netvar1337 DMA Attack Techniques
Overview
This skill covers Direct Memory Access research from the awesome-game-security collection, focusing on FPGA-based PCIe attacks, pcileech usage, physical-memory access workflows, and the defensive limits of software anti-cheat once a hostile device can read memory below the OS.
README Coverage
Cheat > DMAAnti Cheat > Detection:DMAAnti Cheat > Detection: Hacked HypervisorAnti Cheat > Detection:Virtual EnvironmentsAnti Cheat > Detection:HWIDWindows Security Features
Threat Model
External DMA Cheat Architecture
A modern external DMA cheat consists of three components:
1. Cheat PC — runs the cheat application, signature databases,
aim assistance, ESP rendering, and a network/USB link to the gaming PC.
2. DMA Card — an FPGA-based PCIe endpoint installed in the gaming PC
(typically M.2 NVMe slot). Exposes a memory-read/write interface to
the cheat PC. Uses Bus Master capability to issue Memory Read TLPs
against the gaming PC's RAM.
3. Actuator (optional) — a USB HID emulator (microcontroller-based) that
injects keyboard/mouse input on the gaming PC according to commands
from the cheat PC, closing the loop.
The structural property that makes this threat distinctive:
no attacker code executes on the gaming PC. The DMA card performs
hardware-level transactions between the FPGA and the gaming PC's
memory controller, mediated by the chipset and (when configured) the IOMMU.
The gaming PC's OS, drivers, and anti-cheat see only a PCIe device
announcing itself through Configuration Space and performing what looks
like ordinary DMA.
Three Defense Layers
Layer Mechanism What It Catches
─────────────────────────────────────────────────────────────────────────────
PCIe-layer Inspect Config Space & Identity mismatch — spoofed
fingerprinting behavior at the bus level device that doesn't match
real silicon's full signature
IOMMU Use the IOMMU to bound Out-of-domain DMA — device
enforcement what physical memory the trying to read game memory
device can touch it wasn't allocated
External TPM-anchored measured boot, Boot-chain compromise — IOMMU
attestation cloud-verified or kernel itself subverted
PCIe Protocol Stack
Three Protocol Layers
Layer Unit Function
────────────────────────────────────────────────────────────────
Transaction TLP Memory/IO/Config reads & writes,
completions, messages
Data Link DLLP Acknowledgements, flow control
credits, power management
Physical Ordered Sets Link training, equalization,
clock recovery
A real device's behavior is shaped by all three layers.
Many FPGA designs primarily customize Transaction-Layer behavior; Physical and
Data Link behavior may retain implementation fingerprints unless the selected
IP, configuration, and surrounding behavior closely match the claimed device.
Whether a fingerprint is usable must be validated on the actual link.
TLP (Transaction Layer Packet) Format
Every TLP contains a 3 DW (12-byte) or 4 DW (16-byte) base header; optional
TLP Prefixes may precede it. 4 DW headers are used for 64-bit addresses and
certain message types.
First DWord (DW0) encoding:
Bits Field Notes
[31:29] Fmt[2:0] Header format + data presence
[28:24] Type[4:0] TLP type (combined with Fmt)
[22:20] TC[2:0] Traffic Class (default 0)
[18] Attr[2] ID-Based Ordering (IDO)
[15] TD TLP Digest (ECRC trailer)
[14] EP Poisoned data
[13:12] Attr[1:0] Relaxed Ordering, No Snoop
[11:10] AT[1:0] Address Type (critical for ATS bypass)
[9:0] Length[9:0] Payload length in DWords (0x000 = 1024 DW = 4 KB)
Fmt[2:0] encoding:
000 = 3 DW header, no data
001 = 4 DW header, no data
010 = 3 DW header, with data
011 = 4 DW header, with data
100 = TLP Prefix
Key TLP types (Fmt + Type combinations):
Fmt Type TLP
000 0_0000 MRd (Memory Read, 3DW / 32-bit addr)
001 0_0000 MRd (Memory Read, 4DW / 64-bit addr)
010 0_0000 MWr (Memory Write, 3DW)
011 0_0000 MWr (Memory Write, 4DW)
000 0_0100 CfgRd0 (Config read — terminate at this device)
010 0_0100 CfgWr0
000 0_0101 CfgRd1 (Config read — forwarded by bridges)
010 0_0101 CfgWr1
000 0_1010 Cpl (Completion without data)
010 0_1010 CplD (Completion with data)
001 1_0rrr Msg (Message, no data)
011 1_0rrr MsgD (Message with data)
Detection-Relevant DW0 Fields
TC[2:0] — Traffic Class. Default traffic commonly uses TC0, but non-zero TC is
valid when platform and device policy configure it. Compare usage with the
claimed device, driver, and workload rather than flagging it in isolation.
Attr[2:0] — RO/NS/IDO. A device emulating a NIC must follow that NIC's
typical NS/RO usage pattern; mismatches are visible.
AT[1:0] — Address Type:
00 = Untranslated (IOMMU will translate)
01 = Translation Request (ATS only)
10 = Translated (device claims it has already translated via ATS)
This field is the basis of ATS bypass attacks.
