Engineering Resilience: Intentionally Corrupting Flash Memory to Test Satellite-Grade Bootloaders

When developing mission-critical software for satellites, engineers face a unique challenge: How do you test failure modes that only occur when cosmic radiation flips bits in memory? For the MOVE student satellite team at the Technical University of Munich, this question became central to their work on a radiation-tolerant bootloader for STM32 microcontrollers. Their innovative solution? A deliberately engineered flash corruption tool that triggers specific hardware errors to validate system resilience.

The MOVE team's bootloader, written in Rust for the STM32L4R5ZI microcontroller, manages firmware updates for CubeSats already in orbit. With limited flash memory (2MB divided into bootloader, three 500KB firmware slots, and redundant metadata), the system must withstand power failures and radiation-induced bit flips. The team employed formal verification via Kani and hardware-in-loop CI testing, but one problem remained untested: handling Non-Maskable Interrupts (NMIs) triggered by flash ECC double-bit errors.

Flash memory in the STM32L4R5ZI employs Error-Correcting Code (ECC) that detects and corrects single-bit errors. When two or more bits flip in a 64-bit block—a likely scenario in radiation-heavy space environments—the hardware triggers an ECC Detection (ECCD) NMI. Unlike regular interrupts, NMIs cannot be masked and immediately disrupt program flow. An unhandled ECCD event during firmware loading could brick the satellite.

Philipp Erhardt, a MOVE team member, explains the testing challenge: "We needed to validate that our NMI handler could recover from corrupted firmware slots, but microcontrollers don't offer a 'corrupt this memory address' command." The solution emerged from an obscure note in the STM32 reference manual (RM0432): "The contents of the Flash memory are not guaranteed if a device reset occurs during a Flash memory operation."

Erhardt engineered a 'sniper' tool that exploits this behavior:

1. Configure watchdog timer for precise reset timing
2. Busy-wait for calculated intervals
3. Initiate flash write operation near watchdog timeout
4. Trigger reset during write to corrupt specific address

The initial implementation worked sporadically. To achieve deterministic corruption, Erhardt implemented a binary search across resets using the microcontroller's Real-Time Clock (RTC) backup registers (32-bit non-volatile storage) to preserve search state. The algorithm:

• Stores search bounds in RTC registers
• After reset, calculates midpoint wait time
• Attempts flash corruption at that timing
• Uses LED indicators: blue (timing too long), green (successful corruption), red (manual reset needed)

This approach allowed precise targeting of metadata pages and firmware slots. The resulting open-source tool enabled the team to verify that their bootloader could successfully boot even when all but one firmware slot contained corrupted blocks.

For space missions, this validation approach has significant implications:

1. Enables testing of radiation-hardening strategies on Earth
2. Provides deterministic failure injection for safety-critical systems
3. Demonstrates Rust's viability for space-grade firmware
4. Creates a blueprint for testing ECC recovery mechanisms

The MOVE team's work exemplifies how innovative ground testing can compensate for the impossibility of simulating true space conditions. As CubeSats assume more critical roles in scientific and commercial missions, such rigorous validation methods become increasingly essential for mission success 300 kilometers above Earth's surface.