util/e6400-flash-unlock: restore README

when nicholas added this, he removed the README because it's
going on libreboot.org instead. however, i merged a WIP version
of his page for now because i want to get the e6400 going in
libreboot sooner. so, temp-readding this README. will just
link to this on codeberg or something, from the lb docs

NOTE: I didn't write this README, hence author field set
in the commit. Nicholas wrote it, but I (Leah Rowe) am just
adding it. so, git author set to nicholas, not me

util/e6400-flash-unlock/README.md

@@ -1,4 +1,71 @@
+# Dell Latitude E6400 Internal Flashing
 
-This documentation has become part of lbwww. See:
+This utility allows you to use flashrom's internal programmer to program the
+entire BIOS flash chip from software while still running the original Dell
+BIOS, which normally restricts software writes to the flash chip.
 
-<https://libreboot.org/docs/install/e6400_flash_unlock.html>
+## TL;DR
+Run `make` to compile the utility, and then run `sudo ./e6400_flash_unlock` and
+follow the directions it outputs.
+
+## How it works
+There are several ways the firmware can protect itself from being overwritten.
+One way is the Intel Flash Descriptor (IFD) permissions. On Intel systems, the
+flash image is divided into several regions such as the IFD itself, Gigabit
+Ethernet (GBE) non-volative memory, Management Engine (ME) firmware, Platform
+Data (PD), and the BIOS. The IFD contains a section which specifies the
+read/write permissions for each SPI controller (such as the host system) and
+each region of the flash, which are enforced by the chipset.
+
+On the Latitude E6400, the host has read-only access to the IFD, no access to
+the ME region, and read-write access to the PD, GBE, and BIOS regions. In order
+for flashrom to write to the entire flash internally, the host needs full
+permissions to all of these regions. Since the IFD is read only, we cannot
+change these permissions unless we directly access the chip using an external
+programmer, which defeats the purpose of internal flashing.
+
+However, Intel chipsets have a pin strap that allows the flash descriptor
+permissions to be overridden depending on the value of the pin at power on,
+granting RW permissions to all regions. On the ICH9M chipset on the E6400, this
+pin is HDA\_DOCK\_EN/GPIO33, which will enable the override if it is sampled
+low. This pin happens to be connected to a GPIO controlled by the Embedded
+Controller (EC), a small microcontroller on the board which handles things like
+the keyboard, touchpad, LEDs, and other system level tasks. Software can send a
+certain command to the EC, which tells it to pull GPIO33 low on the next boot.
+
+Although we now have full access according to the IFD permissions, we still
+cannot flash the whole chip, due to another protection the firmware uses.
+Before software can update the BIOS, it must change the BIOS Write Enable
+(BIOSWE) bit in the chipset from 0 to 1. However, if the BIOS Lock Enable (BLE)
+bit is also set to 1, then changing the BIOSWE bit triggers a System Management
+Interrupt (SMI). This causes the processor to enter System Management Mode
+(SMM), a highly privileged x86 execution state which operates transparently to
+the operating system. The code that SMM runs is provided by the BIOS, which
+checks the BIOSWE bit and sets it back to 0 before returning control to the OS.
+This feature is intended to only allow SMM code to update the system firmware.
+As the switch to SMM suspends the execution of the OS, it appears to the OS
+that the BIOSWE bit was never set to 1.  Unfortunately, the BLE bit cannot be
+set back to 0 once it is set to 1, so this functionality cannot be disabled
+after it is first enabled by the BIOS.
+
+Older versions of the E6400 BIOS did not set the BLE bit, allowing flashrom to
+flash the entire flash chip internally after only setting the descriptor
+override. However, more recent versions do set it, so we may have hit a dead
+end unless we force downgrade to an older version (though there is a more
+convenient method, as we are about to see).
+
+What if there was a way to sidestep the BIOS Lock entirely? As it turns out,
+there is, and it's called the Global SMI Enable (GBL\_SMI\_EN) bit. If it's set
+to 1, then the chipset will generate SMIs, such as when we change BIOSWE with
+BLE set. If it's 0, then no SMI will be generated, even with the BLE bit set.
+On the E6400, GBL\_SMI\_EN is set to 1, and it can be changed back to 0, unlike
+the BLE bit. But there still might be one bit in the way, the SMI\_LOCK bit,
+which prevents modifications to GBL\_SMI\_EN when SMI\_LOCK is 1. Like the BLE
+bit, it cannot be changed back to 0 once it set to 1. But we are in luck, as
+the vendor E6400 BIOS leaves SMI\_LOCK unset at 0, allowing us to clear
+GBL\_SMI\_EN and disable SMIs, bypassing the BIOS Lock protections.
+
+There are other possible protection mechanisms that the firmware can utilize,
+such as Protected Range Register settings, which apply access permissions to
+address ranges of the flash, similar to the IFD. However, the E6400 vendor
+firmware does not utilize these, so they will not be discussed.