TD — TLP Digest. If set, an ECRC trailer is present.
EP — Poisoned. Indicates data is known-bad.
TLP Routing and Requester ID
Three routing modes:
- Address routing — Memory and IO TLPs, matched against bridge apertures
- ID routing — Config TLPs and Completions, by BDF
- Implicit routing — Some Messages (broadcast, terminate at root)
DW1 carries the Requester ID (16 bits = Bus:Device:Function, "BDF")
and an 8-bit Tag for matching completions to requests.
Requester ID is a key input to IOMMU lookup, ACS source validation, AER source
identification, and interrupt-remapping policy. If spoofed IDs are accepted
without topology or source validation, per-function isolation can be
undermined; the exact effect depends on the platform and remapping path.
Transaction categories:
- Posted (P) — fire-and-forget (Memory Writes, Messages)
- Non-Posted (NP) — requires completion (Memory Reads, IO/Config R/W)
- Completion (Cpl/CplD) — response to Non-Posted requests
Completion Status codes:
000 = Successful Completion (SC)
001 = Unsupported Request (UR)
010 = Configuration Request Retry Status (CRS)
100 = Completer Abort (CA)
UR vs CA distinction matters for spoofing detection — real silicon
responds differently to malformed config accesses vs accesses to
unimplemented offsets. Many spoofed firmwares hard-code one or the other.
Memory Read Completion Splitting
A single Memory Read TLP returns up to Max_Read_Request_Size (MRRS) bytes.
The completer splits the payload at any boundary >= RCB
(Read Completion Boundary, 64 or 128 bytes).
Each fragment cannot exceed Max_Payload_Size (MPS).
Each Completion carries:
- Lower Address[6:0] — lowest 7 bits of first byte address
- Byte Count[11:0] — bytes remaining (last fragment's Byte Count
equals its own payload length)
- BCM — PCI-X compatibility (typically 0)
- Tag — matches originating MRd's Tag
The split pattern (fragment count, boundary positions) is a
strong fingerprint: real memory controllers produce characteristic
distributions of fragment sizes and inter-fragment gaps.
BRAM-backed emulators producing perfectly uniform 64-byte fragments
at constant cadence are anomalous.
Tag Space and Fingerprinting
- 5-bit Tag (original): 32 outstanding non-posted requests per Requester ID
- Extended Tag (PCIe 1.1+, Device Control[8]): 8-bit / 256 outstanding
- 10-Bit Tag (PCIe 4.0+, Device Control 2[12]): 1024 outstanding
Tag turnover discipline — which tags get reissued and how quickly —
reflects the device's internal request tracking pipeline.
Firmware that issues reads with no tag turnover (same tag, or monotonic
beyond negotiated limit) is observably distinct from real silicon.
MPS and MRRS as Fingerprints
Both are negotiated once at link bring-up and fixed for the session.
- Device Capabilities[2:0]: Max_Payload_Size_Supported
(0=128, 1=256, 2=512, 3=1024, 4=2048, 5=4096 bytes)
- Device Control[7:5]: current MPS (must be <= Supported,
set to minimum of all devices in hierarchy)
- Device Control[14:12]: Max_Read_Request_Size (same encoding)
The discriminator is donor consistency: a device claiming a donor
that is known to support larger payloads, different tag behavior,
or a different negotiated profile should match that donor under
the same root-port constraints.
Data Link Layer
DLLPs provide reliable delivery between Physical and Transaction layers.
DLLP Purpose
─────────────────────────────────────────
Ack TLP received correctly
Nak TLP received with error; sender must replay
InitFC1/2 Flow control credit initialization at link bring-up
UpdateFC Ongoing flow control credit updates
PM_* Power management (L0s, L1 entry/exit)
Vendor Vendor-defined
Flow control credits are per TLP category:
- PH / PD — Posted Header / Data
- NPH / NPD — Non-Posted Header / Data
- CplH / CplD — Completion Header / Data
Negotiated credit values are not generally exposed through standard
Link Capabilities register. They are visible in protocol-level traces,
some root-port/vendor performance counters, or FPGA-side debug.
Useful for lab fingerprinting and forensic captures, not normal
runtime config-space detection.
