Ruslan Rakhmetov, Security Vision
Most cyberattacks are traditionally implemented remotely - attackers prefer to act from a distance without risking their physical security at the time of hacking. However, there are a number of dangerous attacks that require physical access to the attacked host - for example, a device can be compromised after connecting a seemingly harmless flash drive. Cyber attacks like Juice jacking (using "charging" USB ports in public places for an attack), the use of BadUSB devices (for example, USB Rubber Ducky), the introduction of mini-PCs like Raspberry Pi or Flipper Zero into attacked infrastructures - all this has become common practice for both pen testers and advanced hackers. In this article, we will discuss direct memory access (DMA) attacks, which also require physical access to the attacked device.
First, you should briefly describe what direct memory access (DMA, Direct Memory Access) is. Memory in this case refers to random access memory (also called RAM - random access memory), which stores data and code (instructions for execution in the CPU - central processing unit). Starting with the Intel 80386 processor, released in 1985, CPUs have received a number of functionalities still in use:
1) Protection Rings security mechanism: the use of privilege sharing in the x86 architecture, in which the OS kernel and kernel-level drivers run at the most privileged level ("Ring 0," kernel mode), other drivers at the "Ring 1" and "Ring 2" levels, and user applications run at the least privileged level "Ring 3" (user mode). At the same time, modern operating systems usually use only "Ring 0" and "Ring 3," and deeper levels are added: a hypervisor of the hardware virtualization subsystem on "Ring -1," System Management Mode (SMM) on "Ring -2," chipset control subsystem with a separate microprocessor (for example, Intel Management Engine Interface or AMD Platform Security Processor) on "Ring -3." In this case, any code running at a higher priority level can access the code and data at a lower priority level, but not vice versa (for example, a hypervisor with "Ring -1" can gain unlimited access to the OS kernel on "Ring 0," and the user program cannot access the OS kernel).
2) The use of virtual memory and its protection: all user programs from "Ring 3" work not with physical memory, but with virtual memory, which allows you to isolate the memory (data and code) of applications from each other. Translation of virtual addresses to physical addresses is performed by a special device - a memory management unit (MMU). Virtual memory protection allows you to prevent a process from accessing memory that is not allocated to it: you can completely deny the process access to a certain area of memory, prohibit only writing to it, or prohibit the execution of the contents of this area of memory. Modern CPUs and operating systems use memory protection technologies such as Data Execution Prevention (DEP), Address Space Layout Randomization (ASLR), Structured Exception Handling Overwrite Protection, SEHOP) and a number of others.
3) The use of page memory (RAM is divided into areas - "pages" of a fixed size, and virtual addresses are related to physical ones page by page) and multitasking (different processes are executed independently of each other).
So, the developers of processors and operating systems have invested significant resources in protecting RAM, since it processes encryption keys, passwords and confidential information, and it is also in it that highly privileged processes store the code of executable commands and record the results of their work. Despite the presence of solutions for encrypting RAM and using secure dedicated cryptoprocessors (Trusted Platform Module, TPM), in most cases, important information that is stored and transmitted in encrypted form is still processed in RAM in clear text, which attracts the attention of cybercriminals. DMA memory access is one method of gaining direct access to RAM, not counting cold boot attack, which is based on storing information in RAM modules after power is turned off, especially at low temperatures.
Direct Memory Access (DMA) technology was developed for good purposes: with an increase in the amount of information stored and processed on external devices, it was required that devices quickly access RAM bypassing the CPU. Under normal conditions, the CPU controls the transfer of data between peripherals and RAM, which leads to a decrease in the speed of solving other tasks. DMA technology accelerates work with devices such as external high-speed drives, external graphics, network and audio cards - the entire periphery, which works with large amounts of information. Such peripherals can be connected via various high-speed interfaces - IEEE 1394 (FireWire), Thunderbolt, USB 3.0/4.0, PCI/PCI Express, SD Express, various adapters and controllers can also be used - for example, USB 3380 (PCI Express - USB 3.0). Sub-external devices can be equipped with their own DMA controller (Bus Mastering mode), which controls the process of data transfer between the device and RAM, interacting with the CPU only at the start and end of this process. To control the operation of the DMA controller with RAM, by analogy with the MMU described above, a special Input-Output Memory Management Unit (IOMMU) was developed. The IOMMU is an intermediary between RAM and the DMA device, telling it not real physical addresses in memory, but virtual ones. In addition, IOMMU in a virtualization environment allows guest operating systems to access host peripherals directly, bypassing the CPU. Processor manufacturers over time formed their specifications for IOMMU blocks, for example, Intel (VT-d technology), AMD (AMD-Vi technology), ARM (SMMU technology), Apple (DART technology).
