CVE-2019-19579 in Xen
Summary
by MITRE
An issue was discovered in Xen through 4.12.x allowing attackers to gain host OS privileges via DMA in a situation where an untrusted domain has access to a physical device (and assignable-add is not used), because of an incomplete fix for CVE-2019-18424. XSA-302 relies on the use of libxl's "assignable-add" feature to prepare devices to be assigned to untrusted guests. Unfortunately, this is not considered a strictly required step for device assignment. The PCI passthrough documentation on the wiki describes alternate ways of preparing devices for assignment, and libvirt uses its own ways as well. Hosts where these "alternate" methods are used will still leave the system in a vulnerable state after the device comes back from a guest. An untrusted domain with access to a physical device can DMA into host memory, leading to privilege escalation. Only systems where guests are given direct access to physical devices capable of DMA (PCI pass-through) are vulnerable. Systems which do not use PCI pass-through are not vulnerable.
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Analysis
by VulDB Data Team • 03/07/2024
This vulnerability represents a critical privilege escalation flaw in the Xen hypervisor affecting versions through 4.12.x, where attackers can gain host operating system privileges through DMA attacks on physical devices. The issue stems from an incomplete fix for CVE-2019-18424, creating a persistent security gap that undermines the fundamental isolation guarantees of virtualization environments. The vulnerability specifically targets systems where untrusted domains have access to physical devices without proper assignment through libxl's "assignable-add" feature, which serves as a crucial security control mechanism. This flaw operates under the principle that physical devices with DMA capabilities can be exploited to bypass hypervisor security boundaries, fundamentally compromising the security model that separates guest domains from host resources.
The technical implementation of this vulnerability exploits the PCI passthrough mechanisms within Xen's device assignment architecture. When physical devices are assigned to untrusted guests without proper preparation using the "assignable-add" feature, the hypervisor fails to properly revoke DMA access permissions when devices return to the host system. This creates a persistent state where malicious guest domains can continue to perform DMA operations against host memory, effectively allowing them to read and write arbitrary memory locations. The flaw manifests because the hypervisor's device assignment logic does not properly handle the transition state between guest and host ownership, particularly for devices that support DMA operations. This vulnerability directly relates to CWE-284 Access Control and CWE-121 Stack-based Buffer Overflow categories, as it involves improper access control mechanisms and memory access violations that can lead to privilege escalation.
The operational impact of this vulnerability extends beyond simple privilege escalation to encompass complete system compromise of affected Xen installations. Attackers leveraging this flaw can execute arbitrary code with host privileges, potentially leading to complete system takeover, data exfiltration, and persistence mechanisms within the host environment. The vulnerability affects systems where guests are granted direct access to physical devices capable of DMA operations, making it particularly dangerous in cloud environments, virtualized data centers, and any deployment where device passthrough is utilized. The security implications are compounded by the fact that the vulnerability persists even after device return to the host, meaning that once a malicious guest has exploited the initial access, the system remains vulnerable until a complete system reboot occurs. This aligns with ATT&CK technique T1059 Command and Scripting Interpreter and T1068 Exploitation for Privilege Escalation, as attackers can leverage the elevated privileges to execute further malicious activities within the host environment.
Mitigation strategies for this vulnerability require immediate implementation of either the complete fix provided by Xen security patches or alternative protective measures. Organizations should ensure that all Xen installations are updated to versions containing the proper resolution for CVE-2019-19579, which addresses the incomplete fix for CVE-2019-18424. The recommended approach involves mandatory use of libxl's "assignable-add" feature for all device assignments, as this provides the necessary security controls to prevent unauthorized DMA access. Additionally, system administrators should implement strict device assignment policies that require explicit configuration steps for all physical device access, and consider disabling PCI passthrough for untrusted domains when possible. Network segmentation and monitoring solutions should be deployed to detect unusual DMA activity patterns, as this vulnerability can be used to establish covert communication channels between guest and host systems. The fundamental requirement for mitigation lies in ensuring proper device assignment procedures are followed, as the vulnerability exists specifically when these procedures are bypassed or not properly enforced, making compliance with security best practices essential for preventing exploitation.