Revoke Secure Boot keys on i.MX¶

This tutorial describes the step-by-step on how to revoke a SRK key on i.MX SoC boards, the purpose of revocations and where to find more information on this topic.

What is Revocation of a Key¶

The process to revoke a SRK key is used when one or more SRK keys become compromised. The SRK keys are keys permanently copied to the hardware and are used during the boot to unsure the Root of Trust.

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

In this tutorial, a root of trust with 4 SRK keys fused to the i.MX machine is used as example. For a different architecture, some commands may differ. Details on how to setup a secure boot on i.MX machines on Secure Boot.

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 on next sections. The value for srk-index is a parameter for the sign-file.sh script used to sign the boot artifacts.

Table 2 **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: this 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 Secure Boot Using HABv4 Guide.

Note

On a HABv4 environment, only the first 3 SRK keys can be revoked. On a AHAB environment, all 4 SRK can be revoked. For more details, check 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 sign process is based on the commands from Secure Boot, 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.

The write access to register SRK_REVOKE is protected by the bit SRK_REVOKE_LOCK which can be configured by CST. The parameter --enable-revoke brings the configuration needed by CST to unlock the write access to SRK_REVOKE and make the revocation possible.

Warning

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

For example, for 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 the write access permission to revoke the key, as the suggestion is to use bootloader.uuu script to load the MFGTool SPL and U-Boot prompt to perform the fuse programming command. For other boot scripts, it might be required to include --enable-revoke to the SPL file signing process (second command).

How to Revoke a Key¶

The suggestion is to use the bootloader.uuu script to access U-Boot prompt to execute 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, for imx6ullevk-sec, to revoke SRK2:

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) and can be fixed by adding --enable-revoke during the sign of boot image:

mxc_ocotp fuse_prog(): Access protect error
ERROR

After the revocation of SRK2, it cannot be used to boot that board never again. Test it by signing again using this SRK key and the boot must fail.

How to Revoke a Key for Devices on a Fleet¶

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

The process can be automatize on a FoundriesFactory 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 on 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 on Production Targets.

After the revocation wave, another firmware update wave is required, now with the bootloader configured to disable the 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 at: http://support.foundries.io/.