However, the use of direct DMA memory access even with a protective IOMMU when implemented in various operating systems has its own characteristics:
1) The developers have put an erroneous assumption in the OS architecture about the ability to trust peripheral devices.
2) Due to the fact that working with DMA devices through the IOMMU slows down data exchange, not all operating systems and motherboard firmware support for IOMMU is enabled by default. In addition, not all operating systems support IOMMU in principle.
3) IOMMU may be subject to Spatial sub-page vulnerability: an IOMMU allocated virtual memory page may contain not only the data required by the DMA device (small I/O buffers), but also OS kernel buffers, which allows the DMA device to access them In addition, pointers to device driver functions or OS objects can reside on the same virtual memory page as the DMA device I/O buffers.
4) The IOMMU may be subject to Temporal deferred invalidation vulnerability: the mapping between virtual and physical memory in the IOMMU is managed by the OS, which, in order to optimize performance, can perform such a reset with some delay, which allows the DMA device to retain access to memory pages.
5) The PCI Express standard provides the ATS (Address Translation Service) function, which allows a compatible DMA device connected via the PCI Express bus to independently translate virtual addresses into physical ones bypassing the IOMMU.
More detailed information about the features of the described vulnerabilities can be obtained from the following publications:
https://thunderclap.io/wp-content/uploads/2024/01/thunderclap-paper-ndss2019.pdf
https://www.cs.tau.ac.il/~mad/publications/eurosys2021-dma.pdf
https://www.usenix.org/system/files/usenixsecurity24-wang-xingkai.pdf
The described features allow the DMA device to gain unlimited access to all RAM to read sensitive information (passwords, encryption keys), to unlock a working PC without a password and to run arbitrary code in the most privileged mode ("Ring 0"). Similar nuances were studied by information security researchers in the mid-2000s: for example, at the CanSecWest conference in 2005, arbitrary code was run on a Windows laptop, to which an iPod with customized Linux firmware was connected via FireWire. Similar vulnerabilities became widespread in the 2010s, for example, in 2016, an information security researcher showed the ability to extract passwords from Mac laptops using a device called PCILeech - this project is still developing to this day and supports the implementation of DMA attacks using the Screamer PCIe Squirrel device. In addition, a number of other tools are available for direct access to RAM, for example, the Inception project allows you to use hosts running Linux or Mac OS X equipped with the FireWire interface for attack. DMA access to RAM is also used for forensics (computer forensics), for example, in Rekall Memory Forensic Framework, Volatility Framework, Velociraptor DFIR solutions.
Various solutions and workarounds can be used to protect against DMA attacks:
1) Microsoft proposes to use the Kernel DMA Protection (kDMAp) function or, if this function is not supported by the CPU or the installed OS version (Windows 10 version 1709 and below), it is proposed to completely block Thunderbolt and IEEE 1394 devices, as well as not to use sleep mode (sleep mode) and, together with BitLocker, apply pre-authentication (Preboot authentication), for example, by entering a pin code when the TPM module is running.
2) For Linux, it is preferable to use kernel version 5.0 and higher, which added support for kDMAp technology.
3) It is recommended to use the current versions of UEFI firmware, include support for IOMMU and Secure Boot and Trusted Boot technologies in UEFI, as well as use modern CPUs with support for IOMMU and kDMAp.
4) For the Thunderbolt interface (version 3 and higher) in UEFI, you can configure the use of security levels SL2 (Secure Connection) or SL3 (DisplayPort and USB only).
5) Apply physical security measures: do not leave devices unattended, lock the workstation even when away for a short time, disable unnecessary interfaces in UEFI with setting a password to change UEFI settings.
6) Implement the use of authentication and pointer signing, for example, similar to Pointer Authentication technology in ARM architecture.
7) Apply full-disk encryption (although this does not apply directly to DMA attacks): when running virtual memory, most operating systems create swap files (swap file in Linux, pagefile.sys in Windows), which may contain information useful to attackers. In addition, when using hibernation mode, a special file is also created on the device's drive, into which all information from RAM is copied before switching to this mode.
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