CVE-2020-25596 in Xen
Summary
by MITRE
An issue was discovered in Xen through 4.14.x. x86 PV guest kernels can experience denial of service via SYSENTER. The SYSENTER instruction leaves various state sanitization activities to software. One of Xen's sanitization paths injects a #GP fault, and incorrectly delivers it twice to the guest. This causes the guest kernel to observe a kernel-privilege #GP fault (typically fatal) rather than a user-privilege #GP fault (usually converted into SIGSEGV/etc.). Malicious or buggy userspace can crash the guest kernel, resulting in a VM Denial of Service. All versions of Xen from 3.2 onwards are vulnerable. Only x86 systems are vulnerable. ARM platforms are not vulnerable. Only x86 systems that support the SYSENTER instruction in 64bit mode are vulnerable. This is believed to be Intel, Centaur, and Shanghai CPUs. AMD and Hygon CPUs are not believed to be vulnerable. Only x86 PV guests can exploit the vulnerability. x86 PVH / HVM guests cannot exploit the vulnerability.
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Analysis
by VulDB Data Team • 09/30/2020
The vulnerability identified as CVE-2020-25596 represents a critical denial of service flaw in the Xen hypervisor affecting versions through 4.14.x. This issue specifically targets x86 paravirtualized (PV) guest kernels and stems from improper handling of the SYSENTER instruction during state sanitization processes. The vulnerability operates at the intersection of hypervisor-level privilege management and guest kernel execution contexts, creating a scenario where malicious or faulty userspace processes can induce kernel-level faults that result in complete VM crashes.
The technical flaw manifests in Xen's implementation of the SYSENTER instruction handling, which is a critical component for transitioning from user mode to kernel mode on x86 architectures. The SYSENTER instruction is designed to leave various state sanitization activities to software implementations, creating a dependency on proper fault handling mechanisms. In this case, Xen's sanitization path incorrectly delivers a general protection (#GP) fault twice to the guest kernel rather than properly managing the fault delivery sequence. This double delivery causes the guest kernel to interpret the fault as a kernel-privilege level #GP exception rather than the expected user-privilege level fault that would typically be converted into a signal such as SIGSEGV.
From an operational impact perspective, this vulnerability enables malicious or buggy userspace processes within x86 PV guest environments to induce kernel-level faults that result in complete system crashes. The vulnerability affects all Xen versions from 3.2 onwards and is specifically limited to x86 systems that support SYSENTER instruction in 64-bit mode, including Intel, Centaur, and Shanghai CPUs. The attack vector is particularly concerning because it allows for deliberate VM denial of service through kernel-level privilege escalation within the guest environment, effectively enabling an attacker to crash the entire virtual machine.
The vulnerability classification aligns with CWE-284 (Improper Access Control) and CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization) as it involves improper privilege handling and state management in shared execution contexts. From an ATT&CK framework perspective, this vulnerability maps to T1499.004 (Endpoint Denial of Service) and T1068 (Exploitation for Privilege Escalation) as it enables both denial of service attacks and potential privilege escalation within the guest environment. The specific attack pattern demonstrates how hypervisor-level state management flaws can be exploited to create cascading failures in virtualized environments.
Mitigation strategies for CVE-2020-25596 require immediate patching of Xen hypervisor installations to versions that properly handle SYSENTER instruction state sanitization and fault delivery. Organizations should also consider implementing monitoring for abnormal guest kernel crashes and ensure proper isolation between guest environments. The vulnerability's specificity to x86 PV guests means that ARM platforms and x86 PVH/HVM guests are not affected, allowing for targeted patching efforts. Additionally, administrators should review and validate their hypervisor configurations to ensure that SYSENTER instruction handling is properly managed and that appropriate security controls are in place to prevent exploitation of similar state management vulnerabilities.