Physical Layer
Two details matter even without PHY-level instrumentation:
LTSSM (Link Training and Status State Machine):
- States: Detect → Polling → Configuration → L0 (operational)
→ L0s, L1, L2 (low-power) → Recovery → Hot Reset → Disabled → Loopback
- Observable via Link Status Register and root-port performance counters
Detection-relevant:
- Negotiated Link Width (Link Status[9:4]):
Compare with slot wiring, platform policy, signal quality, and matched donor
deployments; devices can legitimately train below maximum width
- Current Link Speed (Link Status[3:0]):
A lower negotiated generation is contextual, not a contradiction by itself
- Recovery cycle frequency:
Comparative signal; materially different from donor reference is anomalous
ASPM (Active State Power Management):
- L0s and L1 are link-level low-power states
- Capability does not imply the platform enabled ASPM; evaluate transitions only
under verified policy and workload conditions that should exercise them
Configuration Access Mechanisms
Two mechanisms on x86:
CAM (Legacy I/O-port path):
1. CPU writes to I/O port 0xCF8 (Bus:Device:Function:Register)
2. CPU reads/writes at I/O port 0xCFC
- Reaches only first 256 bytes
- Still used during early BIOS/UEFI boot
ECAM (Enhanced, MMIO path):
1. Read MCFG ACPI table for segment base addresses
2. Compute: addr = base + ((bus << 20) | (dev << 15) | (func << 12) | offset)
3. OS maps physical address into kernel virtual memory
- Required for Extended Configuration Space (0x100–0xFFF)
- Where AER, DSN, LTR, VSEC, ATS, PASID, SR-IOV live
On Windows, supported paths are:
- IRP_MN_READ_CONFIG / IRP_MN_WRITE_CONFIG
- BUS_INTERFACE_STANDARD.GetBusData / SetBusData
Production anti-cheat should use documented bus interfaces;
direct MCFG mapping is a lab-only technique.
PCIe Configuration Space
Legacy 256-Byte Header (Type 0 Endpoint)
Offset Field Notes
0x00 Vendor ID (2B) Chip manufacturer (e.g., 0x8086 Intel)
0x02 Device ID (2B) Specific product
0x04 Command (2B) BME (bit 2), MemSpace (bit 1), IOSpace (bit 0)
0x06 Status (2B) Capabilities List (bit 4)
0x08 Revision ID + Class Code Class triplet: Base / Sub / ProgIF
0x0C Cache Line / Latency / Header Type 0x00 = endpoint,
Header Type / BIST 0x01 = bridge, 0x80 = multi-function
0x10–27 BAR0–BAR5 Memory or I/O windows
0x2C Subsystem Vendor ID Often distinguishes board manufacturers
0x2E Subsystem Device ID
0x30–33 Expansion ROM Base
0x34 Capabilities Pointer Offset of first capability in linked list
0x3C IRQ Line/Pin/Min/Max Legacy INTx routing
BAR encoding (32-bit BAR):
bit 0: 0 = Memory BAR, 1 = I/O BAR
bits 2:1: 00 = 32-bit, 10 = 64-bit (BAR pair)
bit 3: Prefetchable
BAR size discovery: write 0xFFFFFFFF to BAR, read back.
Lower bits (except type bits) come back as 0; rest form a size mask.
Real silicon's size masks are device-specific; a spoofed BAR with
64 KB mask when the donor uses 4 KB is detectable in one operation.
Capabilities Chain
If Status[4] is set, 0x34 points to the first capability.
Each capability has a 2-byte header: [ID | Next].
Next is DWord-aligned in 0x40–0xFF, or 0x00 to terminate.
Common capability IDs:
ID Capability
0x01 PCI Power Management
0x05 MSI
0x10 PCI Express
0x11 MSI-X
0x12 SATA Configuration
0x13 PCI Advanced Features
0x14 Enhanced Allocation
Detection: walk the chain, validate each capability's declared size
doesn't overlap the next, Next is DWord-aligned and within bounds,
no cycle exists. A malformed chain is itself a signal.
PCIe Express Capability (ID 0x10)
The single most important capability for spoofing detection.
Offset Field Notes
+0x02 PCIe Capabilities Cap Version, Device/Port Type, Slot Impl
+0x04 Device Capabilities MPS Supported, FLR, Phantom Functions
+0x08 Device Control MPS current, MRRS, Error Enables
+0x0A Device Status CED, NFED, FED, URD, Transactions Pending
+0x0C Link Capabilities Max Link Speed/Width, ASPM, L0s/L1 latencies
+0x10 Link Control ASPM Control, RCB, Link Disable, Retrain
+0x12 Link Status Current Link Speed/Width, Link Training
+0x24 Device Capabilities 2 Completion Timeout Ranges, AtomicOp,
OBFF, LTR mechanism
+0x28 Device Control 2 Completion Timeout Value, AtomicOp, LTR Enable
+0x2C Link Capabilities 2 Supported Link Speeds Vector
+0x30 Link Control 2 Target Link Speed, Compliance
+0x32 Link Status 2 De-emphasis, EQ Phase status
Detection leverage per field:
- Device Type (+0x02[7:4]): must match donor's role
- MPS Supported (+0x04[2:0]): hard-IP ceiling contradicts donor
- FLR support (+0x04[28]): verify FLR changes same sticky/non-sticky
state as claimed donor; naive firmware acknowledges FLR but continues
unchanged, producing state inconsistent with donor-defined reset semantics
- Link Status (+0x12): Width/Speed are negotiated, observable, hard to
lie about — hard IP reports what LTSSM actually achieved
- Slot Clock Config (+0x12[12]): must match real platform behavior
- Completion Timeout ranges (+0x24): selecting outside claimed ranges
is a discriminator
- AtomicOp (+0x24[6-9]): server-class GPUs/NICs may support; FPGA
support depends on IP generation and configuration; compare advertised and
exercised behavior with the claimed donor
MSI and MSI-X Capabilities
MSI (ID 0x05):
Message Control bits:
[0] MSI Enable
[3:1] Multiple Message Capable (0–5, representing 1–32 vectors)
[6:4] Multiple Message Enable (cannot exceed Capable)
[7] 64-bit Address Capable
[8] Per-Vector Masking Capable
x86 MSI Address: bits [31:20] fixed at 0xFEE (LAPIC prefix)
[19:12] Destination ID, [3] Redirection Hint, [2] Destination Mode
Message Data: [15] Trigger Mode, [10:8] Delivery Mode, [7:0] Vector
MSI-X (ID 0x11):
- Supports up to 2,048 vectors
- Table stored in BAR-mapped region (not Config Space)
- Each entry: 16 bytes (Addr Low, Addr High, Data, Vector Control)
- PBA (Pending Bit Array): bit-per-vector pending state
Naive MSI-X emulation failures:
- Ignores Vector Control Mask writes
- Sets PBA bits but never clears on unmask
- Returns hardcoded PBA values
- Doesn't retire pending interrupts when masks clear
Detection probe: mask vector → induce interrupt condition →
observe PBA bit → unmask → observe interrupt firing.
