105 lines
4.5 KiB
Plaintext
105 lines
4.5 KiB
Plaintext
U-Boot Verified Boot
|
|
====================
|
|
|
|
Introduction
|
|
------------
|
|
Verified boot here means the verification of all software loaded into a
|
|
machine during the boot process to ensure that it is authorised and correct
|
|
for that machine.
|
|
|
|
Verified boot extends from the moment of system reset to as far as you wish
|
|
into the boot process. An example might be loading U-Boot from read-only
|
|
memory, then loading a signed kernel, then using the kernel's dm-verity
|
|
driver to mount a signed root filesystem.
|
|
|
|
A key point is that it is possible to field-upgrade the software on machines
|
|
which use verified boot. Since the machine will only run software that has
|
|
been correctly signed, it is safe to read software from an updatable medium.
|
|
It is also possible to add a secondary signed firmware image, in read-write
|
|
memory, so that firmware can easily be upgraded in a secure manner.
|
|
|
|
|
|
Signing
|
|
-------
|
|
Verified boot uses cryptographic algorithms to 'sign' software images.
|
|
Images are signed using a private key known only to the signer, but can
|
|
be verified using a public key. As its name suggests the public key can be
|
|
made available without risk to the verification process. The private and
|
|
public keys are mathematically related. For more information on how this
|
|
works look up "public key cryptography" and "RSA" (a particular algorithm).
|
|
|
|
The signing and verification process looks something like this:
|
|
|
|
|
|
Signing Verification
|
|
======= ============
|
|
|
|
+--------------+ *
|
|
| RSA key pair | * +---------------+
|
|
| .key .crt | * | Public key in |
|
|
+--------------+ +------> public key ----->| trusted place |
|
|
| | * +---------------+
|
|
| | * |
|
|
v | * v
|
|
+---------+ | * +--------------+
|
|
| |----------+ * | |
|
|
| signer | * | U-Boot |
|
|
| |----------+ * | signature |--> yes/no
|
|
+---------+ | * | verification |
|
|
^ | * | |
|
|
| | * +--------------+
|
|
| | * ^
|
|
+----------+ | * |
|
|
| Software | +----> signed image -------------+
|
|
| image | *
|
|
+----------+ *
|
|
|
|
|
|
The signature algorithm relies only on the public key to do its work. Using
|
|
this key it checks the signature that it finds in the image. If it verifies
|
|
then we know that the image is OK.
|
|
|
|
The public key from the signer allows us to verify and therefore trust
|
|
software from updatable memory.
|
|
|
|
It is critical that the public key be secure and cannot be tampered with.
|
|
It can be stored in read-only memory, or perhaps protected by other on-chip
|
|
crypto provided by some modern SOCs. If the public key can be changed, then
|
|
the verification is worthless.
|
|
|
|
|
|
Chaining Images
|
|
---------------
|
|
The above method works for a signer providing images to a run-time U-Boot.
|
|
It is also possible to extend this scheme to a second level, like this:
|
|
|
|
1. Master private key is used by the signer to sign a first-stage image.
|
|
2. Master public key is placed in read-only memory.
|
|
2. Secondary private key is created and used to sign second-stage images.
|
|
3. Secondary public key is placed in first stage images
|
|
4. We use the master public key to verify the first-stage image. We then
|
|
use the secondary public key in the first-stage image to verify the second-
|
|
state image.
|
|
5. This chaining process can go on indefinitely. It is recommended to use a
|
|
different key at each stage, so that a compromise in one place will not
|
|
affect the whole change.
|
|
|
|
|
|
Flattened Image Tree (FIT)
|
|
--------------------------
|
|
The FIT format is already widely used in U-Boot. It is a flattened device
|
|
tree (FDT) in a particular format, with images contained within. FITs
|
|
include hashes to verify images, so it is relatively straightforward to
|
|
add signatures as well.
|
|
|
|
The public key can be stored in U-Boot's CONFIG_OF_CONTROL device tree in
|
|
a standard place. Then when a FIT is loaded it can be verified using that
|
|
public key. Multiple keys and multiple signatures are supported.
|
|
|
|
See signature.txt for more information.
|
|
|
|
|
|
Simon Glass
|
|
sjg@chromium.org
|
|
1-1-13
|