Bug 12 - Key Manager: Data-Enable FSM Illegal-State Silent Recovery (No Alert)

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Security feature bypassed

Key Manager FSM integrity monitoring - the alert path that must report fault-injected illegal FSM states

Finding

The data-enable FSM uses a 10-bit state encoding (1024 encodings, 7 legal states). The unique case default recovers to the safe state but leaves fsm_err_o = 0 - fault-injected states recover silently:

// keymgr_data_en_state.sv:80,127-132
unique case (state_q)
  ...
  default: begin
    state_d = StIdle;   // Safe recovery - correct
    // BUG: fsm_err_o stays 0 - alert never fires
  end
endcase

Location or code reference

• hw/ip/keymgr/rtl/keymgr_data_en_state.sv:127-132 - default: recovers to StIdle but never sets fsm_err_o
• hw/ip/keymgr/rtl/keymgr_data_en_state.sv:80 - 10-bit sparse FSM state register

The exploit for this bug is linked in the Attachments section below.

New Tools

Yes - module instantiation (Verilator 4.210): keymgr_data_en_state_tb.sv instantiates the keymgr_data_en_state.sv with its genuine dependency chain, exercises the FSM through valid operations, and verifies fsm_err_o stays 0 throughout (see testbench/logs/rtl-test-simulation.log). Also custom VCS fuzzing pipeline (standalone model).

AI Tools

Yes

LLM

Yes

LLM Details

GPT-5.6-Sol from https://agentrouter.org/v1

Online LLM Details

Orchestrator session total: 38,176,115 in / 26,998 out tokens (the agentic session that ran the Lightsaber pipeline, session ses_031497488).

Estimated tokens for this bug’s run (from result.json turn log: 20260805_082405 keymgr agentic run (15 turns, 168,087 bytes) - shared with bugs 03 and 11): ~81,021 in / ~2,015 out. Estimate = file bytes / 4 + ~2,600 system-prompt tokens per turn; the run was part of a larger multi-IP campaign, so the per-bug figure is an approximation.

LLM Prompts

Step-by-step discovery and verification (source: Lightsaber agentic audit):

1. Lightsaber’s keymgr agent (same run 20260805_082405) read keymgr_data_en_state.sv.
2. It flagged that the unique case (state_q) default branch forces state_d = StCtrlDataDis but leaves fsm_err_o=0 - an illegal FSM state is silently recovered with no alert (the sparse-FSM flop macro adds no independent illegal-state alert).
3. Verified on the RTL: FI demo (fi19.v) forces an illegal 10-bit state and observes fsm_err_o stays 0 (fault is SILENT).

Evidence:

• logs/lightsaber/bug-12-fsm/result.json

Agent/session transcript: session transcript line 2455 (finding #5 (keymgr FSM)).

Detection method

Module instantiation (property: illegal states must raise fsm_err_o) + VCS FI demo: fi19.v forces an illegal 10-bit state (10'b1111111111) into the real FSM; it silently recovers with fsm_err_o=0 (see exploit/logs/fi19_run.log: *** BUG #19 FI CONFIRMED: fault is SILENT ***).

Security impact

FSM corruption during key derivation can skip/repeat data-enable stages; the system is unaware - no alert, no log. Key derivation may produce corrupted or predictable output while the security monitors report healthy.

Adversary profile

Type 2 - physical attacker with clock-glitch / EM fault injection on the FSM state register.

Proposed mitigation

default: begin
  state_d = StIdle;
  fsm_err_o = 1'b1;   // raise alert on illegal state
end

Or use Hamming-protected state encoding (distance ≥ 2).

CVSSv3.1 score and severity

4.2 - MEDIUM

CVSSv3.1 Details

CVSS:3.1/AV:P/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N

• AV: Physical - fault injection on the device
• AC: High - precise timing during derivation
• PR: None
• S: Unchanged - within the Key Manager domain
• C: High - corrupted/predictable derived keys

Attachment links

• exploit
• testbench