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Memory Firewalls

Overview

TI System-on-Chips (SoCs) use hardware-backed firewalls to enforce access control. Texas Instruments Foundational Security (TIFS) configures these firewalls to restrict the components that can access specific regions of device-mapped memory. Other components such as Open Portable Trusted Execution Environment (OP-TEE), Arm Trusted Firmware (ATF), Linux, U-Boot, and user-space programs can request TIFS to configure firewalls on their behalf.

Firewalls can restrict access based on:

  • Core: A53, R5, M4
  • Privilege level: privileged or non-privileged
  • Security state: secure or insecure

For example, TIFS might configure a firewall to prevent Linux (running on A53) from accessing a memory region reserved for the secure world.

For more information about firewalls, see the Technical Reference Manual (TRM) for the specific SoC, the TISCI Firewall API, and the TIFS Firewall FAQ.

What Happens During a Firewall Violation

When software attempts an unauthorized access to a memory region that a firewall protects, the firewall blocks the access and triggers an exception. The outcome depends on the type of access:

  • Read access: The firewall blocks the read. The system continues running.
  • Write access: The firewall blocks the write and crashes the Linux kernel. The system halts.

The kernel crash on write violations stops the offending software immediately, preventing further unauthorized access attempts.

TIFS logs information about every firewall exception. The rest of this document explains how to trigger exceptions, enable TIFS logs, access them, and interpret them.

Triggering a Firewall Exception

To test firewall behavior, use k3conf to read from or write to a protected memory region.

Triggering a read exception:

k3conf read <addr> [<size>]

Triggering a write exception (this will crash the kernel):

k3conf write <addr> <value>

Replace <addr> with the address of a firewall-protected region, <size> with the number of bytes to read, and <value> with the value to write.

Enabling TIFS Logs

TIFS does not output logs by default. Enabling TIFS logging requires modifying U-Boot source code, recompiling it, and transferring the new binaries to the boot partition.

Note

Currently, TIFS logs report a firewall read violation during boot. This occurs because A53 speculatively accesses TF-A's memory region, which is protected from non-secure access by a firewall. This occurs only once, since TF-A's memory region is unmapped from the page table afterwards.

Modify U-Boot Configuration

Open :file:`board/ti/<soc_name>/board-cfg.yaml` in the U-Boot source tree. Locate trace_dst_enables and trace_src_enables, which U-Boot sets to 0x0 by default. Change these values as follows:

  • trace_dst_enables: change from 0x0 to 0xD
  • trace_src_enables: change from 0x0 to 0x3F

Build and Deploy U-Boot

After making these changes, compile U-Boot and transfer the resulting binaries to the board's boot partition. See :ref:`u-boot-build-guide-build-k3` for build instructions.

Accessing TIFS Logs

TIFS outputs logs to a separate serial port from the Linux console. If the Linux command line is accessible through :file:`/dev/ttyUSB0`, TIFS logs are typically accessible through :file:`/dev/ttyUSB1`. However, the exact device assignment depends on the hardware setup and the order in which the host enumerates USB devices.

Open the TIFS serial port with a terminal emulator to view the logs.

Interpreting TIFS Logs

For information about interpreting firewall exception logs, see the TIFS Firewall FAQ.