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Trammell Hudson 2016-08-07 13:50:06 -04:00
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@ -24,7 +24,14 @@ significant frustration.
Threat model
---
Heads considers two broad classes of threats:
* Attackers with physical access to the system
** Customs officials, LEO, etc with brief access
** "Evil maid" attacks with longer, but still limited access (sans password)
** Stolen machines, with unlimited physical access without password
** Insider attacks with unlimited time, with password
** Insider attacks with unlimited time, with password and without regard for the machine
* Attackers with ring0 code execution on the runtime system
The first is hardest to deal with since it allows an attacker to
@ -43,9 +50,27 @@ a software attack, we have better hopes of handling with some harware
modifications. The SPI flash chip's boot block protection modes can
be locked on and the WP# pin grounded, which will prevent any software
attacks from overwriting that portion of the boot ROM. This gives us
a better root of trust than any of the other x86 boot processes,
since we now have
a better root of trust than the EFI configurations, most of which do
not lock the boot ROM.
Even if they are not able to write to the ROM, the attackers might
be able to use their software code execution to modify the system
software or boot partition on the drive. The recommended OS
configuration is a read-only `/boot` and `/` filesystem, with
only the user data directories writable. Additional protection
comes from using dm-verity on the file systems, which will
detect any writes to the filesystem through a hash tree
that is signed by the user's (offline) key.
Updates to `/` or `/boot` will require a special boot mode,
which can be selected by the boot firmware. After the file
systems are updated, the user can sign the new hashes with their
key on a different machine and store the signed root hash on the
drive. TPM keys might need to be migrated as well for the recovery
boot mode. On next boot the firmware will mount the drives read-only
and verify that the correct key was used to sign the changes,
and the TPM should be able to unseal the secrets for TPMTOTP
as well as the drive decryption.