A conforming implementation should satisfy the relevant MSI-X semantics;
incomplete emulations may fail, while sophisticated emulations can pass.
AER Extended Capability (ID 0x0001)
Three error classes:
- Correctable: Receiver Error, Bad TLP, Bad DLLP, Replay Timer Timeout
- Uncorrectable Non-Fatal: Completion Timeout, Completer Abort, UR, ACS Violation
- Uncorrectable Fatal: Malformed TLP, DLL Protocol Error, Surprise Down
Each has Status (sticky, W1C), Mask, and Severity registers.
Header Log (16B) captures full TLP header of first logged uncorrectable error.
Detection:
- Absence of AER when donor model is known to expose it = mismatch
- Zero correctable-error count over long window when donor's silicon
normally produces a baseline rate = anomalous
- Anomalous UR response patterns to probes of unimplemented offsets
Extended Capabilities
4-byte header at each offset:
[31:20] Next Capability Offset (0 to terminate)
[19:16] Capability Version
[15:0] Extended Capability ID
Key Extended Capability IDs:
0x0001 AER
0x0002 Virtual Channel (VC)
0x0003 DSN (Device Serial Number, 8 bytes)
0x000B Vendor-Specific Extended Capability (VSEC)
0x000D ACS (Access Control Services)
0x000E ARI
0x000F ATS (Address Translation Services)
0x0010 SR-IOV
0x0015 Resizable BAR (RBAR)
0x0018 LTR (Latency Tolerance Reporting)
0x001B PASID
0x001D DPC (Downstream Port Containment)
0x001E L1 PM Substates
0x001F Precision Time Measurement (PTM)
Detection-relevant:
- DSN: 8-byte unique serial; donor-cloned firmware can collide
with another player's identical card
- VSEC: Xilinx PCIe IP optionally emits VSEC blocks with
characteristic Vendor ID + VSEC ID combinations
- ATS/PASID/SR-IOV presence on consumer-class donor is
demographically suspicious — rare outside server-class hardware
IOMMU Architecture
Translation Flow
1. Device issues Memory TLP with target IOVA.
TLP header carries 16-bit Requester ID (BDF).
2. TLP travels upstream through switches/bridges to root complex.
3. IOMMU intercepts, uses Requester ID to look up translation context.
4. IOMMU walks device's I/O page tables: IOVA → physical address.
5. Permission bits (Read, Write) checked against access type.
6. Success: TLP forwarded with translated physical address.
7. Failure: fault logged, device receives UR or CA completion.
Intel VT-d Internals
Two-level table lookup:
BDF → Root Table (256 entries, 16B each, indexed by Bus)
→ Context Table (256 entries, 16B each, indexed by Dev:Func)
→ Second-Level Page Tables (3–5 levels)
→ Final 4 KB physical page
Context Entry fields:
- SLPTPTR: Second-Level Page Table Pointer
- Domain ID: 16-bit (multiple devices can share a domain)
- AW: Address Width (3/4/5-level = 39/48/57-bit IOVA)
- T: Translation Type (untranslated-only, translated-only, or both)
- P: Present
- FPD: Fault Processing Disable
Page table entries (PTE, EPT-like format):
[0] R - Read permission
[1] W - Write permission
[7] PS - Page Size (1=leaf super-page, 0=next-level table)
[N-1:12] Physical address of next-level table or 4 KB page
Super-pages: level-2 leaf = 2 MB, level-3 leaf = 1 GB.
Scalable Mode (VT-d 3.0+):
Context Entry → PASID Directory → PASID Table → per-PASID
first-level page-table roots. Enables Shared Virtual Memory (SVM).
Check RTADDR_REG.TTM to determine which mode is in effect.
AMD-Vi Internals
Single-level Device Table indexed directly by BDF:
BDF → Device Table Entry (32 bytes)
→ I/O Page Tables (1–6 levels)
→ Final page
DTE encodes:
- Page Table Root Pointer
- Mode (0–6, selects paging levels)
- Domain ID (16 bits)
- IR, IW — Default Read/Write permission
- GV — Guest Valid (nested translation)
- PASID-related fields
Page sizes: 4 KB, 2 MB, 1 GB.
