Ramblings & ephemera

Virtual-machine based rootkits

From Samuel T. King, Peter M. Chen, Yi-Min Wang, Chad Verbowski, Helen J. Wang, & Jacob R. Lorch’s “SubVirt: Implementing malware with virtual machines
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We evaluate a new type of malicious software that gains qualitatively more control over a system. This new type of malware, which we call a virtual-machine based rootkit (VMBR), installs a virtual-machine monitor underneath an existing operating system and hoists the original operating system into a virtual machine. Virtual-machine based rootkits are hard to detect and remove because their state cannot be accessed by software running in the target system. Further, VMBRs support general-purpose malicious services by allowing such services to run in a separate operating system that is protected from the target system. We evaluate this new threat by implementing two proof-of-concept VMBRs. We use our proof-of-concept VMBRs to subvert Windows XP and Linux target systems, and we implement four example malicious services using the VMBR platform. Last, we use what we learn from our proof-of-concept VMBRs to explore ways to defend against this new threat. We discuss possible ways to detect and prevent VMBRs, and we implement a defense strategy suitable for protecting systems against this threat. …

A major goal of malware writers is control, by which we mean the ability of an attacker to monitor, intercept, and modify the state and actions of other software on the system. Controlling the system allows malware to remain invisible by lying to or disabling intrusion detection software.

Control of a system is determined by which side occupies the lower layer in the system. Lower layers can control upper layers because lower layers implement the abstractions upon which upper layers depend. For example, an operating system has complete control over an application’s view of memory because the operating system mediates access to physical memory through the abstraction of per-process address spaces. Thus, the side that controls the lower layer in the system has a fundamental advantage in the arms race between attackers and defenders. If the defender’s security service occupies a lower layer than the malware, then that security service should be able to detect, contain, and remove the malware. Conversely, if the malware occupies a lower layer than the security service, then the malware should be able to evade the security service and manipulate its execution.

Because of the greater control afforded by lower layers in the system, both security services and rootkits have evolved by migrating to these layers. Early rootkits simply replaced user-level programs, such as ps, with trojan horse programs that lied about which processes were running. These user-level rootkits were detected easily by user-level intrusion detection systems such as TripWire [29], and so rootkits moved into the operating system kernel. Kernel-level rootkits such as FU [16] hide malicious processes by modifying kernel data structures [12]. In response, intrusion detectors also moved to the kernel to check the integrity of the kernel’s data structures [11, 38]. Recently, researchers have sought to hide the memory footprint of malware from kernel-level detectors by modifying page protections and intercepting page faults [43]. To combat such techniques, future detectors may reset page protections and examine the code of the page-fault handler. …

Our project, which is called SubVirt, shows how attackers can use virtual-machine technology to address the limitations of current malware and rootkits. We show how attackers can install a virtual-machine monitor (VMM) underneath an existing operating system and use that VMM to host arbitrary malicious software. The resulting malware, which we call a virtual- machine based rootkit (VMBR), exercises qualitatively more control than current malware, supports general-purpose functionality, yet can completely hide all its state and activity from intrusion detection systems running in the target operating system and applications. …

A virtual-machine monitor is a powerful platform for malware. A VMBR moves the targeted system into a virtual machine then runs malware in the VMM or in a second virtual machine. The targeted system sees little to no difference in its memory space, disk space, or execution (depending on how completely the machine is virtualized). The VMM also isolates the malware’s state and events completely from those of the target system, so software in the target system cannot see or modify the malicious software. At the same time, the VMM can see all state and events in the target system, such as keystrokes, network packets, disk state, and memory state. A VMBR can observe and modify these states and events—without its own actions being observed—because it completely controls the virtual hardware presented to the operating system and applications. Finally, a VMBR provides a convenient platform for developing malicious services. A malicious service can benefit from all the conveniences of running in a separate, general-purpose operating system while remaining invisible to all intrusion detection software running in the targeted system. In addition, a malicious service can use virtual-machine introspection to understand the events and states taking place in the targeted system. …

In the overall structure of a VMBR, a VMBR runs beneath the existing (target) operating system and its applications (Figure 2). To accomplish this, a VMBR must insert itself beneath the target operating system and run the target OS as a guest. To insert itself beneath an existing system, a VMBR must manipulate the system boot sequence to ensure that the VMBR loads before the target operating system and applications. After the VMBR loads, it boots the target OS using the VMM. As a result, the target OS runs normally, but the VMBR sits silently beneath it.

To install a VMBR on a computer, an attacker must first gain access to the system with sufficient privileges to modify the system boot sequence. There are numerous ways an attacker can attain this privilege level. For example, an attacker could exploit a remote vulnerability, fool a user into installing malicious software, bribe an OEM or vendor, or corrupt a bootable CD-ROM or DVD image present on a peer-to-peer network. On many systems, an attacker who attains root or Administrator privileges can manipulate the system boot sequence. On other systems, an attacker must execute code in kernel mode to manipulate the boot sequence. We assume the attacker can run arbitrary code on the target system with root or Administrator privileges and can install kernel modules if needed. …

VMBRs use a separate attack OS to deploy malware that is invisible from the perspective of the target OS but is still easy to implement. None of the states or events of the attack OS are visible from within the target OS, so any code running within an attack OS is effectively invisible. The ability to run invisible malicious services in an attack OS gives intruders the freedom to use user-mode code with less fear of detection.

