CVE-2018-7542 in Xen
Summary
by MITRE
An issue was discovered in Xen 4.8.x through 4.10.x allowing x86 PVH guest OS users to cause a denial of service (NULL pointer dereference and hypervisor crash) by leveraging the mishandling of configurations that lack a Local APIC.
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Analysis
by VulDB Data Team • 02/16/2023
The vulnerability identified as CVE-2018-7542 represents a critical denial of service flaw within the Xen hypervisor affecting versions 4.8.x through 4.10.x. This issue specifically targets x86 Paravirtualized Hypervisor (PVH) guest operating systems and exploits a fundamental weakness in how the hypervisor handles configurations lacking Local APIC (Advanced Programmable Interrupt Controller) components. The flaw manifests when guest operating systems attempt to utilize certain interrupt handling mechanisms without proper Local APIC initialization, creating a scenario where the hypervisor fails to properly validate guest configurations before processing interrupt-related operations.
The technical root cause of this vulnerability stems from improper input validation and error handling within the hypervisor's interrupt management subsystem. When a PVH guest OS operates without a Local APIC configuration, the hypervisor's interrupt handling code path encounters a NULL pointer dereference condition. This occurs because the hypervisor assumes certain interrupt controller structures will be properly initialized and populated, but when these structures remain uninitialized due to the missing Local APIC configuration, subsequent memory access operations result in null pointer dereferences. The vulnerability is categorized under CWE-476 as NULL pointer dereference, which represents a classic programming error where a null pointer is dereferenced without proper validation, leading to system crashes and potential denial of service conditions.
The operational impact of CVE-2018-7542 extends beyond simple service disruption as it can compromise the entire virtualization environment. When exploited, this vulnerability allows malicious or malformed guest operating systems to crash the hypervisor kernel, potentially affecting all virtual machines running on the same physical host. This creates a significant risk for cloud service providers and data center environments where multiple tenants share the same hypervisor infrastructure, as a single compromised guest could potentially cause cascading failures across the entire virtualized platform. The vulnerability operates at the ATT&CK technique T1499.001, which involves network denial of service, but in this case the attack vector is specifically targeting the hypervisor layer rather than network connectivity.
Mitigation strategies for CVE-2018-7542 focus on both immediate patching and operational hardening measures. Organizations should prioritize upgrading to Xen versions 4.11.0 or later, where the vulnerability has been addressed through proper input validation and null pointer checks in the interrupt handling code. Additionally, administrators can implement runtime configurations that enforce proper Local APIC initialization for all PVH guests, though this approach may not be feasible in all deployment scenarios. The vulnerability demonstrates the importance of robust input validation in hypervisor code, particularly when dealing with guest configurations that may vary significantly from expected defaults. Security teams should monitor for exploitation attempts through hypervisor logs and implement intrusion detection systems that can identify anomalous interrupt handling patterns that may indicate exploitation attempts. The fix implemented by Xen developers involved adding proper null pointer checks and graceful error handling in the interrupt controller initialization path, preventing the hypervisor from crashing when encountering malformed guest configurations. This vulnerability highlights the critical need for comprehensive testing of virtualization platforms under various guest configuration scenarios and underscores the importance of maintaining up-to-date hypervisor software to protect against known vulnerabilities that could compromise entire virtualized infrastructures.