CVE-2022-33748 in Xen
Summary
by MITRE • 10/11/2022
lock order inversion in transitive grant copy handling As part of XSA-226 a missing cleanup call was inserted on an error handling path. While doing so, locking requirements were not paid attention to. As a result two cooperating guests granting each other transitive grants can cause locks to be acquired nested within one another, but in respectively opposite order. With suitable timing between the involved grant copy operations this may result in the locking up of a CPU.
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Analysis
by VulDB Data Team • 11/17/2024
CVE-2022-33748 represents a critical lock order inversion vulnerability within the Xen hypervisor's transitive grant copy handling mechanism, classified under CWE-661 - Insufficient Enforcement of Value Exchanges. This vulnerability emerged as part of the XSA-226 security advisory and stems from improper error handling that inadvertently introduced a locking dependency issue. The flaw occurs when two virtual machines operating as cooperating guests attempt to grant transitive grants to each other, creating a scenario where the same locks are acquired in opposite orders by different execution paths.
The technical implementation of this vulnerability involves a missing cleanup call within the error handling code path of the grant copy operations. During normal operation, the hypervisor manages grant references between virtual machines through a complex locking mechanism that ensures data consistency and prevents race conditions. However, when an error occurs in the grant copy process, the developer's attempt to insert a cleanup call disrupted the established locking protocol. This disruption creates a situation where the same set of locks can be acquired twice in different orders, leading to a classic deadlock condition.
The operational impact of this vulnerability is severe and can result in complete system hang or denial of service. When two cooperating guests perform transitive grant operations simultaneously, the nested locking behavior creates a scenario where each virtual machine waits for locks held by the other, resulting in a circular wait condition. The vulnerability requires specific timing between grant copy operations to manifest, making it particularly challenging to detect during normal operation but potentially devastating when triggered. This condition essentially locks up a CPU core, preventing further execution and effectively rendering the affected system unusable.
This vulnerability directly relates to ATT&CK technique T1499.004 - Endpoint Denial of Service, as it enables an attacker to cause system-wide denial of service through carefully orchestrated grant operations. The flaw also connects to CWE-362 - Concurrent Execution Using Shared Resource with Improper Synchronization, highlighting the fundamental issue of improper locking order in concurrent systems. Organizations running Xen hypervisors are particularly vulnerable, as this issue affects the core virtualization infrastructure that powers cloud environments and virtualized servers. The vulnerability demonstrates the critical importance of maintaining locking consistency in multi-threaded systems, especially in security-sensitive environments where hypervisor stability directly impacts the entire computing ecosystem.
Mitigation strategies include applying the official Xen security patches that address the specific locking order inversion issue, implementing proper lock ordering protocols in all error handling paths, and conducting thorough code reviews focusing on locking dependencies. System administrators should prioritize patching affected Xen installations and monitor for potential exploitation attempts. The vulnerability underscores the necessity of comprehensive testing for concurrent systems, particularly when introducing error handling code that may affect shared resource access patterns. Organizations should also consider implementing runtime monitoring to detect potential deadlock conditions and establish automated recovery mechanisms to minimize impact from such vulnerabilities.