i.MX security

Parsing x.509 in firmware code is spoooky. Calling code from a previous boot stage is scary. But what other options do we have?

Slightly related topics:

• using eFPGAs
• distributed trust

Secure Boot HAB

i.mx8M still uses HAB, other imx8 variants use AHAB.

Commands remained the same:

• burn some fuse
• call hab_image_auth
• check hab_status
• if ok, burn more

How to automate:

• Buildroot: diy scripts called by post-image.sh
• Yocto: meta-variscite-hab

#!/usr/bin/env bash

KEYS_PATH="${BINARIES_DIR}/../../../keys"

# Code Signing tool is customized to load dek.bin from the env var KEYS_PATH
export KEYS_PATH

# Load template
cp "$(dirname $0)/templates/u-boot.csf_template" "${BINARIES_DIR}/uboot.csf"

# Fill out key location
sed -i "s:KEY_LOCATION:${KEYS_PATH}:g" "${BINARIES_DIR}/uboot.csf"

# Fill out image location
sed -i "s:IMAGES:${BINARIES_DIR}:g" "${BINARIES_DIR}/uboot.csf"

# Read u-boot.imx.log to check uboot size
HAB_LINE=$(cat "${BINARIES_DIR}/../build/uboot-2018.05/u-boot.imx.log" | grep HAB)
read -ra UBOOT_HAB <<< $HAB_LINE

# Fill out DECRYPT_SIZE in CSF template - 0xc10 = IVT + DCD is the data we need to sign
UBOOT_DECRYPT_SIZE=$( printf '%x\n' $(( ${UBOOT_HAB[4]} - 0xc10 )) )
sed -i "s/DECRYPT_SIZE/${UBOOT_DECRYPT_SIZE}/g" "${BINARIES_DIR}/uboot.csf"

# Fill out BLOB_LOCATION in CSF template
BLOB_LOCATION=$( printf '%x\n' $(( 0x177ff400 + ${UBOOT_HAB[4]} + 0x1F00 )) )
sed -i "s/BLOB_LOCATION/${BLOB_LOCATION}/g" "${BINARIES_DIR}/uboot.csf"

# Backup uboot.imx, because CST modifies the file 
cp "${BINARIES_DIR}/u-boot.imx" "${BINARIES_DIR}/u-boot.imx.back"

${HOST_DIR}/cst-2.3.2/cst_encrypt_custom --o "${BINARIES_DIR}/u-boot_csf.bin" --i "${BINARIES_DIR}/uboot.csf"

objcopy -I binary -O binary --pad-to=0x1F00 --gap-fill=0x00 ${BINARIES_DIR}/u-boot_csf.bin ${BINARIES_DIR}/u-boot_csf.bin

cat "${BINARIES_DIR}/u-boot.imx.back" "${BINARIES_DIR}/u-boot_csf.bin" > "${BINARIES_DIR}/u-boot-signed.imx" 

Patch code signing tool and load DEK key from custom path:

Who wrote this? …

diff --git a/cst-2.3.2/code/back_end/src/adapt_layer_openssl.c b/cst-2.3.2/code/back_end/src/adapt_layer_openssl.c
index c0ed926..ba9a1f8 100755
--- a/cst-2.3.2/code/back_end/src/adapt_layer_openssl.c
+++ b/cst-2.3.2/code/back_end/src/adapt_layer_openssl.c
@@ -719,9 +719,21 @@ int32_t generate_dek_key(uint8_t * key, int32_t len)
   /*  if (gen_random_bytes(key, len) != CAL_SUCCESS) {
         return CAL_CRYPTO_API_ERROR;
     } */
+    char key_path[256];
+    char* pPath;
+    FILE* fp;

-    FILE* fp= fopen("dek_pre.bin","rb");
-
+    pPath = getenv("KEYS_PATH");
+    if (pPath !=NULL) strcpy(key_path, pPath);
+    else {
+   printf("KEYS_PATH variable not exists!\n");
+   return CAL_CRYPTO_API_ERROR;
+
+    }
+    strcat(key_path,"/dek.bin");
+    printf("%s\n",key_path);
+    fp = fopen(key_path,"rb");
+
     if (!fp) return CAL_CRYPTO_API_ERROR;

     else {

Trusted Firmware

https://trustedfirmware-a.readthedocs.io/en/latest/design/firmware-design.html

verify using secure boot eg. imx HAB - bl31.bin

NXP and PUF

https://www.nxp.com/docs/en/application-note/AN12324.pdf