IOTLB and Invalidation
Translations cached in IOTLB (I/O Translation Lookaside Buffer).
When mappings change, IOTLB must be invalidated.
Two distinct caches when ATS is in use:
- IOMMU's own IOTLB
- Device-side TLB (DevTLB) caching prior translations
Full invalidation with ATS requires:
1. IOMMU invalidates own IOTLB
2. IOMMU sends ATS Invalidate Request Message to device
3. Device drops affected DevTLB entries, replies with Invalidate Completion
If step 2 or 3 is skipped, device retains stale translations
and can DMA to unmapped addresses.
VT-d invalidation granularities:
- Global: flush entire IOTLB
- Domain-Selective: flush all entries for a Domain ID
- Page-Selective: flush specific IOVA range in a domain
Strict vs lazy invalidation:
Lazy mode defers IOTLB invalidation, batching them for performance.
Opens a window where stale translations remain valid — a device
whose driver has unmapped a buffer can still DMA to the old IOVA.
Fault Recording
VT-d: Fault Recording Registers — circular array capturing
Requester ID, faulting IOVA, fault reason, TLP type.
AMD-Vi: Event Log Buffer — producer-consumer ring buffer of
IO_PAGE_FAULT, INVALID_DEVICE_REQUEST, ATS-related events.
Both surface faults via interrupts and event-log entries.
On Windows, some IOMMU violations observable through WHEA/bug-check
paths and Driver Verifier DMA-violation telemetry.
Per-device fault rate is one of the most operationally useful
IOMMU-layer signals, but its baseline is platform-, device-, driver-, firmware-,
and workload-specific. Sustained faults show failed or invalid DMA requests;
driver bugs, resets, stale mappings, or hardware faults must be excluded before
attributing malicious out-of-domain access.
RMRR/IVMD:
ACPI DMAR table contains RMRR (Reserved Memory Region Reporting)
sub-tables declaring physical ranges devices need identity-mapped.
AMD-Vi has analogous IVMD (I/O Virtualization Memory Definition)
in the IVRS table. A defender should enumerate these and reject
configurations where suspect BDFs appear in RMRR scope or
RMRR ranges overlap game memory regions.
IOMMU Topology and Isolation
IOMMU Groups
Devices in the same IOMMU group may not be safely isolated from
one another. Group membership determined by:
- PCIe topology — devices behind a switch share a group
unless the switch supports and enables ACS
- ACS state of upstream bridges
- Quirks for known-broken hardware
Linux: /sys/kernel/iommu_groups/N/devices/
Windows: equivalent constraints but no simple public group filesystem
ACS (Access Control Services, Extended Cap ID 0x000D)
ACS is a PCIe capability that switches/root ports advertise
to declare they can enforce isolation between downstream ports.
ACS Capability register enable bits:
Bit Feature Effect
0 Source Validation (SV) Drop TLPs with wrong Requester ID
1 Translation Blocking (TB) Block AT=10 (Translated) TLPs
2 P2P Request Redirect (RR) Force P2P requests upstream for IOMMU
3 P2P Completion Redirect (CR) Force P2P completions upstream
4 Upstream Forwarding (UF) Forward upstream regardless
5 P2P Egress Control (EC) Allow/deny P2P routing per-port
6 Direct Translated P2P (DT) Allow P2P with translated addresses
Critical for untrusted endpoints: SV, TB, RR, and CR.
A switch missing Source Validation lets a malicious device spoof
its Requester ID, defeating per-BDF IOMMU translation.
A switch missing P2P Request Redirect allows devices on the same
switch to DMA directly to each other without IOMMU involvement.
Peer-to-Peer DMA
Devices on the same PCIe tree can send Memory TLPs directly to
each other's BAR ranges without involving system memory.
Without ACS redirection, peer traffic may remain below the root complex and
bypass the host IOMMU translation path. Routing behavior is topology- and
platform-specific, so confirm it with the actual root complex and switches.
Plausible P2P DMA targets for cheat:
- GPU framebuffer — rendered game state
- Network adapter ring buffers — game traffic
- USB controller queues — input device data
Mitigation: ACS Translation Blocking + P2P Request Redirect
on every intermediate bridge. Defender must walk topology and
confirm both bits are active.
Interrupt Remapping
MSI/MSI-X interrupts are Memory Writes to 0xFEE00000–0xFEEFFFFF.
Without Interrupt Remapping (IR), any device with Bus Master enabled
can write to this range and trigger arbitrary interrupts — NMIs, SMIs,
or vectors targeting wrong CPU.
With IR enabled, IOMMU validates MSI/MSI-X writes and uses
remapping-table state to determine permitted destination.
IR is part of VT-d's broader DMA Remapping architecture.
Both VT-d and AMD-Vi have integrated equivalents.
Both should be mandatory in any anti-cheat threat model.
