From 5d6946c42cf8faa64f609155cf3b121ff8d78474 Mon Sep 17 00:00:00 2001 From: Nicholas Chin Date: Thu, 12 Oct 2023 17:57:06 -0600 Subject: util/e6400-flash-unlock: Rename to dell-flash-unlock This more accurately describes the scope of the utility. Signed-off-by: Nicholas Chin --- util/dell-flash-unlock/README.md | 102 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 102 insertions(+) create mode 100644 util/dell-flash-unlock/README.md (limited to 'util/dell-flash-unlock/README.md') diff --git a/util/dell-flash-unlock/README.md b/util/dell-flash-unlock/README.md new file mode 100644 index 00000000..ba45ddcc --- /dev/null +++ b/util/dell-flash-unlock/README.md @@ -0,0 +1,102 @@ +# Dell Laptop Internal Flashing + +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. It seems like +this works on any Dell laptop that has an EC similar to the SMSC MEC5035 on the +E6400, which mainly seem to be the Latitude and Precision lines starting from +around 2008 (E6400 era). + +## TL;DR +Run `make` to compile the utility, and then run `sudo ./dell_flash_unlock` and +follow the directions it outputs. + +## Confirmed supported devices +- Latitude E6400 +- Latitude E6410 +- Latitude E4310 +- Latitude E6430 +- Precision M6800 + +It is likely that any other Latitude/Precision laptops from the same era as +devices specifically mentioned in the above list will work as Dell seems to use +the same ECs in one generation. + +## Detailed device specific behavior +- On GM45 era laptops, the expected behavior is that you will run the utility + for the first time, which will tell the EC to set the descriptor override on + the next boot. Then you will need to shut down the system, after which the + system will automatically boot up. You should then re-run the utility to + disable SMM, after which you can run flashrom. Finally, you should run the + utility a third time to reenable SMM so that shutdown works properly + afterwards. +- On 1st Generation Intel Core systems such as the E6410 and newer, run the + utility and shutdown in the same way as the E6400. However, it seems like the + EC no longer automatically boots the system. In this case you should manually + power it on. It also seems that the firmware does not set the BIOS Lock bit + when the descriptor override is set, making the 2nd run after the reboot + technically unnecessary. There is no harm in rerunning it though, as the + utility can detect when the flash is unlocked and perform the correct steps + as necessary. + +## 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. -- cgit v1.2.1