Laser Your Way into Debug Mode on the RP2350

The RP2350 is actually a pretty secure chip, all things considered. It has secure boot, ARMv8’s TrustZone to split secure and non-secure execution, and you can permanently disable debug — …. Laser Your Way into Debug Mode on the RP2350.

It has secure boot, ARMv8’s TrustZone to split secure and non-secure execution, and you can permanently disable debug — the Pi Foundation even included glitch detection, meaning the traditional ‘zap the chip until it obeys’ technique is blocked. That’s why the [Ledger Donjon] security team went full Bond Villain and strapped everyone’s favourite fruit-flavoured microcontroller to a table with a slowly-approaching laser beam. The bench setup to do all this is pretty impressive– and came with an impressive 250,000 USD price tag.

Okay — movie clichés aside, the laser was in fact very carefully focused on target before they turned it on. That target was the register that enables the 2350’s debug features. Said register was located by decapsulating the chip and examining the die with photon-emission electron microscopy; the actual attack was carried out on a chip that had been decapped on the back side, with IR shining through the silicon wafer.

There was probably more than a little trial-and-error to figure out exactly where on the die adjacent to the register to zap with the laser to flip those bits.