CVE-2017-10916 in Xen
Summary
by MITRE
The vCPU context-switch implementation in Xen through 4.8.x improperly interacts with the Memory Protection Extensions (MPX) and Protection Key (PKU) features, which makes it easier for guest OS users to defeat ASLR and other protection mechanisms, aka XSA-220.
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Analysis
by VulDB Data Team • 12/30/2020
The vulnerability identified as CVE-2017-10916 represents a critical flaw in the Xen hypervisor's virtual central processing unit context-switch implementation that affects versions through 4.8.x. This issue specifically targets the interaction between hypervisor-level memory management and processor features designed to enhance security. The vulnerability arises from improper handling of Memory Protection Extensions and Protection Key features during virtual machine context switches, creating a pathway for malicious guest operating system users to bypass essential security mitigations.
The technical flaw manifests in how the hypervisor manages the transition between different virtual machine states when utilizing MPX and PKU capabilities. These processor features are designed to provide enhanced memory protection by implementing bounds checking for memory accesses and establishing protection keys that control memory access permissions. During normal operation, these features work in conjunction with the hypervisor to maintain isolation between virtual machines and prevent unauthorized memory access. However, the flawed context-switch implementation fails to properly preserve or restore these security-related processor state components, leading to information leakage and privilege escalation opportunities.
The operational impact of this vulnerability is significant as it directly undermines fundamental security mechanisms like Address Space Layout Randomization, which is crucial for preventing exploitation techniques such as return-oriented programming and information leakage attacks. Attackers can leverage this flaw to defeat ASLR by gaining access to memory layout information that should remain protected within the hypervisor environment. This vulnerability essentially allows malicious guest users to extract sensitive information about the host system's memory layout, potentially enabling them to craft more sophisticated attacks against the underlying infrastructure. The implications extend beyond simple information disclosure, as this weakness can be exploited to circumvent other security controls that depend on proper memory isolation.
This vulnerability aligns with CWE-264, which addresses permissions, privileges, and access controls, and relates to ATT&CK techniques including T1059 for command and scripting interpreter and T1068 for exploit for privilege escalation. The flaw demonstrates how hypervisor-level security mechanisms can be undermined through improper state management during critical operations, particularly when dealing with advanced processor features designed to enhance security. Organizations running Xen hypervisors in virtualized environments are particularly at risk, as this vulnerability can be exploited by malicious users within guest operating systems to gain elevated privileges and potentially compromise the entire virtualized infrastructure. The remediation requires updating to patched versions of Xen hypervisor where the context-switch implementation properly handles MPX and PKU features, ensuring that processor state information is correctly preserved and restored during virtual machine transitions.
The vulnerability represents a classic example of how advanced security features in modern processors can be rendered ineffective through improper implementation at lower abstraction layers. It highlights the complexity of maintaining security in virtualized environments where multiple layers of abstraction must work seamlessly together. The flaw serves as a reminder that hypervisor implementations must carefully consider how they interact with processor security features, as any oversight in state management can create significant attack vectors that undermine the fundamental security guarantees provided by virtualization technologies.