CVE-2021-28702 in Xen
Summary
by MITRE • 10/06/2021
PCI devices with RMRRs not deassigned correctly Certain PCI devices in a system might be assigned Reserved Memory Regions (specified via Reserved Memory Region Reporting, "RMRR"). These are typically used for platform tasks such as legacy USB emulation. If such a device is passed through to a guest, then on guest shutdown the device is not properly deassigned. The IOMMU configuration for these devices which are not properly deassigned ends up pointing to a freed data structure, including the IO Pagetables. Subsequent DMA or interrupts from the device will have unpredictable behaviour, ranging from IOMMU faults to memory corruption.
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Analysis
by VulDB Data Team • 10/10/2021
The vulnerability described in CVE-2021-28702 represents a critical flaw in how virtualization platforms handle PCI device assignment and deallocation, particularly concerning Reserved Memory Regions. This issue specifically affects systems where PCI devices are assigned through Reserved Memory Region Reporting mechanisms, which are typically used for platform-level functions such as legacy USB emulation and other hardware-specific tasks. The flaw occurs when these devices are passed through to virtual machines, creating a scenario where proper resource management fails during the guest shutdown process.
The technical root cause of this vulnerability stems from improper handling of IOMMU (Input-Output Memory Management Unit) configurations when PCI devices are removed from virtual machines. When a device with an associated RMRR is passed through to a guest system and subsequently shut down, the virtualization platform fails to correctly deassign the device from the IOMMU domain. This leaves behind stale IOMMU configuration entries that reference freed memory structures, including IO Pagetables that are essential for DMA operations. The underlying mechanism involves the failure to properly invalidate or clean up the IOMMU mappings that were established for the device during its assignment.
This vulnerability has significant operational impact as it creates a persistent security and stability risk within virtualized environments. The unpredictable behavior that results from this flaw can manifest as IOMMU faults that prevent legitimate DMA operations from completing successfully, or more dangerously, lead to memory corruption that could be exploited by malicious actors. The potential for memory corruption is particularly concerning as it could allow privilege escalation attacks or arbitrary code execution within the virtualized environment. According to CWE classification, this vulnerability maps to CWE-121 for buffer overflow conditions and CWE-284 for improper access control, while ATT&CK framework references this as T1068 for exploit for privilege escalation and T1499 for endpoint denial of service.
The mitigation strategies for this vulnerability require careful attention to virtualization platform updates and proper configuration management. System administrators should ensure that all virtualization software is updated to versions that address this specific IOMMU deallocation issue, particularly in environments where PCI passthrough is actively used. The recommended approach involves implementing proper device deassignment procedures during guest shutdown sequences, ensuring that IOMMU mappings are correctly invalidated before memory structures are freed. Additionally, monitoring systems should be configured to detect anomalous DMA behavior or IOMMU faults that might indicate this vulnerability is being exploited, as these could serve as early warning indicators of potential security breaches.