CVE-2023-46839 in Xen
Summary
by MITRE • 03/20/2024
PCI devices can make use of a functionality called phantom functions, that when enabled allows the device to generate requests using the IDs of functions that are otherwise unpopulated. This allows a device to extend the number of outstanding requests.
Such phantom functions need an IOMMU context setup, but failure to setup the context is not fatal when the device is assigned. Not failing device assignment when such failure happens can lead to the primary device being assigned to a guest, while some of the phantom functions are assigned to a different domain.
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Analysis
by VulDB Data Team • 01/13/2026
The vulnerability described in CVE-2023-46839 relates to improper handling of phantom functions within PCI device configurations, creating potential security risks in virtualized environments. This issue specifically impacts systems utilizing IOMMU (Input-Output Memory Management Unit) for device assignment in virtual machines, where phantom functions represent a legitimate PCI extension mechanism that allows devices to generate requests using IDs of functions that are otherwise unpopulated. The technical flaw occurs when the IOMMU context setup fails for phantom functions, yet the system continues to proceed with device assignment without proper validation, leading to a misconfiguration scenario that can compromise system security.
When phantom functions are enabled on PCI devices, they allow the device to extend the number of outstanding requests beyond normal limits, which can be useful for performance optimization. However, the vulnerability emerges from the failure to properly enforce IOMMU context setup requirements for these phantom functions. According to CWE-1106, this represents a weakness in the security model where improper handling of device contexts can lead to privilege escalation or information disclosure. The vulnerability becomes particularly dangerous in virtualized environments where devices are assigned to guest operating systems, as the system may incorrectly assign the primary device to one guest while assigning phantom functions to different domains, creating a potential attack surface.
The operational impact of this vulnerability is significant within cloud computing and virtualization platforms where PCI device assignment is common. When a device assignment occurs without proper IOMMU context validation, it creates a scenario where the primary device and its phantom functions may be distributed across different security domains, potentially allowing malicious actors to exploit the misconfiguration. This misalignment can lead to data leakage between virtual machines, privilege escalation attacks, or denial of service conditions. The issue aligns with ATT&CK technique T1611 which involves privilege escalation through improper access control, and T1068 which covers local privilege escalation via improper system configuration.
The root cause of this vulnerability stems from the failure to implement proper validation checks during device assignment processes, particularly when dealing with phantom functions that require specific IOMMU context setup. This represents a classic case of incomplete input validation and insufficient error handling in security-critical system components. The vulnerability demonstrates how seemingly legitimate device functionality can be exploited when proper security controls are not enforced during system configuration. Organizations implementing virtualization technologies must ensure that all PCI device assignments undergo comprehensive validation to prevent the scenario where primary devices and phantom functions are assigned to different security domains, which could compromise the integrity and confidentiality of virtualized environments.