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feat(linux): Add fTPM based filesystem encryption rst
Add new security subsection for fTPM based filesystem encryption feature. Enable this documentation for AM62X, AM62AX, AM62PX, AM62LX as per testing done with 12.0 SDK. Signed-off-by: Shiva Tripathi <s-tripathi1@ti.com>
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‎configs/AM62AX/AM62AX_linux_toc.txt‎

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linux/Foundational_Components/System_Security/SELinux
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linux/Foundational_Components/System_Security/Auth_boot
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linux/Foundational_Components/System_Security/Memory_Firewalls
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linux/Foundational_Components/System_Security/Filesystem_Encryption
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linux/Foundational_Components_Kernel_Users_Guide
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linux/Foundational_Components_Kernel_LTP-DDT_Validation

‎configs/AM62LX/AM62LX_linux_toc.txt‎

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#linux/Foundational_Components/System_Security/SELinux
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linux/Foundational_Components/System_Security/Auth_boot
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linux/Foundational_Components/System_Security/Memory_Firewalls
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linux/Foundational_Components/System_Security/Filesystem_Encryption
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linux/Foundational_Components_Kernel_Users_Guide
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linux/Foundational_Components_Kernel_LTP-DDT_Validation

‎configs/AM62PX/AM62PX_linux_toc.txt‎

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linux/Foundational_Components/System_Security/SELinux
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linux/Foundational_Components/System_Security/Auth_boot
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linux/Foundational_Components/System_Security/Memory_Firewalls
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linux/Foundational_Components/System_Security/Filesystem_Encryption
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linux/Foundational_Components_Kernel_Users_Guide
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linux/Foundational_Components_Kernel_LTP-DDT_Validation

‎configs/AM62X/AM62X_linux_toc.txt‎

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linux/Foundational_Components/System_Security/SELinux
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linux/Foundational_Components/System_Security/Auth_boot
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linux/Foundational_Components/System_Security/Memory_Firewalls
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linux/Foundational_Components/System_Security/Filesystem_Encryption
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linux/Foundational_Components_PRU_Subsystem
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linux/Foundational_Components/PRU-ICSS-Linux-Drivers

