From 041bca5ba3adb48750d0a438cb3b1356a0c2e603 Mon Sep 17 00:00:00 2001 From: Simon Glass Date: Thu, 13 Jun 2013 15:10:11 -0700 Subject: Add verified boot information and test Add a description of how to implement verified boot using signed FIT images, and a simple test which verifies operation on sandbox. The test signs a FIT image and verifies it, then signs a FIT configuration and verifies it. Then it corrupts the signature to check that this is detected. Signed-off-by: Simon Glass --- doc/uImage.FIT/verified-boot.txt | 104 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 104 insertions(+) create mode 100644 doc/uImage.FIT/verified-boot.txt (limited to 'doc/uImage.FIT') diff --git a/doc/uImage.FIT/verified-boot.txt b/doc/uImage.FIT/verified-boot.txt new file mode 100644 index 0000000..3c83fbc --- /dev/null +++ b/doc/uImage.FIT/verified-boot.txt @@ -0,0 +1,104 @@ +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 ben 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 alreay 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 it 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 -- cgit v1.1