ATS, PASID, and Address Translation Trust
ATS (Address Translation Services, Extended Cap ID 0x000F)
ATS lets a device cache IOMMU translations locally:
1. Device issues Translation Request TLP (AT=01) with IOVA
2. IOMMU translates and responds with Translation Completion
carrying physical address
3. Device caches translation in Device-side TLB (DevTLB)
4. Subsequent accesses issued with AT=10 (Translated) —
IOMMU bypasses page-walk, trusting device's cached translation
5. On mapping changes, IOMMU sends Invalidation Request
Attack surface: malicious device claiming ATS can present
arbitrary AT=10 TLPs whose addresses were never approved
by the IOMMU. The IOMMU forwards them trusting the device's claim.
PASID (Extended Cap ID 0x001B)
Extends ATS to per-process address spaces. 20-bit PASID carried
in a TLP Prefix. IOMMU uses (Requester ID, PASID) jointly
to select translation context.
PASID enables Shared Virtual Memory (SVM) — primarily found in
datacenter NICs, AI accelerators. Presence on a consumer card
is anomalous.
ATS Trust Model and "ATS Untrusted" Mode
The fundamental trust assumption: device honestly reports
translations it has been granted. Unreasonable for external
Thunderbolt enclosures, FPGAs in M.2 slots, or untrusted
accelerator cards.
Modern OS/IOMMU stacks can treat endpoints as ATS-untrusted:
ATS is disabled, blocked by policy, or stripped.
Linux: pci=noats plus per-device quirks.
Windows: Kernel DMA Protection / DMAGuard matters, but don't
treat "Kernel DMA Protection: On" as proof every internal
endpoint is ATS-untrusted. Verify ATS state per endpoint.
Driver–IOMMU Contract and Bypass Catalog
Legitimate DMA Path (Windows)
1. Acquire DMA adapter: IoGetDmaAdapter / WDF wrapper
2. Allocate buffer: MmAllocateContiguousMemorySpecifyCacheNode
or WdfCommonBufferCreate
3. Map for DMA: AllocateCommonBuffer / MapTransferEx
- OS allocates IOVA from device's domain
- Creates IOMMU page-table entries: [IOVA, IOVA+size) → physical pages
- Returns IOVA to driver
4. Program device: driver writes IOVA into device's BAR registers
5. Device DMAs: TLPs arrive at IOMMU with BDF + IOVA
6. IOMMU translates: page-walk produces physical address
7. Completion and unmap: teardown IOMMU entries + IOTLB invalidation
In this model, device can DMA only to addresses the driver
explicitly mapped. Game memory is not in that range.
Six Paths to Out-of-Domain Access
1. IOMMU not active or not applied to this path
VT-d/AMD-Vi disabled, OS not enforcing, device outside protected ports
2. Pre-boot DMA injection
Inject before IOMMU initialized; requires firmware-level exploit
3. Identity-mapped / passthrough domains
Legacy drivers request 1:1 mapping; modern strict-mode rejects it
4. Driver mapping over-allocation (Thunderclap class)
OS maps full 4 KB page when buffer is smaller; adjacent kernel data exposed
5. Legitimate-path data exfiltration
Cheat spoofed as NIC; OS network stack passes game packets through
NIC's RX ring buffer (legitimately IOMMU-mapped). Cheat reads game data
without leaving allowed mappings. Undetectable at IOMMU layer.
6. IOMMU page-table manipulation via kernel compromise
BYOVD / vulnerable driver reprograms IOMMU tables.
Requires code execution on gaming PC.
Approaches 1–3 are the foundation of most current DMA cheats.
IOMMU Bypass Catalog (16 Techniques)
# Technique Mechanism Mitigation
─────────────────────────────────────────────────────────────────────────────────
1 IOMMU disabled VT-d/AMD-Vi off in BIOS Refuse misconfigured platforms
2 Pre-boot DMA Firmware leaves injection window UEFI updates; verify ACPI indicators
3 Identity/passthrough 1:1 IOVA-to-physical mapping Strict-mode IOMMU policy
4 Driver over-allocation Full 4 KB page, adjacent data OS bounce buffers; strict mappings
5 ATS abuse AT=10 TLPs with arbitrary addrs ATS Untrusted mode for non-allowlisted
6 ACS missing on bridge P2P or spoofed Requester ID Verify ACS state on all bridges
7 Lazy IOTLB invalidation Stale translations valid briefly Strict invalidation mode
8 FLR race FLR/Hot Reset race window Synchronized FLR handling
9 SMM bypass SMM code exempt from IOMMU Boot Guard / Platform Secure Boot
10 DMA-remapping driver bugs Bugs in OS IOMMU manager OS patching
11 Hypervisor escape Compromised hypervisor VBS / measured boot; TPM attestation
12 Interrupt injection (no IR) Write arbitrary interrupts Mandatory IR enforcement
13 RMRR/IVMD scope abuse Fake ACPI tables cover attacker Measured boot; runtime RMRR audit
physical ranges
14 Snoop-bit manipulation Stale cache lines visible Strict snoop enforcement
15 PASID confusion Misconfigured PASID Table PASID-aware IOMMU programming
16 DMAR/IVRS spoofing Compromised firmware, fake tables Measured boot covering firmware
Techniques 1–6: active attack surface for current commercial DMA cheats
Techniques 7–13: academic, APT, firmware-level contexts
Techniques 14–16: largely theoretical
FPGA Hardware
Xilinx PCIe Integrated Block
Hardened IP block handling:
- Physical Layer (PHY, 8b/10b or 128b/130b, LTSSM, equalization)
- Data Link Layer (sequence numbers, replay buffer, flow control)
- Transaction Layer framing and parsing
- Subset of Configuration Space
IP core documentation:
- PG054 for 7-series
- PG156 for UltraScale Gen3
- PG213 for UltraScale+ Gen4
User logic interfaces over AXI-Stream (TX/RX) and separate
config management: cfg_mgmt_* (7-series), cfg_ext_* (UltraScale).
