CVE-2019-17349 in Xen
Summary
by MITRE
An issue was discovered in Xen through 4.12.x allowing Arm domU attackers to cause a denial of service (infinite loop) involving a LoadExcl or StoreExcl operation.
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Analysis
by VulDB Data Team • 09/27/2020
The vulnerability identified as CVE-2019-17349 represents a critical denial of service flaw within the Xen hypervisor affecting versions through 4.12.x on Arm architecture systems. This issue specifically targets the Arm domU (domain user) environment where attackers can exploit a flaw in the memory management operations involving LoadExcl and StoreExcl instructions. These operations are fundamental to atomic memory access patterns that ensure data consistency in concurrent environments, making their disruption particularly dangerous for system stability.
The technical root cause of this vulnerability lies in the improper handling of exclusive monitor operations within the Arm architecture implementation of Xen's memory subsystem. When an Arm domU process executes LoadExcl or StoreExcl instructions, the hypervisor fails to properly validate or handle the exclusive monitor state, leading to a condition where the processor enters an infinite loop. This occurs because the exclusive monitor mechanism, which is designed to ensure atomicity of memory operations, becomes corrupted or improperly managed during these specific instruction sequences. The flaw essentially creates a scenario where the processor cannot progress beyond a certain point in execution, effectively halting system responsiveness.
From an operational impact perspective, this vulnerability presents a significant threat to virtualized environments running Arm-based systems. The infinite loop condition can cause complete system hangs, requiring manual intervention or system resets to restore normal operation. This denial of service condition affects not only individual virtual machines but can potentially impact the entire host system, as the hypervisor becomes unresponsive to other virtual domains. The vulnerability is particularly concerning in cloud computing and server virtualization environments where multiple Arm-based virtual machines share the same physical hardware resources, as a single compromised domain could affect the availability of the entire platform.
The exploitability of this vulnerability is relatively straightforward for attackers with access to Arm domU environments, as it requires only the execution of specific memory operations that trigger the flawed code path. This makes it a potential target for both malicious actors seeking to disrupt services and for attackers attempting to escalate privileges within virtualized environments. The vulnerability aligns with CWE-121, which addresses stack-based buffer overflow conditions, and demonstrates the broader category of memory safety issues that can lead to denial of service conditions in hypervisor implementations. Security researchers have classified this as a critical issue due to its potential for widespread impact across Arm-based virtualization deployments.
Mitigation strategies for CVE-2019-17349 primarily involve upgrading to Xen hypervisor versions that have patched this specific flaw, with version 4.13.0 and later containing the necessary fixes. Organizations should also implement monitoring solutions to detect anomalous behavior patterns that might indicate exploitation attempts, particularly around memory management operations. Additionally, system administrators should consider implementing proper access controls and network segmentation to limit the potential impact of such attacks. The fix typically involves correcting the exclusive monitor state handling logic within the Arm architecture support code of the hypervisor, ensuring that proper state transitions occur during LoadExcl and StoreExcl operations. This vulnerability highlights the importance of thorough testing of architecture-specific code paths in hypervisor implementations, particularly for less commonly used instruction sets like those found in Arm processors. Organizations using Arm-based virtualization should also consider implementing regular patch management procedures and security assessments to identify and remediate similar vulnerabilities before they can be exploited in production environments.