DMA Attack Surface & IOMMU Protection

Kernel DMA Protection

DMA buffer window 10 pages

Request rate 6 / s

No kernel-memory access attempted yet.

Live Readouts

Allowed

Blocked

Kernel pages touched

Legend

Ordinary memory page Kernel / OS memory Secret (disk-key) page Device's mapped DMA window

How it works

A Thunderbolt / USB4 port exposes raw PCIe, and PCIe devices can issue Direct Memory Access (DMA) — reads and writes straight to physical RAM, bypassing the CPU. Without protection, any plugged-in device can address the full physical space:

no IOMMU:  device_addr → physical_addr   (identity, unchecked)

An IOMMU (Intel VT-d / AMD-Vi) sits between the port and memory and translates every DMA request through a per-device page table, exactly like the CPU's MMU does for processes:

with IOMMU: allowed  ⇔  device_addr ∈ [win_start, win_start + win_size)
            physical_addr = IOMMU_PageTable[device][device_addr]
            else → DMA remapping fault, request dropped in hardware

IOMMU toggle — with it off, every request reaches memory (the pre-2020 default on many laptops; this is exactly the class of bug behind the Thunderclap and BitLocker cold-boot-adjacent DMA attacks).
Device: Trusted / Malicious — a trusted peripheral only ever touches its own assigned buffer; a malicious or firmware-compromised one scans arbitrary physical addresses looking for secrets.
DMA buffer window — how large a region the OS mapped for this device. Even under IOMMU enforcement, a wider window is a wider attack surface for a malicious device landing lucky hits inside it — this is why real drivers request the smallest buffer they need.
Kernel pages touched — should stay at zero whenever IOMMU is on, because the mapped window is placed away from kernel/secret pages by construction. Watch it climb the moment you switch IOMMU off.

Real-world relevance: Windows' "Kernel DMA Protection" and macOS's Thunderbolt access controls both implement exactly this IOMMU gate by default on modern laptops — this simulation shows why turning it off for "compatibility" reopens a direct hardware path into RAM.

Allowed request Blocked at IOMMU gate

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