Detection consequences:
- Default fingerprints leak through: hard block populates Config Space
with Xilinx-characteristic byte patterns
- 7-series firmware authors who don't understand cfg_mgmt_* leave
subtle behavioral differences (some CfgTLPs return hard-block defaults)
FPGA Family Hierarchy
Artix-7 (consumer/mid-range, GTP transceivers, PCIe Gen2):
Chip LUTs BRAM(Kbit) PCIe Hard Block
XC7A35T 20,800 1,800 Gen2 x4
XC7A50T 32,600 2,700 Gen2 x4
XC7A75T 46,200 3,780 Gen2 x4
XC7A100T 63,400 4,860 Gen2 x4
XC7A200T 134,600 13,140 Gen2 x4
(Smaller than T35 have no hard PCIe block)
Kintex-7 (high-end, GTX transceivers):
XC7K70T 41,000 4,860 Gen2 x8
XC7K160T 101,400 11,700 Gen2 x8
XC7K325T 203,800 16,020 Gen2 x8 / Gen3 x4
XC7K410T 254,200 28,620 Gen3 x8
Zynq UltraScale+ (ARM Cortex-A53 cores, GTH/GTY):
ZU2EG/CG ~47,000 ~5.3M Gen3 x4
ZU3EG/CG ~70,000 ~7.6M Gen3 x4
ZU4EG/EV ~88,000 ~11.0M Gen3 x8
ZU5EG/EV ~117,000 ~18.0M Gen3 x8
ZU6EG/CG ~230,000 ~32.1M Gen3 x16
(EV-suffixed: hardened H.265 codec for DMA + video-capture boards)
Resource Constraints and Capability
BRAM size caps:
shadow config + writable overlay + BAR emulation + state machines.
T35 (1.8 Mbit) struggles with full 4 KB shadow + 64 KB BAR + jitter buffers.
T100 (4.86 Mbit) fits comfortably.
Zynq ZU3 (7+ Mbit) has effectively unlimited room.
LUT count caps behavioral complexity:
Each subsystem (MSI generator, ASPM FSM, AER counter, BAR responder)
costs thousands of LUTs. T35 holds 1–2; T100 the full set;
Kintex/Zynq adds runtime-reconfigurable parameter tables.
PHY transceiver family (GTP/GTX/GTH/GTY) has measurably different
signal characteristics; can sometimes be inferred from root-port
performance counters independent of firmware spoofing.
Form Factors
Form Factor Description Detection
────────────────────────────────────────────────────────────────────
M.2 NGFF Key M Internal NVMe slot Dominant modern form;
physically invisible
M.2 + USB3 bridge M.2 board with FT601 Gaming PC sees only M.2
PCIe x1/x4 add-in Traditional add-in card More physically visible
External USB3 USB3-to-PCIe (legacy) Mostly obsolete
Combo boards DMA + HDMI capture + Complex device tree;
input injection HDMI activity is fingerprint
M.2 slot populations are partially auditable from software through
PCI topology, ACPI, SMBIOS, storage inventory, and vendor board databases.
SMBIOS slot records are often incomplete for M.2, so detection should
be probabilistic and board-model-aware.
pcileech Framework
Project Lineage
Five upstream repositories:
- pcileech: Host-side C application with attack modules
- pcileech-fpga: FPGA firmware in Verilog/SystemVerilog, per-board variants
- MemProcFS: Virtual filesystem mounting target memory as /proc-like tree
- LeechCore: Low-level device abstraction library
- vmm: Memory analysis engine (vmm.dll API)
Pipeline: FPGA → LeechCore → PCILeech attack modules / MemProcFS analysis
FPGA Firmware Architecture
Key modules:
- pcileech_pcie_a7.v / _us.v: Top-level Artix-7 / UltraScale integration
- pcileech_pcie_tlps128_bram_rdwr.v: 128-bit TLP source/sink (AXI-Stream)
- pcileech_pcie_cfgspace_shadow.v: Shadow config space in BRAM
- pcileech_cfgspace.coe: Init data (stock: Xilinx 10EE:0666)
- pcileech_bar_impl_zerowrite4k.v: Default BAR — absorbs writes, returns zero
- pcileech_bar_impl_loopaddr.v: Alternative BAR — echoes address
- pcileech_bar_impl_none.v: Disables BAR (returns UR)
- pcileech_pcie_cfg_a7.v: Config management via cfg_mgmt_*
- pcileech_mux.v: TLP multiplexer
- pcileech_fifo.v: Internal staging FIFO
Two key architectural choices:
1. Shadow config is spoofable but not spoofed by default.
.coe ships with placeholder Xilinx IDs. User must overwrite
with real donor's dump and resynthesize.
