CVE-2021-28709 in Xen
Summary
by MITRE • 11/24/2021
issues with partially successful P2M updates on x86 T[his CNA information record relates to multiple CVEs; the text explains which aspects/vulnerabilities correspond to which CVE.] x86 HVM and PVH guests may be started in populate-on-demand (PoD) mode, to provide a way for them to later easily have more memory assigned. Guests are permitted to control certain P2M aspects of individual pages via hypercalls. These hypercalls may act on ranges of pages specified via page orders (resulting in a power-of-2 number of pages). In some cases the hypervisor carries out the requests by splitting them into smaller chunks. Error handling in certain PoD cases has been insufficient in that in particular partial success of some operations was not properly accounted for. There are two code paths affected - page removal (CVE-2021-28705) and insertion of new pages (CVE-2021-28709). (We provide one patch which combines the fix to both issues.)
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Analysis
by VulDB Data Team • 11/27/2021
The vulnerability described in CVE-2021-28709 relates to a critical flaw in hypervisor memory management within x86 HVM and PVH virtual machine environments. This issue specifically impacts the populate-on-demand (PoD) functionality that allows virtual machines to dynamically request additional memory resources. The vulnerability exists in the handling of page-to-machine (P2M) updates, where virtual machines can control certain aspects of individual memory pages through hypercalls. These hypercalls operate on page ranges specified by page orders, which results in power-of-2 quantities of pages being processed. When hypervisor operations need to handle large page requests, they split these into smaller manageable chunks for processing, creating a complex interaction between the guest operating system and the underlying hypervisor infrastructure.
The technical flaw manifests in insufficient error handling mechanisms during partial success scenarios within PoD operations. When hypervisor operations process page requests, they may encounter situations where only partial success occurs, meaning some pages are successfully processed while others fail. The vulnerability specifically affects the handling of these partial success cases, where the hypervisor fails to properly account for operations that succeed partially rather than completely. This improper accounting creates a potential attack surface where malicious or compromised virtual machines could exploit the inconsistent state management to gain unauthorized access to memory resources or potentially escalate privileges. The issue affects two distinct code paths within the hypervisor implementation, with CVE-2021-28705 addressing page removal operations and CVE-2021-28709 specifically targeting the insertion of new pages, though both vulnerabilities are resolved through a combined patch solution.
The operational impact of this vulnerability extends beyond simple memory management issues, creating potential security risks that could allow virtual machine escape or privilege escalation attacks. When partial success scenarios are not properly handled, the hypervisor's memory management state can become inconsistent, potentially enabling attackers to manipulate memory mappings in ways that violate the isolation boundaries between virtual machines. This vulnerability particularly affects systems running x86 HVM and PVH guests that utilize the populate-on-demand feature, making it relevant to cloud computing environments, virtualization platforms, and any infrastructure relying on hypervisor memory management. The flaw aligns with CWE-248 Unhandled Exception and CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization, both of which relate to improper handling of partial success conditions in concurrent systems. From an ATT&CK perspective, this vulnerability could be leveraged for privilege escalation through the T1068 Exploitation for Privilege Escalation technique, potentially enabling attackers to gain elevated privileges within the virtualized environment.
Mitigation strategies for this vulnerability require immediate patching of affected hypervisor implementations, as the flaw exists at the core memory management layer of virtualization infrastructure. System administrators should prioritize updating their hypervisor software to versions containing the combined fix for both CVE-2021-28705 and CVE-2021-28709, ensuring that all virtual machines running x86 HVM and PVH configurations receive the necessary security updates. Additionally, organizations should implement monitoring for unusual memory allocation patterns or hypercall behavior that might indicate exploitation attempts. The vulnerability's nature suggests that regular security assessments of virtualization environments are essential, particularly focusing on hypervisor memory management components. Organizations should also consider implementing network segmentation and access controls to limit the potential impact of successful exploitation, as the vulnerability could potentially allow attackers to move laterally within virtualized environments. Regular vulnerability scanning and penetration testing of virtualization infrastructure should be conducted to identify and remediate similar memory management flaws that might exist in other components of the virtualization stack.