We classify malicious services into three categories: those that need not interact with the target system at all, those that observe information about the target system, and those that intentionally perturb the execution of the target system. In the remainder of this section, we discuss how VMBRs support each class of service.

The first class of malicious service does not communicate with the target system. Examples of such services are spam relays, distributed denial-of-service zombies, and phishing web servers. A VMBR supports these services by allowing them to run in the attack OS. This provides the convenience of user-mode execution without exposing the malicious service to the target OS.

The second class of malicious service observes data or events from the target system. VMBRs enable stealthy logging of hardware-level data (e.g., keystrokes, network packets) by modifying the VMM’s device emulation software. This modification does not affect the virtual devices presented to the target OS. For example, a VMBR can log all network packets by modifying the VMM’s emulated network card. These modifications are invisible to the target OS because the interface to the network card does not change, but the VMBR can still record all network packets. …

The third class of malicious service deliberately modifies the execution of the target system. For example, a malicious service could modify network communication, delete e-mail messages, or change the execution of a target application. A VMBR can customize the VMM’s device emulation layer to modify hardware-level data. A VMBR can also modify data or execution within the target through virtual-machine introspection.

Using our proof-of-concept VMBRs, we developed four malicious services that represent a range of services a writer of malicious software may want to deploy. We implemented a phishing web server, a keystroke logger, a service that scans the target file system looking for sensitive files, and a defense countermeasure that defeats a current virtual-machine detector. …

To avoid being removed, a VMBR must protect its state by maintaining control of the system. As long as the VMBR controls the system, it can thwart any attempt by the target to modify the VMBR’s state. The VMBR’s state is protected because the target system has access only to the virtual disk, not the physical disk.

The only time the VMBR loses control of the system is in the period of time after the system powers up until the VMBR starts. Any code that runs in this period can access the VMBR’s state directly. The first code that runs in this period is the system BIOS. The system BIOS initializes devices and chooses which medium to boot from. In a typical scenario, the BIOS will boot the VMBR, after which the VMBR regains control of the system. However, if the BIOS boots a program on an alternative medium, that program can access the VMBR’s state.

Because VMBRs lose control when the system is powered off, they may try to minimize the number of times full system power-off occurs. The events that typically cause power cycles are reboots and shutdowns. VMBRs handle reboots by restarting the virtual hardware rather than resetting the underlying physical hardware. By restarting the virtual hardware, VMBRs provide the illusion of resetting the underlying physical hardware without relinquishing control. Any alternative bootable medium used after a target reboot will run under the control of the VMBR.

In addition to handling target reboots, VMBRs can also emulate system shutdowns such that the system appears to shutdown, but the VMBR remains running on the system. We use ACPI sleep states [3] to emulate system shutdowns and to avoid system power-downs. ACPI sleep states are used to switch hardware into a low-power mode. This low-power mode includes spinning down hard disks, turning off fans, and placing the monitor into a power-saving mode. All of these actions make the computer appear to be powered off. Power is still applied to RAM, so the system can come out of ACPI sleep quickly with all memory state intact. When the user presses the power button to “power-up” the system, the computer comes out of the low-power sleep state and resumes the software that initiated the sleep. Our VMBR leverage this low-power mode to make the system appear to be shutdown; when the user “powers-up” the system by pressing the power button the VMBR resumes. If the user attempts to boot from an alternative medium at this point, it will run under the control of the VMBR. We implemented shutdown emulation for our VMware-based VMBR. …

We first measure the disk space required to install the VMBR. Our Virtual PC-based VMBR image is 106 MB compressed and occupies 251 MB of disk space when uncompressed. Our VMware-based VMBR image is 95 MB compressed and occupies 228 MB of disk space uncompressed. The compressed VMBR images take about 4 minutes to download on a 3 Mb/s cable modem connection and occupy only a small fraction of the total disk space present on modern systems. …

The installation measurements include the time it takes to uncompress the attack image, allocate disk blocks, store the attack files, and modify the system boot sequence. Installation time for the VMware-based VMBR is 24 seconds. Installation for the Virtual PC-based VMBR takes longer (262 seconds) because the hardware used for this test is much slower and has less memory. In addition, when installing a VMBR underneath Windows XP, we swap the contents of the disk blocks used to store the VMBR with those in the beginning of the Windows XP disk partition, and these extra disk reads/writes further lengthen the installation time.