2. BAR controller is functionally inert.
zerowrite4k doesn't emulate device behavior.
Active BAR probing catches stock builds in one operation.
Host-Side MemProcFS
Mounts target memory as filesystem:
M:\
├── pid\1234\
│ ├── name.txt
│ ├── modules\ ← loaded module list
│ ├── handles\
│ ├── vad\ ← virtual address descriptors
│ ├── memmap.txt
│ └── minidump\
├── sys\
├── name\game.exe\ ← lookup by process name
└── forensic\
├── yara\
├── timeline\
└── registry\
One possible development/use pattern:
1. Development phase: MemProcFS, signature search, cross-references
→ slow, broad scanning to find entity manager / player array / view matrix
2. Execution phase: custom app via vmm.dll/LeechCore,
narrower periodic or batched reads of known offsets
Behavioral analysis can test for this pattern, but implementations may cache,
randomize, or use different access strategies.
Stock Firmware Fingerprints
Common or older vanilla pcileech-fpga configurations may exhibit the following;
verify the exact commit, FPGA IP configuration, and synthesized design:
- VID/DID 10EE:0666 (Xilinx placeholder)
- Xilinx 7-series PCIe IP signature bytes at characteristic offsets
- DSN Extended Capability absent or default
- No AER, LTR, ARI, ATS, or SR-IOV capabilities
- BAR0 mapped (DMA window); BAR1–5 disabled or all-ones
- BAR reads return zero (zerowrite4k) or echo address (loopaddr)
- MSI capability present but expected interrupts are not generated
- Config reads complete in deterministically uniform time
(BRAM lookup with fixed pipeline depth, near-zero variance)
- no observed ASPM transitions under a workload and policy that should enter
lower-power states
- AER correctable-error count stays at zero
- power state remains D0 under tested conditions
- Class Code matches donor placeholder but no class-specific behavior
Configuration Space Spoofing
Bridge vs Emulated Firmware
Bridge firmware:
Patches identity fields via Vivado's PCIe IP Core GUI
(VID, DID, Subsystem IDs, Class Code, sometimes DSN).
Fast to produce, but 7-series hard IP generates internal capability
blocks at characteristic offsets that retain FPGA-specific fingerprints.
Emulated (1:1) firmware:
Implements complete shadow Configuration Space in BRAM.
Entire 4 KB extended config space initialized from real donor device hex dump.
When OS issues CfgRd TLP, firmware responds from BRAM.
The design attempts to prevent IP-core defaults from appearing on the bus.
Common bugs in emulated firmware:
- First 16 bytes still come from IP block (mux priority)
- Type 1 config reads not intercepted
- Capability blocks bypassed in GUI still leak defaults
Shadow Configuration Space Implementation
Requirements:
1. Intercept incoming CfgRd0/CfgWr0 TLPs
2. Decode target offset
3. Look up value in BRAM
4. Build Completion TLP with correct Completer ID, status, payload
5. Send Completion through hard IP block
4 KB coverage at 4-byte granularity = 1,024 entries × 4 bytes = 4 KB BRAM.
Well within even T35's resources.
Overlay RAM and Writable Register Emulation
Real devices have writable registers. Firmware that returns correct
values on reads but drops writes creates detectable inconsistency.
Detection probe:
write Command[BME] = 1 → read Command[BME]
write Command[BME] = 0 → read Command[BME]
Real silicon: bit toggles. Naive shadow: bit stays at BRAM init value.
Overlay RAM merges at read time:
response = (base_value & ~writable_mask) | (overlay_value & writable_mask)
The catch: writable mask is register-specific:
- Command Register: different reserved bits than Device Control
- MSI Address Low: bits [1:0] reserved-zero
- BAR: type bits in [3:0] depend on I/O/memory, prefetchable
- Status Register: W1C bits — writing 1 clears, writing 0 no change
- AER Status: W1C across the board
Naive implementations with single global mask fail because
reserved-bit and W1C behavior diverges. Detection probes
W1C cases: write 0x00000000 to Correctable Error Status,
then write known-1 patterns, verify read-back semantics.
Donor Card Extraction
A common emulation strategy starts with a donor device and clones more than
VID/DID: configuration-space layout, capabilities, BAR masks, interrupt-table
layout, and device serial behavior where present. Completeness varies by design.
Extraction tools:
- lspci -d [VID:DID] -vvv -xxxx (full ECAM dump)
- setpci -s <BDF> ECAP_AER+0x4.L
- Direct ECAM mapping in kernel-mode tools
- VFIO passthrough + programmatic dump
Donor selection pitfalls:
- Duplicating hardware already in gaming PC → caught instantly
- Must match FPGA's actual PCIe capability (x16 Gen4 on Gen2 x1 fails)
- Silicon generati
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