Trying to simulate secure boot on MCIMX6ULL-EVK

[JohnKlug]

I have a SEGGER j-Link Plus, and we have enabled J-Link debugging on the evaluation board, and it can set the shadow registers for the fuses.  I then do serial boot from USB.

Is there any documentation on how to do secure boot with this board?

I am using CST-3.3.1 and u-boot 2020.04.  I have chosen the following answers in the Code signing tool:

existing CA Key: n
Elliptic Curve Crypto: n
bits for PKI tree: 2048
PKI tree duration: 10
# of Super Root Keys: 4
SRK has CA flag set: y

 
The signing tool gives no errors.

If I ask U-Boot after booting for hab_status, I see:

Secure boot disabled

HAB Configuration: 0xf0, HAB State: 0x66

--------- HAB Event 1 -----------------
event data:
        0xdb 0x00 0x14 0x42 0x69 0x0a 0xc0 0x00
        0xca 0x00 0x0c 0x00 0x01 0xc5 0x1d 0x00
        0x00 0x00 0x07 0xe4

STS = HAB_WARNING (0x69)
RSN = HAB_UNS_ENGINE (0x0A)
CTX = HAB_CTX_COMMAND (0xC0)
ENG = HAB_ENG_ANY (0x00)


--------- HAB Event 2 -----------------
event data:
        0xdb 0x00 0x1c 0x42 0x69 0x0a 0xc0 0x00
        0xca 0x00 0x14 0x00 0x02 0xc5 0x1d 0x00
        0x00 0x00 0x0d 0x3c 0x87 0x7f 0xf4 0x00
        0x00 0x0a 0x1c 0x00

STS = HAB_WARNING (0x69)
RSN = HAB_UNS_ENGINE (0x0A)
CTX = HAB_CTX_COMMAND (0xC0)
ENG = HAB_ENG_ANY (0x00)


--------- HAB Event 3 -----------------
event data:
        0xdb 0x00 0x14 0x42 0x33 0x0c 0xa0 0x00
        0x00 0x00 0x00 0x00 0x00 0x91 0x00 0x00
        0x00 0x00 0x02 0x00

STS = HAB_FAILURE (0x33)
RSN = HAB_INV_ASSERTION (0x0C)
CTX = HAB_CTX_ASSERT (0xA0)
ENG = HAB_ENG_ANY (0x00)

If I corrupt the image, (changed the date from 2021 to 9021), and I run hab_status in U-Boot, I see the first two warning events are the same, and event 3 I see two changes:

RSN = HAB_INV_SIGNATURE (0x18)
CTX = HAB_CTX_COMMAND (0xC0)

In addition, the corrupted image sees this error in events 4 through 8:

event data:
        0xdb 0x00 0x14 0x42 0x33 0x0c 0xa0 0x00
        0x00 0x00 0x00 0x00 0x87 0x7f 0xf4 0x00
        0x00 0x00 0x00 0x20

STS = HAB_FAILURE (0x33)
RSN = HAB_INV_ASSERTION (0x0C)
CTX = HAB_CTX_ASSERT (0xA0)
ENG = HAB_ENG_ANY (0x00)

So I believe something must be right about my image.

So are there fuse settings I need to prevent HAB_UNS_ENGINE and HAB_INV_ASSERTION?  Or is something wrong with my signing process?

[JohnKlug]

I found a way to get rid of the warning about HAB_UNS_ENGINE.

I removed the following line from my u-boot.csf file:

Engine = CAAM

Note that Engine = CAAM is mentioned in many examples with the i.MX6.  How does one know what to do?

That still leaves this error:

HAB Configuration: 0xf0, HAB State: 0x66

--------- HAB Event 1 -----------------
event data:
        0xdb 0x00 0x14 0x42 0x33 0x0c 0xa0 0x00
        0x00 0x00 0x00 0x00 0x00 0x91 0x00 0x00
        0x00 0x00 0x02 0x00

STS = HAB_FAILURE (0x33)
RSN = HAB_INV_ASSERTION (0x0C)
CTX = HAB_CTX_ASSERT (0xA0)
ENG = HAB_ENG_ANY (0x00)

 
Looking at the HABV4 doc with 3.3.1 CST, I see the following:

In this case the context is the hab_rvt.assert()API. An assertion event means that one of
the following required areas is not signed as documented in the Operation section for
authenticate_image() API:
• IVT;
• DCD (if provided);
• Boot Data (initial byte - if provided);
• Entry point (initial word).

How do I fix this problem?  Why would one of the above not be signed as documented?  Is there a bug in the Code Signing Tool?

[Yuri · Accepted solution]

@JohnKlug 
Hello,

   i.MX 6ULL does not have CAAM, therefore Engine = SW should be applied.

  You mentioned about shadow registers for the fuses.
Perhaps it makes sense to burn the SRK.

Regards,
Yuri.

[JohnKlug]

What is interesting to note is that if I set SEC_CONFIG[1], closed, USB (external) boot works with a corrupted image.  So this device does not have enforceable security over external boot?

U-Boot results with corrupted image:

U-Boot 2020.04-5.4.24-2.1.0+g4979a99482 (May 11 9021 - 22:11:57 +0000)

=> hab_status

Secure boot enabled

...

RSN = HAB_INV_SIGNATURE (0x18)

SEC_CONFIG[1] is set:

=> fuse read 0 6  
Reading bank 0:

Word 0x00000006: 00000002