CVE-2021-28705 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-2021-28705 relates to insufficient error handling within the Xen hypervisor's implementation of populate-on-demand (PoD) functionality for x86 hardware virtual machine (HVM) and PVH guests. This issue specifically affects the management of page-to-machine (P2M) mappings during memory operations, where guests can control certain aspects of individual pages through hypercalls. The PoD mode allows guests to defer memory allocation until actual usage occurs, providing efficient memory management but introducing complex state management challenges. When guests request memory operations using page orders that result in power-of-2 page counts, the hypervisor may split these requests into smaller chunks for processing. This splitting mechanism creates potential points of failure where partial success conditions are not properly accounted for during error handling.
The technical flaw manifests in two distinct code paths that handle different aspects of P2M updates. The first path involves page removal operations where partial failures in the PoD mechanism were not properly detected or handled, allowing inconsistent state to persist in the hypervisor's memory management tables. The second path concerns the insertion of new pages into the guest's memory space, where similar error handling deficiencies existed. Both scenarios create opportunities for memory management inconsistencies that could potentially be exploited by malicious guests to manipulate the hypervisor's memory mappings. The vulnerability stems from inadequate state tracking during partial success conditions, where the hypervisor fails to properly account for operations that succeed partially while others fail within the same request sequence. This issue falls under the CWE category of insufficient error handling and can be classified as a memory corruption vulnerability that affects hypervisor integrity.
The operational impact of CVE-2021-28705 extends beyond simple memory management inconsistencies to potentially compromise the security isolation between virtual machines and the underlying hypervisor. Attackers could exploit this vulnerability to manipulate P2M mappings in ways that might lead to information disclosure, privilege escalation, or denial of service conditions. The vulnerability is particularly concerning in environments where multiple tenants share the same hypervisor infrastructure, as a malicious guest could potentially interfere with other guests' memory spaces or gain unauthorized access to hypervisor resources. The partial success handling issue creates a state where the hypervisor maintains inconsistent memory mappings, which could be leveraged to bypass security controls or create unexpected behavior in the virtualized environment. This vulnerability aligns with ATT&CK techniques related to privilege escalation and defense evasion through hypervisor manipulation, as it enables attackers to corrupt the fundamental memory management structures that maintain virtual machine isolation.
Mitigation strategies for CVE-2021-28705 focus on implementing comprehensive error handling for partial success conditions in the PoD implementation. The recommended approach involves ensuring that all P2M update operations properly account for partial failures and maintain consistent state regardless of whether individual operations succeed or fail within a batch request. This includes implementing robust rollback mechanisms when partial failures occur, ensuring that memory mappings remain consistent and that no partial state is left in an inconsistent condition. System administrators should apply the combined patch that addresses both CVE-2021-28705 and CVE-2021-28709 simultaneously, as they represent related aspects of the same underlying issue. Additional monitoring should be implemented to detect unusual memory allocation patterns or P2M mapping changes that might indicate exploitation attempts. Organizations should also consider implementing hypervisor hardening measures and regular security assessments to identify potential exploitation vectors that could leverage this vulnerability for more severe attacks.