CVE-2018-19967 in Xen
Summary
by MITRE
An issue was discovered in Xen through 4.11.x on Intel x86 platforms allowing guest OS users to cause a denial of service (host OS hang) because Xen does not work around Intel's mishandling of certain HLE transactions associated with the KACQUIRE instruction prefix.
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Analysis
by VulDB Data Team • 04/19/2020
The vulnerability identified as CVE-2018-19967 represents a critical denial of service flaw within the Xen hypervisor affecting versions through 4.11.x on Intel x86 platforms. This issue stems from the hypervisor's inadequate handling of specific Hardware Lock Elision (HLE) transactions that occur when guest operating systems utilize the KACQUIRE instruction prefix. The root cause lies in Intel's processor implementation where certain HLE transactions are mishandled, creating a scenario where malicious or unintended guest activities can trigger system-wide disruptions.
The technical flaw manifests when guest operating systems execute code containing the KACQUIRE instruction prefix, which is part of Intel's HLE instruction set designed to improve performance by allowing hardware to handle certain lock operations without software intervention. However, Intel's implementation contains a bug where specific combinations of HLE transactions can cause the processor to enter an inconsistent state that the hypervisor cannot properly manage or recover from. This results in the host operating system becoming unresponsive or hanging, effectively rendering the entire virtualization environment non-functional.
From an operational perspective, this vulnerability poses significant risks to virtualized environments where multiple guest operating systems share the same physical host. An attacker with access to a guest OS can exploit this flaw to perform a denial of service attack against the host system, potentially affecting all other virtual machines running on the same physical hardware. The impact extends beyond simple service disruption as it can lead to complete system unavailability, requiring manual intervention to restore normal operations and potentially causing cascading failures in cloud environments or data centers that rely heavily on virtualization technologies.
The vulnerability aligns with CWE-119, which addresses improper access to memory locations, and relates to the broader category of hardware-assisted virtualization flaws that can be exploited to compromise system stability. From an ATT&CK framework perspective, this vulnerability maps to the T1499.004 technique for Network Denial of Service, as the attack can be executed remotely through guest access and can cause network-wide service disruption. Organizations using Xen hypervisors must consider this vulnerability as part of their broader security posture assessment, particularly in environments where guest isolation is critical for maintaining service availability.
Mitigation strategies should include immediate patching of Xen hypervisor versions to address the specific handling of HLE transactions and KACQUIRE instruction prefix usage. System administrators should also implement monitoring solutions that can detect anomalous behavior patterns associated with HLE transaction processing and consider disabling HLE instruction sets in guest operating systems where possible. Additionally, organizations should conduct thorough vulnerability assessments to identify all systems running affected Xen versions and establish incident response procedures for handling potential exploitation attempts. The fix implemented by Xen developers typically involves adding proper workarounds for Intel's HLE implementation issues and ensuring that the hypervisor can gracefully handle and recover from the problematic transaction states that would otherwise cause system hangs.