Revoke Secure Boot Keys on i.MX

This page covers how to revoke a SRK key on i.MX SoC boards. This includes the purpose of revocations, and where to find more information on this topic.

Revocation of a Key: Overview

Revoking a SRK key should be done when one or more SRK keys become compromised. The SRK keys are keys permanently copied to the hardware and are used during boot to unsure the Root of Trust.

When a SRK key is revoked, the key is considered unreliable. Thus, any image signed with that key will not be executed during boot. This is a permanent change to the hardware.

The example used on this page is a root of trust with 4 SRK keys fused to an i.MX machine. For different architectures, some commands may differ. Details on how to setup Secure Boot on i.MX machines can be found under Secure Boot (Hardware Root of Trust).

The following table lists the values for each SRK key. The value for SRK_REVOKE mask is used for the revocation command line, and is detailed in the following sections. The value for srk-index is a parameter for sign-file.sh, used to sign the boot artifacts.

Table 8 **SRK Revoke i.MX**
SRK key	Source index	SRK_REVOKE[3:0]	SRK_REVOKE[hex]	srk-index
SRK0	0	0001	0x1	1
SRK1	1	0010	0x2	2
SRK2	2	0100	0x4	3
SRK3	3	1000	0x8	4

srk-index is a sign-file.sh parameter that defines a SRK key to be used to sign the SPL.

Columns 1, 2 and 3 are from Table 4 from the Secure Boot Using HABv4 Guide.

Note

In a HABv4 environment, only the first 3 SRK keys can be revoked. In a AHAB environment, all 4 SRK can be revoked. For more details, check the Code-Signing Tool User’s Guide. When there is no available SRK key, the board cannot boot!

How to Sign the Boot Image for Revoking a Key

The first step is to make sure there are other SRK keys available for the boot. A key can only be revoked after a secure boot with a different key is executed, with the permission to unlock SRK_REVOKE write access. After that a fuse is burned. In short:

boot with an available SRK key, different from the one to revoke

unlock SRK_REVOKE

fuse a register according to the SRK key being revoked.

The signing process is based on the commands from Secure Boot (Hardware Root of Trust), adding two parameters:

--srk-index = <value> to choose the SRK key to be used on that boot sequence. For the right value, consult SRK Revoke i.MX.

--enable-revoke to unlock the key revocation register write access.

Write access to register SRK_REVOKE is protected by the bit SRK_REVOKE_LOCK. This can be configured by CST. The parameter --enable-revoke brings the configuration needed by CST to unlock write access to SRK_REVOKE, making revocation possible.

Warning

After revoking a SRK key, it cannot be used to boot the board again. A board with no remaining reliable SRK keys does not boot.

For example, for a imx6ullevk-sec SPL image to be signed with SRK1, use the following command:

#Sign the MFGTool SPL file
./sign-file.sh --engine SW --key-dir $KEY_PATH \
            --cst $CST_PATH \
            --spl $SPL_PATH/mfgtool-files-imx6ullevk-sec/SPL-mfgtool \
            --fix-sdp-dcd \
            --srk-index 2 \
            --enable-revoke

#Sign the SPL file
./sign-file.sh --engine SW --key-dir $KEY_PATH \
            --cst $CST_PATH \
            --spl $SPL_PATH/SPL-imx6ullevk-sec \
            --srk-index 2

Warning

In the example, only the SPL from MFGtool has write access to revoke the key. The suggestion is to use bootloader.uuu to load the MFGTool SPL, and then U-Boot prompt to perform the fuse programming command. For other boot scripts,you may be required to include --enable-revoke to the SPL file signing process (second command).

How to Revoke a Key

The suggestion is to use bootloader.uuu to access U-Boot prompt for executing the following command:

fuse prog <bank> <word> <hexval>

The values for <bank> and <word> for the register SRK_REVOKE can be found on the SoC Reference Manual. The value for <hexval> is from SRK Revoke i.MX.

For example, to revoke SRK2 for imx6ullevk-sec:

fuse prog 5 7 0x4
Programming bank 5 word 0x00000007 to 0x00000004...
Warning: Programming fuses is an irreversible operation!
        This may brick your system.
        Use this command only if you are sure of what you are doing!

Really perform this fuse programming? <y/N>
y

The following error happens when the key revocation write access is not available (SRK_REVOKE is not unlocked). This can be fixed by adding --enable-revoke during the signing of the boot image:

mxc_ocotp fuse_prog(): Access protect error
ERROR

After the revocation of SRK2, it can never be used to boot that board again. Test it by signing again using this SRK key and the boot must fail.

How to Revoke a Key for Devices in a Fleet

The method suggested here describes the commands needed to revoke a key from the SoC perspective. It requires serial download and console and bootloader access, which are not always accessible on devices in the field. However, this is the base procedure to be used on a fleet.

The process can be automated in a Factory by creating a signed SPL using another SRK key and enabling the SRK_REVOKE write access. While on this, the bootcmd can be customized to perform the fusing command needed to revoke the compromised key.

The fusing can be performed in Linux® Kernel mode instead, when the system is configured to allow this kind of execution.

Then the firmware update is performed in a Wave—described in detail under Production Targets.

After the revocation wave, another firmware update wave is required. This time, with the bootloader configured to disable write access to the SRK_REVOKE, and still using the reliable SRK key.

This is a two-steps process which is highly dependent on the device configuration and access, and requires caution. The revoke fusing command can make the device unavailable if not executed properly.

To get help with the revocation automatization, open a support ticket.