CVE-2018-7540 in Xen
Summary
by MITRE
An issue was discovered in Xen through 4.10.x allowing x86 PV guest OS users to cause a denial of service (host OS CPU hang) via non-preemptable L3/L4 pagetable freeing.
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Analysis
by VulDB Data Team • 02/16/2023
The vulnerability identified as CVE-2018-7540 represents a critical flaw in the Xen hypervisor affecting versions through 4.10.x, specifically impacting x86 paravirtualized guest operating systems. This vulnerability stems from improper handling of pagetable memory management operations within the hypervisor's memory subsystem, creating a condition where guest users can manipulate the host system's CPU execution behavior. The flaw manifests when guest operating systems execute certain memory management operations that trigger non-preemptable pagetable freeing at the L3/L4 levels of the x86 memory management hierarchy, leading to a complete system hang on the host machine.
The technical root cause of this vulnerability lies in the hypervisor's pagetable management logic where it fails to properly account for the preemptability of memory freeing operations during page table traversal. When a paravirtualized guest executes operations that result in L3/L4 pagetable structures being freed, the hypervisor's memory management code path does not adequately handle the potential for these operations to become non-preemptable. This creates a scenario where the host CPU can become indefinitely blocked while attempting to process these memory management operations, as the system cannot properly yield control back to other processes or the scheduler during the critical memory freeing sequence. The vulnerability specifically affects x86 architecture systems running paravirtualized guest operating systems where the guest has access to memory management operations that can trigger this condition.
The operational impact of CVE-2018-7540 extends beyond simple denial of service to potentially compromising the entire hosting environment's availability and stability. When exploited, this vulnerability allows malicious or compromised guest operating systems to induce a complete CPU hang on the host system, effectively rendering the hypervisor unresponsive and preventing it from servicing other virtual machines running on the same physical hardware. This creates a cascading effect where multiple virtual machines may become unavailable simultaneously, leading to significant service disruption for cloud providers and infrastructure operators. The vulnerability is particularly concerning in multi-tenant environments where guest isolation is paramount, as it represents a potential escape from guest confinement that could allow an attacker to impact the broader hosting infrastructure.
Mitigation strategies for CVE-2018-7540 primarily focus on upgrading to patched versions of the Xen hypervisor where the memory management operations have been properly corrected to handle pagetable freeing in a preemptable manner. System administrators should prioritize immediate deployment of Xen versions 4.11.0 and later, which contain the necessary fixes for this vulnerability. Additionally, implementing monitoring solutions to detect anomalous memory management patterns from guest operating systems can provide early warning of potential exploitation attempts. Network segmentation and guest isolation measures can help limit the impact if exploitation occurs, though these are not complete defenses against this specific vulnerability. The fix addresses the underlying CWE-367 issue related to time-of-check to time-of-use vulnerabilities in memory management operations, and aligns with ATT&CK technique T1499.001 for network denial of service, though it specifically targets hypervisor-level execution rather than network-based disruption. Organizations should also consider implementing hypervisor-level resource controls and monitoring to detect unusual CPU utilization patterns that might indicate exploitation attempts.