‎source/linux/Foundational_Components/System_Security/Filesystem_Encryption.rst‎

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.. _filesystem-encryption:
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################################
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File System Encryption with fTPM
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################################
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************
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Introduction
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************
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Data security is essential in modern embedded systems, be it industrial,
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automotive or IoT applications. This guide provides a reference
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implementation for root filesystem encryption by using Trusted Platform
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Module (TPM) protected keys on TI K3 platforms.
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**Yocto-Based Implementation:**
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This is a Yocto based implementation integrated into the Processor SDK.
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It provides recipes and configuration for LUKS2 full-disk encryption
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with automatic first-boot encryption and following boots decryption,
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all controlled through Yocto recipes.
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The implementation here leverages a firmware TPM (fTPM) running
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in OP-TEE to protect encryption keys, with secure persistent storage
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in TPM's persistent handles. The solution provides strong data-at-rest
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protection without requiring discrete TPM hardware.
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************
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Key features
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************
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- **TPM-protected keys**: Firmware TPM generates and seals encryption
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keys during first boot
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- **Automatic In-place encryption**: First-boot encryption (in-place)
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and next boot decryption happen automatically
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- **Secure key storage**: Keys stored in TPM persistent storage
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accessed through TPM 2.0 APIs
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********
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Concepts
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********
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Root Filesystem Encryption
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==========================
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Root filesystem encryption protects data at rest by encrypting the
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filesystem (root partition of SD card). This protection exists even if
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the storage device is physically removed from the system.
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The Linux kernel uses **dm-crypt** (Device Mapper Crypt) for
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block-level encryption, with **LUKS** (Linux Unified Key Setup) managing
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encryption parameters and key slots.
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Firmware TPM (fTPM)
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===================
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A firmware TPM (fTPM) implements the TPM 2.0 specification as a
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Trusted Application running within OP-TEE's secure environment:
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- **Key generation**: Creates cryptographically strong keys using
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hardware entropy
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- **Key sealing**: Protects keys so only authorized parties can access
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them in a known secure state
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- **Secure storage**: Stores TPM state in tamper-resistant eMMC
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Replay Protected Memory Block (RPMB)
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eMMC RPMB
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=========
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Replay Protected Memory Block (RPMB) is a secure partition in eMMC storage:
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- **Authentication**: Only accepts authenticated write operations
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- **Replay protection**: Prevents replay attacks with write counters
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- **Limited access**: Only accessible through OP-TEE secure environment
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The fTPM stores its persistent state in eMMC RPMB through OP-TEE's secure
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backend.
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**********************
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Implementation Details
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**********************
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System Architecture
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===================
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.. figure:: ./images/LUKS_ftpm.png
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The filesystem encryption implementation consists of several components
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working together across the boot process:
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- **Boot loader** : U-Boot loads kernel, initramfs and DTBs into memory
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from unencrypted boot partition
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- **Linux Kernel**: Provides the core cryptographic functionality through
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the Device Mapper subsystem
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- **Initramfs**: Contains the early boot environment where
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encryption/decryption occurs
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- **OP-TEE**: Secure operating system running in TrustZone that hosts the
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firmware TPM
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- **eMMC Storage**: provides tamper-resistant key storage with RPMB
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Boot Process Flow
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=================
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The encryption system operates during the Linux boot process:
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#. **Boot loader Stage**: U-boot loads the kernel and initramfs into memory
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#. **Early Boot**: The kernel starts and mounts the initramfs as a
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temporary root filesystem
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#. **TPM Initialization**: The firmware TPM is initialized within OP-TEE
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#. **Encryption Detection**: The system checks if the root partition is
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already encrypted
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#. **Encryption/Decryption**: Based on the detection result, the system
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either:
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- Performs first-time in-place encryption of the root filesystem
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- Retrieves the key from TPM and decrypts the existing encrypted filesystem
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#. **Root Switch**: Control is transferred to the actual root filesystem
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Key Management Flow
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===================
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The encryption key lifecycle is managed securely:
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#. **Key Generation**: During first boot, the TPM generates a random
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encryption key
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#. **Key Sealing**: The key is sealed by the TPM, protecting it from
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unauthorized access
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#. **Key Storage**: Sealed key data is stored in eMMC RPMB through the
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TPM's secure storage
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#. **Key Retrieval**: During later boots, the key is unsealed by
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the TPM
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Encryption Process
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==================
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The in-place encryption process follows these steps:
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#. **Filesystem Preparation**: The filesystem is checked and resized to
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make room for LUKS headers
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#. **Space Verification**: The system ensures at least 32MB is available
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for LUKS metadata
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#. **Encryption Initialization**: LUKS headers are written to the beginning
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of the partition
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#. **Block Encryption**: Data blocks are read, encrypted, and written back
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to storage
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#. **Filesystem Expansion**: After encryption, the filesystem is expanded
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to use available space
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*****
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Setup
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*****
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The fTPM based filesystem encryption support is available in Yocto. The
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following section acts as the guide for setting it up.
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Please use :ref:`Processor SDK - Building the SDK with Yocto
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<building-the-sdk-with-yocto>` as reference while following the below
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steps specific to LUKS:
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#. Use the latest :ref:`oe-config file <yocto-layer-configuration>`, using
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the "luks" specific config.
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#. Before building the SDK image, there are few **prerequisites**:
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- **Writing keys to eMMC RPMB** : The implementation here uses RPMB keys
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for secure persitance storage. Writing keys into RPMB is a one-time
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and non-reversible step, follow :ref:`secure-storage-with-rpmb`
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- Once the keys are written to RPMB, the optee-os and optee-client
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components in yocto setup should be configured to make use of these
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hardware keys.
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Following can be used in yocto for the same:
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- for **optee-os**: under meta-ti layer:
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*"meta-ti-bsp/recipes-security/optee/optee-os-ti-overrides.inc"*
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.. code-block:: console
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EXTRA_OEMAKE:append:k3 = " \
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CFG_REE_FS=n \
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CFG_RPMB_FS=y \
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CFG_RPMB_WRITE_KEY=y \
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CFG_RPMB_ANNOUNCE_PROBE_CAP=n \
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"
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- for **optee-client**: disable RPMB emulation mode. under meta-ti layer:
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*"meta-ti-bsp/recipes-security/optee/optee-client_%.bbappend"*
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.. code-block:: console
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EXTRA_OECMAKE:append = " -DRPMB_EMU=OFF"
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- **u-boot configuration**: The kernel Image and dtbs are read from the
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root partition of SD by default. But since this implemenation encrypts the root
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filesystem, u-boot needs to be configured to pick kernel image, dtbs
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and initramfs from the boot partition. This can be done using such
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following change in uboot (can be a patch in u-boot meta-ti layer):
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.. ifconfig:: CONFIG_part_variant in ('AM62LX')
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.. code-block:: console
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CONFIG_BOOTCOMMAND="setenv bootargs console=ttyS0,115200n8 earlycon=ns16550a,mmio32,0x02800000 root=/dev/mmcblk1p2 rootwait rootfstype=ext4; load mmc 1:1 0x82000000 /Image; load mmc 1:1 0x88080000 /ti-core-initramfs.cpio.xz; setenv initrd_size ${filesize}; load mmc 1:1 0x88000000 /k3-am62l3-evm.dtb; booti 0x82000000 0x88080000:${initrd_size} 0x88000000"
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.. ifconfig:: CONFIG_part_variant not in ('AM62LX')
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.. code-block:: console
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CONFIG_BOOTCOMMAND="setenv bootargs console=ttyS2,115200n8 earlycon=ns16550a,mmio32,0x02800000 root=/dev/mmcblk1p2 rootwait rootfstype=ext4; load mmc 1:1 0x82000000 /Image; load mmc 1:1 0x88080000 /ti-core-initramfs.cpio.xz; setenv initrd_size ${filesize}; load mmc 1:1 0x88000000 /<DTB_NAME>.dtb; booti 0x82000000 0x88080000:${initrd_size} 0x88000000"
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- **Additional configs**: The following can be added in local.conf of yocto build:
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- To copy dtbs to boot patition for post encryption boot:
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.. code-block:: console
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IMAGE_BOOT_FILES:append = " *.dtb"
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- Adding free space in rootfs for LUKS encryption (since LUKS expects
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atleast 32MB of free space for header post resize2fs operations):
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.. code-block:: console
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IMAGE_ROOTFS_EXTRA_SPACE = "65536"
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- Some other useful configurations (**not mandatory** to have):
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- In order to use tpm2 tools in Linux command line, add following in
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local.conf:
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.. code-block:: console
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IMAGE_INSTALL:append = " \
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tpm2-tools \
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tpm2-tss \
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libtss2-tcti-device \
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"
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- Size of initramfs image can be reduced by using the busybox
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optimizations, for reference:
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.. code-block:: console
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VIRTUAL-RUNTIME_init_manager:pn-packagegroup-ti-core-initramfs = "busybox"
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VIRTUAL-RUNTIME_dev_manager:pn-packagegroup-ti-core-initramfs = "busybox"
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********************
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dm-crypt performance
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********************
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- The first boot involves encryption of complete root filesystem using the
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ARM aes-generic (software implmentation), giving around 17.0 MB/s of
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performance. This makes use of "cryptsetup reencrypt" which reads,
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encrypts and writes back data. Therefore, the first boot is expected
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to take more time dependending on the size of filesystem.
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- The next boots involve decryption of data, giving around 86 MB/s
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of decryption throughput.
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***********************
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Security Considerations
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***********************
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Reference Implementation
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========================
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This implementation serves as a reference design that demonstrates the
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integration of filesystem encryption with firmware TPM. It is **not intended
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for direct use in production environments without appropriate security
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review** and customization including:
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- **Threat model evaluation**: a thorough threat assessment should be
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conducted before deployment
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- **Key management**: The default TPM persistent handle (0x81080001)
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should be reviewed for security requirements
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- **Boot process hardening**: The initramfs module may need modifications
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to align with specific security policies
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- **Recovery mechanisms**: Production implementations may require key
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recovery procedures
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Further Enhancements
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====================
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Implementation of this reference design can have following enhancements:
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- **Integrating with secure boot**: Establish a verified chain of trust
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from ROM to filesystem
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- **Passphrase Recovery**: Incase the TPM keys become inaccessible during
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boot, the current implementation doesn't use any backup passphrase
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recovery method resulting in **potential data loss**. Using a passphrase
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would reduce risk of data loss but introduces additional security
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considerations.
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- **Measured boot**: Add TPM PCR measurements to bind keys to verified
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software state, the current reference doesn't use PCR measurements
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- **Audit logging**: Add secure logging of encryption/decryption
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operations for compliance purposes
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‎source/linux/Foundational_Components/System_Security/Security_overview.rst‎

