[컴] 썬더클랩 Thunderclap

썬더볼트 취약점 /


paper : http://thunderclap.io/thunderclap-paper-ndss2019.pdf

썬더클랩(Thunderclap)

• 이 취약점은 물리적인 접근을 한 공격자에게 Thunderbolt(썬더볼트) 포트를 이용해서 해당기계를 위험하게 한다.
• 공격자가 가장높은 권한으로 임의의 코드를 실행시킬 수 있게 해준다. 그래서 잠재적으로 암호, 은행 로그인, 암화화 키, 개인파일등에 접근할 수 있게 해준다.
• 또한, 공격자가 이 취약점을 이용하면 겉으로 무해해 보이는 충전기, 프로젝터 같은 주변기기가 충전이나, 영상을 쏘아 주는 사이에 "host 머신"을 위험하게 할 수 있다.(이러면, 우리가 제3자의 충전기도 마음대로 사용하기 힘들어진다. 마치 공용 wifi 같이)

동작

네트워크 카드의 동작

1. OS 가 network interface card(NIC) 에 packet 을 보내라고 요청할 때
2. OS 는 NIC에 보낼 data 에 대한 address 를 제공해 준다.
3. 그러면 NIC 의 payload 함수는 plaintext data를 찾으려고 memory 근처를 검색할 것이다.

Thunderclap 방법

When an IOMMU is in operation, the most obvious way touse it for protection requires some changes to ring buffer usage. 

• First,  packet  data  must  be  allocated  from  a  pool  of  physical memory  that  allows  exposure  to  devices  (some  memory  maybe  inaccessible  due  to  hardware  limitations).  
• Second,  before a data  block  is  placed  in  the  ring  buffer  for  transmission,  a window must be opened for it to be accessible by the device. This involves creating a mapping for the block in the IOMMU page table.
• Third, the address written into the ring buffer is now the I/O virtual address of the mapping, rather than the physical address. 
• Finally, when the device is finished with the data, the operating system should close the window again, revoking the mapping from the IOMMU page table and IOTLB.

How do the Thunderclap vulnerabilities work?

The Thunderclap vulnerabilities stem from the fact that computer peripherals such as network cards and GPUs have traditionally been trusted parts of a computer system: they have direct memory access (DMA), which allows them to read and write all of system memory without operating system oversight. DMA allows peripherals to bypass operating system security policies, and DMA attacks abusing this access have been widely employed by hackers and the intelligence community to take control of and exfiltrate sensitive data from target machines. This means passwords, banking logins, private files and browser activity are all exposed, and an attacker can inject any code they wish onto your machine.

Current systems feature input-output memory management units (IOMMUs), protection mechanisms that allow the operating system to restrict peripheral-device memory access. With IOMMU usage enabled, operating systems can protect against DMA attacks by restricting memory access to peripherals that perform legitimate functions and only allowing access to non-sensitive regions of memory. Unfortunately, IOMMU protection is turned off by default in many systems.

Our work leverages vulnerabilities in operating system IOMMU usage to compromise a target system via DMA, even in the presence of an IOMMU that is enabled and configured to defend against DMA attacks. The novel Thunderclap security evaluation platform, built on field-programmable gate array (FPGA) hardware, mimics the functionality of a legitimate peripheral device to convince a target operating system to grant it access to regions of memory. It then examines those regions of memory to find a rich and nuanced attack surface of vulnerable structures that can be exploited to take control of the system.

The rise of hardware interconnects like Thunderbolt 3 over USB-C that combine power input, video output, and peripheral device DMA over the same port greatly increases the real-world applicability of Thunderclap vulnerabilities. Thunderbolt can allow potentially malicious devices to hotplug into a running machine and obtain direct memory access, which makes DMA attacks against temporarily unattended targets feasible. Furthermore, the confusion of power, video, and DMA facilitates the creation of malicious charging stations or projectors that take control of connected machines.

Additionally, our work shows that the Thunderclap vulnerabilities can also be exploited by compromised firmware on existing PCI Express devices, for example network cards or baseboard management controllers (BMCs) integrated into servers. A firmware compromise might be introduced via a firmware vulnerability or a compromise in the device supply chain or factory.

How does this work differ from earlier DMA attacks such as Inception?

Early DMA attacks relied on the absence of an IOMMU. They involved scanning all of a system’s memory for sensitive data from devices that did not appear to the system as legitimate peripherals. These attacks were addressed by the introduction of IOMMUs, which block all memory access from unrecognized devices.

Some previous DMA attacks have taken advantage of weaknesses in IOMMU configuration or setup to disable IOMMU protections. Thunderclap explores serious vulnerabilities that are present even once the IOMMU is configured correctly.

Technical details of the Thunderclap platform

The Thunderclap platform consists of an FPGA that runs the Thunderclap application. The FPGA then plugs into a computer via PCI Express or Thunderbolt. The Thunderclap application makes the FPGA behave to the computer like a genuine Ethernet card (the Intel 82574L network interface card or NIC). The operating system will identify the ethernet peripheral, load drivers, allow the device to access memory (via DMA and an IOMMU if enabled), and ask it to send and receive packets.

With this deep interaction with the operating system, Thunderclap’s device model provides hooks that allow payload functions to be added to device behavior. For example, when the operating system asks the NIC to send a packet, it provides the NIC with the address of the data to send. A payload function might search nearby memory looking for plaintext data that was intended for a different network device.

The Thunderclap application runs on Intel/Altera FPGA boards:

1. Intel Arria 10 SoC Development Kit ($4500) with Samtec HDR-181157-01-PCIEC cable (available from Samtec direct) - currently recommended
2. Enclustra Mercury+ AA1 module (ME-AA1-270-3E4-D11E) on PE1 carrier board (~EUR 800) - work in progress
3. Terasic DE5-Net board (Stratix V) with BERI soft-CPU - no longer supported
4. As far as we can ascertain, Xilinx, Lattice and Intel Cyclone FPGAs don’t allow us to replace the vendor-supplied implementation of configuration registers with our own (Intel calls it ‘config bypass’ mode) which we require.

It is composed of several pieces:

1. The underlying FPGA bitfile, containing the hardware that receives PCIe packets (TLPs) and delivers them to software. The FPGA contains an Arm Cortex A9 CPU (hard processor system or HPS) to run our software stack. GitHub repo
2. The Ubuntu 16.04 operating system running on the on the Arm, including kernel, device tree and u-boot bootloader (which also loads the FPGA bitfile at boot time). Automated build scripts (work in progress): GitHub repo
3. The Thunderclap application, which is a substantially cut down version of QEMU, based on its e1000e device. This runs in Ubuntu on the ARM core and connects directly to the PCIe queues provided by the hardware. GitHub repo