160 lines
7.4 KiB
Markdown
160 lines
7.4 KiB
Markdown
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---
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title: Secure Boot Shenanigans
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date: 2016-11-11
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layout: Post
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tags:
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- linux
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hero: /assets/2016-11-11/banner.a6acff235ea8.jpg
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classes:
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header: header-black-gradient
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---
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I bought a [Razer Blade Stealth](http://www.razerzone.com/gaming-systems/razer-blade-stealth)
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recently as an ultrabook I can take to lectures and just generally use when I'm out and about
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as a companion to the Pixel C I'm already using. Generally just for those annoying edgecases
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where it's not quite enough to just have Android ;)
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---
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Anyway, it's dual-booting Arch and Windows 10. I wanted to see how much I could lock the platform
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down in terms of security, so, first steps...
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## Secure Boot
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My end goal here is to use my own PKI to sign binaries which I deem acceptable to run on my system.
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I'm not going to go into the details of the PKI used for Secure Boot, but more... the specifics of
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modifying my laptop's firmware to allow me to substitute my own PKI for the factory shipped one.
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However: Secure Boot on the Razer Blade Stealth is... a tad lacking. Why?
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Razer have hidden the option to get to the key management options on the Razer Blade Stealth,
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for some reason. I'm too lazy to contact Razer support to get a modified firmware, and I don't know
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if they would even oblige, or just tell me to disable Secure Boot.
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*Note that I went back and took this picture afterwards, which is why this is already in User mode and the Vendor Keys are not active.*
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But that's OK. The Razer Blade Stealth uses [AptioV from AMI](https://ami.com/products/bios-uefi-firmware/aptio-v/)
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as its firmware, which is fairly well understood...
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### Notes
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* Using KeyTool.efi, I quickly discovered that the PK shipped with my system was the AMI
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Test PK: `DO NOT TRUST - AMI Test PK`. *sigh*
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### Modifying the firmware, or "this way leads to insanity and a voided warranty"
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```txt
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WARNING WARNING WARNING: There's a great potential to make your shiny new ultrabook into a £999+ brick.
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I'm not responsible if you do this.
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WARNING WARNING: This will almost certainly void your warranty. Maybe don't do this if you
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like your warranty?
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WARNING: if you do this, you probably want to not have Bitlocker enabled first, or at the very least
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to have your Bitlocker recovery key first. Replacing your system's firmware *will* change the TPM
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measurements and cause unsealing to fail (by design!).
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```
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#### Dumping the firmware
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Using AMI's AFUWIN tool [(download)](https://www.wimsbios.com/amiflasher.jsp), it's possible to
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dump (hint: the `Save` button is your friend!) the BIOS included with the system, since Razer have
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not published any firmware updates for this system yet, so we can't just download it.
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BACK THIS FILE UP. You never know when you might need it(!).
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OK, great, you have a dump of your system's firmware (excluding the Management Engine's firmware).
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What now?
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#### Taking a peek under the hood
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Well, using [UEFITool](https://github.com/LongSoft/UEFITool) it's possible to unpack the ROM image
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that you've dump and make modifications to it. If you're following along, then as of writing, you
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should use the older [0.21.5](https://github.com/LongSoft/UEFITool/releases/tag/0.21.5) version
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which supports repacking the ROM with changes.
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Once you've done that, you need to locate the UEFI binary which is the configuration utility
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(i.e. the thing with all the settings which most people think of when you say "BIOS"). The easiest
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way to find this for me was to just search for anything containing the Unicode string "Key Management"(!)
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Having found this binary, you can then dump it to a separate file (using **Extract body...** not
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**Extract as is...**). As a brief overview, this binary contains the setup menus, and often contains
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a bunch of functionality that's been masked out by the OEM, but which is still compiled in to the binary.
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This includes, thankfully, the options for resetting the system back into Secure Boot Setup Mode.
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A tool called [Universal IFR Extractor](https://github.com/donovan6000/Universal-IFR-Extractor) can help
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shine some light on when this is the case, by showing you a textual representation of what the *I*nternal
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*F*orms *R*epresentation tree looks like. It's super helpful, because it also displays the hex representation
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alongside, as well as displaying the location in the binary where it was found.
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In my case, I discovered that there's a check to see if the variable `0xB0E` is set to `0x0`, and if
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so then the interesting menu items are hidden. Boo. We can fix that using a hex editor ;)
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#### Hmm... I want... ALL OF THEM
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By replacing the byte sequence `0A 82 12 06 0E 0B 00 00` (suppress if: variable 0xB0E has value 0x0)
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with `0A 82 12 06 0E 0B 00 FF` (suppress if: variable 0xB0E has value 0xFF00), then we can force
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all of these previously-hidden menu items to be shown. I'm being a bit rash here and just hoping that
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this doesn't appear in the text section of the binary. Feel free to be a bit more surgical ;)
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Having done that, you can now repack the ROM using UEFITool (using **Replace body...**), and then
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save it (*not* over the top of your clean ROM!).
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#### The Dangerous Bit
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**You should reread the warnings I wrote above before doing this. It's not my fault if you brick your
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system, even if you follow all of these instructions to the letter.**
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Again using AFUWIN you can take your freshly unlocked ROM and flash it to your system.
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Once that's done, cross your fingers and reboot! Hopefully you should now seen the `Key Management`
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options under `Secure Boot` in the options :)
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## Drive Encryption
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### Windows
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Windows 10 Pro ships with Bitlocker, and the Razer Blade Stealth has a TPM, which means
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that you can attest to the state of the system using the TPM to ensure that you're being booted
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on the hardware (and with the software configuration) you think you're being booted on.
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However, the RBS only ships with Windows 10 Home, so after entering my W10 Pro license key, and
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waiting what felt like an age for it to apply the additional features, and reboot (twice!), I
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managed to Bitlocker my drive.
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For additional security, you can optionally [set it up in TPM+PIN mode](http://www.howtogeek.com/262720/how-to-enable-a-pre-boot-bitlocker-pin-on-windows/),
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but I'm not entirely convinced this provides any serious additional benefit beyond a BIOS/disk
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unlock password (with TPMed Bitlocker).
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### Arch
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Arch has dm-crypt support, which is both fairly standard and boring. I'm using LUKS+dm-crypt,
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with a reasonably long passphrase. It looks there's some interest in
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[TrustedGrub](https://projects.sirrix.com/trac/trustedgrub/) to use the TPM measurements to
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then unseal a key, similarly to Bitlocker, but I haven't done this.
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### UEFI
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I added a disk unlock password, as well as a UEFI administrator password to avoid unauthorised
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changes to the boot order or to the UEFI configuration. Boot from USB is also disabled and removed
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from the boot order.
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## Bonus Pictures
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Before:
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...and after:
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As well as my new boot splash:
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