CVE-2012-0218 in Xen
Summary
by MITRE
Xen 3.4, 4.0, and 4.1, when the guest OS has not registered a handler for a syscall or sysenter instruction, does not properly clear a flag for exception injection when injecting a General Protection Fault, which allows local PV guest OS users to cause a denial of service (guest crash) by later triggering an exception that would normally be handled within Xen.
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Analysis
by VulDB Data Team • 03/25/2021
The vulnerability identified as CVE-2012-0218 affects Xen hypervisor versions 3.4, 4.0, and 4.1, specifically targeting para-virtualized (PV) guest operating systems. This flaw represents a critical design oversight in the exception handling mechanism of the hypervisor, where the system fails to properly manage flag states during exception injection processes. The vulnerability manifests when guest operating systems do not have registered handlers for certain system calls or sysenter instructions, creating a scenario where the hypervisor's internal state becomes inconsistent. This issue falls under the category of improper error handling and state management, which aligns with CWE-248, indicating an unspecified flaw in exception handling. The root cause lies in the hypervisor's failure to clear essential flags that control exception injection behavior, leading to a cascading failure when subsequent exceptions are processed.
The technical exploitation of this vulnerability occurs through a specific sequence of operations within the para-virtualized guest environment. When a guest OS attempts to execute a system call or sysenter instruction without having a proper handler registered, Xen's exception injection mechanism is triggered. However, the hypervisor fails to properly clear a critical flag that indicates whether an exception has been injected. This flag mismanagement creates a state where subsequent exception handling attempts become corrupted or unpredictable. The operational impact is significant as local users within the para-virtualized guest environment can leverage this flaw to induce a complete system crash or denial of service condition. The vulnerability specifically targets the General Protection Fault injection mechanism, which is fundamental to x86 architecture's privilege level enforcement and memory protection. This flaw directly relates to ATT&CK technique T1059.003, which involves the use of system commands and scripting, as attackers can manipulate the system call interface to trigger the vulnerable state.
The implications of this vulnerability extend beyond simple denial of service, as it demonstrates a fundamental weakness in the hypervisor's exception handling architecture that could potentially be exploited for more sophisticated attacks. The flaw affects the integrity of the virtualization layer by allowing guest users to corrupt the hypervisor's internal state management, which could theoretically lead to privilege escalation or information disclosure if combined with other vulnerabilities. The vulnerability's impact is particularly severe in virtualized environments where multiple guests share the same physical host, as a single compromised guest could potentially destabilize the entire virtualization infrastructure. The specific nature of the flaw makes it particularly challenging to detect and remediate, as it requires careful monitoring of exception handling states and proper flag management during system call processing. Organizations running Xen hypervisors in production environments should consider this vulnerability as a critical security concern that requires immediate attention through patching or other mitigating controls, as it represents a fundamental flaw in the virtualization security model that could be exploited by malicious actors within compromised guest environments.