machine Unique Data (MUD)
+- Hashes of the IFD, GbE firmware and MUD, the hash algorithm is unknown
+- A copy of the bootblock, UEFI PEI stage, and microcode
+
+If the IFD of the system flash does not match the hash in the private flash, for example,
+modifying the IFD with ``ifdtool -u`` or ``me_cleaner -S``, the EC will recover the IFD.
+
+If the content of the private flash is lost, the EC firmware will still copy the IFD,
+bootblock and PEI to the private flash. However, the IFD is not protected after that.
+
+HP Sure Start also verifies bootblock, PEI, and microcode without using the private flash.
+EC firmware reads them from an absolute address of the system flash chip, which is
+hardcoded in the EC firmware. It looks like this verification is done with a digital
+signature. If the PEI volume is modified, EC firmware will recover it using the copy
+in the private flash. If the private flash has no valid copies of the PEI volume, and
+the PEI volume is modified, the machine will refuse to boot with the CapsLock LED blinking.
+
+## Bypassing HP Sure Start
+
+First search the mainboard for the flash chips. If there are two flash chips,
+the smaller one may be the private flash.
+
+For Intel boards, try to modify the IFD with ``ifdtool -u``, power on and shut down
+the machine, then read the flash again. If the IFD is not modified, it is likely to
+be recovered from the private flash. Find the private flash and erase it, then the IFD
+can be modified.
+
+To bypass the bootblock and PEI verification, we can modify the IFD to make the
+BIOS region not overlap with the protected region. Since the EC firmware is usually
+located at the high address of the flash chip (and in the protected region),
+we can leave it untouched, and do not need to extract the EC firmware to put it in
+the coreboot image.
+
+[HP Sure Start Technical Whitepaper]: http://h10032.www1.hp.com/ctg/Manual/c05163901
diff --git a/Documentation/mainboard/index.md b/Documentation/mainboard/index.md
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 ### EliteBook series
 
 - [HP Laptops with KBC1126 EC](hp/hp_kbc1126_laptops.md)
+- [HP Sure Start](hp/hp_sure_start.md)
 - [EliteBook 2560p](hp/2560p.md)
 - [EliteBook 8760w](hp/8760w.md)