We next measure boot time, which we define as the amount of time it takes for an OS to boot and reach an initial login prompt. Booting a target Linux system without a VMBR takes 53 seconds. After installing the VMware-based VMBR, booting the target system takes 74 seconds after a virtual reboot and 96 seconds after a virtual shutdown. It takes longer after a virtual shutdown than after a virtual reboot because the VMM must re-initialize the physical hardware after coming out of ACPI sleep. In the uncommon case that power is removed from the physical system, the host OS and VMM must boot before loading the target Linux OS. The VMware-based VMBR takes 52 seconds to boot the host OS and load the VMM and another 93 seconds to boot the target Linux OS. We speculate that it takes longer to boot the target OS after full system power-down than after a virtual reboot because some performance optimizations within the VMware VMM take time to warm up.

Booting a target Windows XP system without a VMBR takes 23 seconds. After installing the Virtual PC-based VMBR, booting the target system takes 54 seconds after a virtual reboot. If power is removed from the physical system, the Virtual PC-based VMBR takes 45 seconds to boot the host OS and load the VMM and another 56 seconds to boot the target Windows XP OS. …

Despite using specialized guest drivers, our current proof-of-concept VMBRs use virtualized video cards which may not export the same functionality as the underlying physical video card. Thus, some high-end video applications, like 3D games or video editing applications, may experience degraded performance.

The physical memory allocated to the VMM and attack OS is a small percentage of the total memory on the system (roughly 3%) and thus has little performance impact on a target OS running above the VMBR. …

In this section, we explore techniques that can be used to detect the presence of a VMBR. VMBRs are fundamentally more difficult to detect than traditional malware because they virtualize the state seen by the target system and because an ideal VMBR modifies no state inside the target system. Nonetheless, a VMBR does leave signs of its presence that a determined intrusion detection system can observe. We classify the techniques that be used to detect a VMBR by whether the detection system is running below the VMBR, or whether the detection system is running above the VMBR (i.e., within the target system). …

There are various ways to gain control below the VMBR. One way to gain control below the VMBR is to use secure hardware. Intel’s LaGrande [25], AMD’s platform for trustworthy computing [2], and Copilot [36] all propose hardware that can be used to develop and deploy low-layer security software that would run beneath a VMBR.

Another way to gain control below the VMBR is to boot from a safe medium such as a CD-ROM, USB drive or network boot server. This boot code can run on the system before the VMBR loads and can view the VMBR’s quiescent disk state. …

A third way to gain control below the VMBR is to use a secure VMM [17]. Like alternative bootable media, secure VMMs gain control of the system before the operating system boots. Running a secure VMM does not by itself stop a VMBR, as a VMBR can still insert itself between the VMM and the operating system. However, a secure VMM does retain control over the system as it runs and could easily add a check to stop a VMBR from modifying the boot sequence above the secure VMM. …

There are several reasons why traditional techniques (e.g., having the target system scan memory or disk) may not be able to detect a VMBR. First, the target system is confined to its virtual machine. Since the VMBR stores its state outside that virtual machine, the target system will see no suspicious changes in its own state. Second, even if the target system did see something amiss, the VMBR could tamper with the execution of the detector and force it to report incorrect results. …

A fundamental perturbation of the VMBR is that it uses machine resources such as CPU time, memory and disk space, and possibly network bandwidth. A VMBR adds CPU overhead to trap and emulate privileged instructions, as well as to run any malicious services. …

In addition to CPU overhead, a VMBR uses memory and disk space. A detector can detect this overhead by running a program that requires the entire machine’s memory or disk space. A VMBR can hide its space overhead in several ways. It can hide its memory overhead by mapping target pages with the same contents to the same physical page [47] or by paging some data to disk. While this extra paging activity may increase the running time of the program, the memory overhead of VMware was too small to measure in our tests. A VMBR can hide its disk overhead by compressing a portion of the disk, sharing disk blocks with the same contents, or pretending that the disk space it uses have bad sectors. …

Another type of perturbation is caused by the variety of I/O devices on the computer. Recall that a VMM typically virtualizes all I/O devices. However, virtualizing the I/O device without changing the target’s view of the device requires one to understand the interface and semantics of each device. This is a formidable task, given the wide variety of I/O devices present on today’s computing platforms. Instead, current VMMs emulate a small number of virtual devices (often with customized interfaces to improve performance [1, 34]). The target OS must then use the drivers for the emulated virtual devices. …

A final source of perturbations are caused by the imperfect virtualization of today’s x86 processors. Sensitive, non-privileged instructions like sidt leak information about the VMM yet do not trap to the VMM [31, 37]. …

We expect future enhancements to the x86 platform to reduce these perturbations. Upcoming virtualization support from Intel [45] and AMD [7] will enable more efficient virtualization. These enhancements eliminate sensitive, non-privileged instructions so they cannot be used from the CPU’s user-mode to detect the presence of a VMM. These enhancements may also accelerate transitions to and from the VMM, and this may reduce the need to run specialized guest drivers. …

However, VMBRs have a number of disadvantages compared to traditional forms of malware. When compared to traditional forms of malware, VMBRs tend to have more state, be more difficult to install, require a reboot before they can run, and have more of an impact on the overall system. Although VMBRs do offer greater control over the compromised system, the cost of this higher level of control may not be justified for all malicious applications.

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