CVE-2020-15566 in Xen
Summary
by MITRE
An issue was discovered in Xen through 4.13.x, allowing guest OS users to cause a host OS crash because of incorrect error handling in event-channel port allocation. The allocation of an event-channel port may fail for multiple reasons: (1) port is already in use, (2) the memory allocation failed, or (3) the port we try to allocate is higher than what is supported by the ABI (e.g., 2L or FIFO) used by the guest or the limit set by an administrator (max_event_channels in xl cfg). Due to the missing error checks, only (1) will be considered an error. All the other cases will provide a valid port and will result in a crash when trying to access the event channel. When the administrator configured a guest to allow more than 1023 event channels, that guest may be able to crash the host. When Xen is out-of-memory, allocation of new event channels will result in crashing the host rather than reporting an error. Xen versions 4.10 and later are affected. All architectures are affected. The default configuration, when guests are created with xl/libxl, is not vulnerable, because of the default event-channel limit.
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Analysis
by VulDB Data Team • 07/08/2020
The vulnerability identified as CVE-2020-15566 represents a critical flaw in the Xen hypervisor affecting versions through 4.13.x, where guest operating systems can exploit incorrect error handling during event-channel port allocation to cause host system crashes. This issue stems from inadequate validation mechanisms that fail to properly handle multiple failure scenarios during the allocation process, creating a pathway for malicious or unintended behavior that can compromise system stability and availability. The flaw manifests when event-channel port allocation encounters various failure conditions including port conflicts, memory allocation failures, or exceeding supported ABI limits, yet only the first condition is properly recognized as an error while other scenarios are treated as successful allocations leading to subsequent host crashes.
The technical implementation of this vulnerability demonstrates a clear breakdown in defensive programming practices within the Xen hypervisor's event-channel management subsystem, where error handling logic is incomplete and fails to account for all possible failure modes during resource allocation. When the hypervisor attempts to allocate event-channel ports, it should validate that all allocation conditions are met before returning a valid port identifier, but instead, it proceeds with invalid allocations that will later cause memory access violations and system crashes. This pattern of behavior aligns with common software security weaknesses documented in CWE-252, which describes "Unchecked Return Value" vulnerabilities where programs fail to check the results of operations that may fail, leading to unexpected program states and potential system instability.
The operational impact of this vulnerability extends beyond simple denial-of-service conditions, as it creates a potential attack vector where unprivileged guest users can execute arbitrary code to crash the host system, potentially leading to complete system compromise or service disruption in virtualized environments. Attackers can exploit this vulnerability by configuring guest operating systems to request more than the default 1023 event channel limit, which when processed through the flawed allocation logic results in host crashes. This vulnerability affects all supported architectures within the Xen hypervisor, making it particularly dangerous in diverse computing environments where multiple processor architectures are in use. The default configuration of Xen installations using xl/libxl creates a protective barrier against exploitation since these default settings limit event channel allocation to safe parameters, but custom configurations that exceed these defaults create the vulnerability window.
Security practitioners should note that this vulnerability operates at the hypervisor level, making it particularly dangerous as it can be exploited by any guest user without requiring elevated privileges or specific authentication credentials. The flaw represents a classic case of insufficient input validation and error handling that can be mapped to ATT&CK technique T1499.004, which covers "Endpoint Denial of Service" through hypervisor manipulation. Organizations running Xen hypervisors must implement immediate mitigations including restricting guest configurations to prevent excessive event channel allocation, monitoring for unusual allocation patterns, and ensuring that all systems are updated to versions that address this specific error handling flaw. The vulnerability also highlights the importance of comprehensive testing of error handling paths in hypervisor components, as proper validation of all allocation outcomes is essential for maintaining system stability and preventing unauthorized privilege escalation through resource manipulation attacks.
This vulnerability serves as a reminder of the critical importance of robust error handling in hypervisor implementations, where the failure to properly validate allocation results can create cascading failures that compromise entire virtualized infrastructures. The security implications extend beyond immediate system crashes to potential data loss and service disruption in environments where virtualization is critical for business operations, emphasizing the need for regular security assessments of hypervisor components and adherence to security best practices in virtualization management.