Missing Immutable Root of Trust in Hardware
A missing immutable root of trust in the hardware results in the ability to bypass secure boot or execute untrusted or adversarial boot code.
Description
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.
Demonstrations
The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.
Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.
Example One
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.
In general, if the boot code, key materials and data that enable "Secure Boot" are all mutable, the implementation is vulnerable.
Good architecture defines RoT as immutable in hardware. One of the best ways to achieve immutability is to store boot code, public key or hash of the public key and other relevant data in Read-Only Memory (ROM) or One-Time Programmable (OTP) memory that prevents further programming or writes.
Example Two
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.
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;
...The vulnerable code shows an insecure implementation of the bootrom where bootrom can be written directly by enabling write enable, we_i, and using write address, addr_i, and write data, wdata_i.
To mitigate this issue, remove the write access to bootrom memory. [REF-1349]
...
always_ff @(posedge clk_i) begin
if (req_i) begin
raddr_q <= addr_i[$clog2(RomSize)-1+3:3];
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;
...See Also
Weaknesses in this category are related to protection mechanism failure.
Weaknesses in this category are related to improper design of full-system security flows, including but not limited to secure boot, secure update, and hardware-device ...
This view (slice) covers all the elements in CWE.
This view (slice) lists weaknesses that can be introduced during implementation.
This view (slice) lists weaknesses that can be introduced during design.
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