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| | execution of security-sensitive applications and services | |
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+-------------------------+-----------------------------------------------------------+--------------------------------------+
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| **Memory Firewalls** | Prevents unauthorized access through hardware-enforced | :ref:`memory-firewalls` |
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| | security boundaries | |
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| | security boundaries | |
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+-------------------------+-----------------------------------------------------------+--------------------------------------+
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|**fTPM based** | Yocto reference implemenation of filesystem encryption | :ref:`filesystem-encryption` |
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|**Filesystem Encryption**| using LUKS2 with TPM-sealed keys | |
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+-------------------------+-----------------------------------------------------------+--------------------------------------+
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| | execution of security-sensitive applications and services | |
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+-------------------------+-----------------------------------------------------------+--------------------------------------+
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| **Memory Firewalls** | Prevents unauthorized access through hardware-enforced | :ref:`memory-firewalls` |
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| | security boundaries | |
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| | security boundaries | |
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+-------------------------+-----------------------------------------------------------+--------------------------------------+
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|**fTPM based** | Yocto reference implemenation of filesystem encryption | :ref:`filesystem-encryption` |
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|**Filesystem Encryption**| using LUKS2 with TPM-sealed keys | |
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+-------------------------+-----------------------------------------------------------+--------------------------------------+
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.. ifconfig:: CONFIG_part_variant not in ('AM62X', 'AM62PX', 'AM62AX', 'AM62LX')

‎source/linux/Foundational_Components/System_Security/images/LUKS_ftpm.png‎

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‎source/linux/Foundational_Components_Security.rst‎

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Foundational_Components/System_Security/SELinux
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Foundational_Components/System_Security/Auth_boot
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Foundational_Components/System_Security/Memory_Firewalls
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Foundational_Components/System_Security/Filesystem_Encryption

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