A missing immutable root of trust in the hardware results in the ability to bypass secure boot or execute untrusted or adversarial boot code.
A System-on-Chip (SoC) implements secure boot by verifying or authenticating signed boot code. The signing of the code is achieved by an entity that the SoC trusts. Before executing the boot code, the SoC verifies that the code or the public key with which the code has been signed has not been tampered with. The other data upon which the SoC depends are system-hardware settings in fuses such as whether "Secure Boot is enabled". These data play a crucial role in establishing a Root of Trust (RoT) to execute secure-boot flows. One of the many ways RoT is achieved is by storing the code and data in memory or fuses. This memory should be immutable, i.e., once the RoT is programmed/provisioned in memory, that memory should be locked and prevented from further programming or writes. If the memory contents (i.e., RoT) are mutable, then an adversary can modify the RoT to execute their choice of code, resulting in a compromised secure boot. Note that, for components like ROM, secure patching/update features should be supported to allow authenticated and authorized updates in the field.
Threat Mapped score: 0.0
Industry: Finiancial
Threat priority: Unclassified
| Phase | Note |
|---|---|
| Architecture and Design | N/A |
| Implementation | Such issues could be introduced during policy definition, hardware architecture, design, manufacturing, and/or provisioning. They can be identified later during testing or system configuration phases. |
Intro: The RoT is stored in memory. This memory can be modified by an adversary. For example, if an SoC implements "Secure Boot" by storing the boot code in an off-chip/on-chip flash, the contents of the flash can be modified by using a flash programmer. Similarly, if the boot code is stored in ROM (Read-Only Memory) but the public key or the hash of the public key (used to enable "Secure Boot") is stored in Flash or a memory that is susceptible to modifications or writes, the implementation is vulnerable.
Body: In general, if the boot code, key materials and data that enable "Secure Boot" are all mutable, the implementation is vulnerable.
Intro: The example code below is a snippet from the bootrom of the HACK@DAC'19 buggy OpenPiton SoC [REF-1348]. The contents of the bootrom are critical in implementing the hardware root of trust.
Body: It performs security-critical functions such as defining the system's device tree, validating the hardware cryptographic accelerators in the system, etc. Hence, write access to bootrom should be strictly limited to authorized users or removed completely so that bootrom is immutable. In this example (see the vulnerable code source), the boot instructions are stored in bootrom memory, mem. This memory can be read using the read address, addr_i, but write access should be restricted or removed.
... always_ff @(posedge clk_i) begin if (req_i) begin if (!we_i) begin raddr_q <= addr_i[$clog2(RomSize)-1+3:3]; end else begin mem[addr_i[$clog2(RomSize)-1+3:3]] <= wdata_i; end end end ... // this prevents spurious Xes from propagating into the speculative fetch stage of the core assign rdata_o = (raddr_q < RomSize) ? mem[raddr_q] : '0; ...