ROM Controller Technical Specification

Overview

This document describes the ROM controller (rom_ctrl). This module attaches as a peripheral to the system bus, and thus follows the Comportability Specification.

The ROM controller interfaces between the system bus and the mask ROM. This ROM has scrambled contents (scrambled with a fixed key, derived from a global constant). The controller is responsible for descrambling these contents on memory fetches.

Unlike the SRAM controller, which performs the equivalent task for SRAM, the ROM controller also contains a ROM checker block. This ROM checker is used to compute a cryptographic hash of the ROM contents just after boot, detecting any malicious changes that have been made to the mask ROM when the system was at rest.

Features

• Logic for memory and address descrambling
• Post-boot ROM integrity check
• Alert trigger and status CSRs for ROM integrity errors or FSM glitches.

Theory of Operations

Block Diagram

The image below shows a high-level block diagram of the module. Blue boxes are instantiations of generic primitives that are used elsewhere on the chip. Green boxes are simple operations; the meat of the design is in the grey boxes.

The upper half of the diagram shows paths for ROM reads when the system is in normal operation. The lower half of the diagram shows the ROM checker. This is triggered by the power manager early in the chip boot sequence to check validity of the ROM image. It runs exactly once, and releases the green multiplexer when it is done.

ROM access when chip is in operation

Once the chip has booted, ROM accesses are requested over the system TL-UL bus. These come in through the TL-UL SRAM adapter (top-left of block diagram). In normal operation, the green multiplexer will give access to these TL reads. The address is scrambled at the first substitution-permutation network (marked S&P in the diagram).

In parallel with the ROM access, a reduced prim_prince primitive (5 rounds with latency 1; equivalent to the cipher used for SRAM) computes a 39-bit truncated keystream for the block. On the following cycle, the scrambled data from ROM goes through a substitution-permutation network and is then XOR’d with the keystream. This scheme is the same as that used by the SRAM controller, but is much simplified because the ROM doesn’t have to deal with writes, byte accesses or key changes.

The output from the XOR is the unscrambled 32-bit data, plus seven ECC bits. This data is passed straight through the TL-UL SRAM adapter; the ECC bits are used as a signal integrity check by the system bus.

The following diagram shows the timing of the different signals. The time from the req output from the tlul_adapter_sram to the response that appears on its rvalid input is one cycle. The “scrambling scheme” for addresses in the diagram is to reverse their digits. The word stored at address 21 in the ROM is denoted w21. The keystream value for address 12 is denoted k12. The unscrambled ROM data for (logical) address 12 is denoted d12.

The prim_prince primitive and the two substitution-permutation networks are all parameterised by “keys”. For rom_ctrl, these keys are global randomised netlist constants: they are assumed to be difficult to recover, but aren’t considered secret data.

The startup ROM check

The ROM checker runs immediately after reset. Until it is done, it controls ROM address requests (through the green multiplexer). The select signal for this multiplexer has a redundant encoding to protect it against fault injection attacks. If the select signal has an invalid value, this will trigger a fatal alert. Before starting to read data, it starts a cSHAKE operation on the KMAC module using kmac_cmd_o.

The checker reads the ROM contents in address order, resulting in a scattered access pattern on the ROM itself because of the address scrambling. Each read produces 39 bits of data, which are padded with zeros to 64 bits to match the interface expected by the KMAC block. The checker FSM loops through almost all the words in ROM (from bottom to top), passing each to the KMAC block with the ready/valid interface and setting the kmac_rom_last_o bit for the last word that is sent. Once the last word has been sent, the FSM releases the multiplexer; this now switches over permanently to allow access through the TL-UL SRAM adapter.

The top eight words in ROM (by logical address) are interpreted as a 256-bit expected hash. Unlike the rest of ROM, their data is not stored scrambled, so the expected hash can be read directly. This is taken by the checker FSM (ignoring ECC bits) and will be compared with the digest that is read back from the KMAC block.

Once it comes back, the digest is forwarded directly to the Key Manager. It is also compared with the hash that was read from the top eight words of ROM. On a match, check_good_o is signalled as On. In either case, check_done_o goes high when the calculation is complete.

The diagram below shows the operation of the simple FSM.

What does the ROM check do?

One of the possible physical attacks on a system like OpenTitan is to subvert the mask ROM. The regular structure of a mask ROM is useful because it makes metal fixes easy, but (for the same reasons) it makes the ROM quite an easy target for an attacker. See [SKO-05]¹, section 2.1.1, for a description of mask ROMs and attacks on them.

Since the code in ROM is the first thing to execute, an attacker that modifies it undetected can completely subvert the chain of trust. As such, OpenTitan needs some form of ROM integrity checking and the ROM checker is the module in charge of providing it.

After bringing the ROM controller module out of reset, the power manager must wait until check_done_o is asserted before starting the host processor. In PROD state, it will also check that check_good_o is On: if not, it will refuse to boot. This provides an extra safety check, but the real security comes from key manager integration described below.

The simple KMAC interface assumes that KMAC is pre-configured to run the cSHAKE algorithm with a prefix specific to the ROM checker. The ROM checker will not assert kmac_rom_vld_o after finishing the one and only hash computation, but KMAC may wish to ignore the signal after then to allow very simple arbitration that still has robustness against fault injection attacks.

The integration with the key manager is based on forwarding the digest data in kmac_digest_share0_i and kmac_digest_share1_i as keymgr_digest_data_o. This 256-bit digest will be incorporated into the CreatorRootKey. The key manager should only allow one transaction (of 256 bits / 32 bits = 8 beats) after reset to pass this information across. On future messages, it should raise an alert, defeating an attacker that tries to trigger extra transactions before or after the real one.

CreatorRootKey forms the first key in the chain described in Identities and Root Keys. An attacker who modifies the ROM will perturb CreatorRootKey (to avoid doing so would require a preimage attack on the ROM checksum calculation or the KM_DERIVE function). The result is that, while the chip will function, it will have the “wrong” root key and the chain of trust used for attestation will be broken.

Hardware Interfaces

Referring to the Comportable guideline for peripheral device functionality, the module rom_ctrl has the following hardware interfaces defined.

Primary Clock: clk_i

Other Clocks: none

Bus Device Interfaces (TL-UL): regs_tl, rom_tl

Bus Host Interfaces (TL-UL): none

Peripheral Pins for Chip IO: none

Interrupts: none

Security Alerts:

Parameters

Signals

The table below lists other ROM controller inter-module signals.

Programmer’s Guide

Software will mostly interact with the ROM controller by fetching code or loading data from ROM. For this, the block looks like a block of memory, accessible through a TL-UL window. However, there are a few registers that are accessible. Other than the standard ALERT_TEST register, all are read-only.

The FATAL_ALERT_CAUSE register might change value during operations (if an alert is signalled), but the other registers will all have fixed values by the time any software runs.

To get the computed ROM digest, software can read DIGEST_0 through DIGEST_7. The ROM also contains an expected ROM digest. Unlike the rest of the contents of ROM, this isn’t scrambled. As such, software can’t read it through the standard ROM interface (which would try to unscramble it again, resulting in rubbish data that woud cause a failed ECC check). In case software needs access to this value, it can be read at EXP_DIGEST_0 through EXP_DIGEST_7.

Register Table

Registers visible under device interface regs

Registers visible under device interface rom