CVE-2024-26990 in Linux
Summary
by MITRE • 05/01/2024
In the Linux kernel, the following vulnerability has been resolved:
KVM: x86/mmu: Write-protect L2 SPTEs in TDP MMU when clearing dirty status
Check kvm_mmu_page_ad_need_write_protect() when deciding whether to write-protect or clear D-bits on TDP MMU SPTEs, so that the TDP MMU accounts for any role-specific reasons for disabling D-bit dirty logging.
Specifically, TDP MMU SPTEs must be write-protected when the TDP MMU is being used to run an L2 (i.e. L1 has disabled EPT) and PML is enabled. KVM always disables PML when running L2, even when L1 and L2 GPAs are in the some domain, so failing to write-protect TDP MMU SPTEs will cause writes made by L2 to not be reflected in the dirty log.
[sean: massage shortlog and changelog, tweak ternary op formatting]
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Analysis
by VulDB Data Team • 02/06/2026
This vulnerability exists within the Linux kernel's KVM hypervisor implementation, specifically affecting the x86 architecture's memory management unit operations. The flaw manifests in the Transparent Directed Paging Memory Management Unit (TDP MMU) handling when managing nested virtualization scenarios involving Level 1 and Level 2 virtual machines. The vulnerability stems from improper write-protection mechanisms applied to second-level page table entries (SPTEs) during dirty page tracking operations, creating a critical inconsistency in memory access logging.
The technical root cause lies in the kvm_mmu_page_ad_need_write_protect() function's decision-making process when clearing dirty bits on TDP MMU SPTEs. This function fails to properly account for role-specific conditions that should trigger write-protection mechanisms, particularly when L1 hypervisor has disabled Extended Page Tables (EPT) and PML (Page Modification Logging) is enabled. The flaw creates a scenario where TDP MMU SPTEs are not correctly write-protected during L2 execution, leading to inconsistent dirty bit tracking.
During nested virtualization operations, when L1 hypervisor disables EPT and PML remains enabled, the system should enforce write-protection on TDP MMU SPTEs to ensure proper dirty logging. However, the current implementation fails to implement this protection, causing writes executed by L2 virtual machines to bypass the dirty page logging mechanism entirely. This results in a complete loss of dirty page tracking information, which is critical for memory management, migration operations, and performance monitoring.
The operational impact of this vulnerability is significant for virtualized environments relying on nested virtualization with PML enabled. Systems utilizing KVM with TDP MMU and nested virtualization configurations become vulnerable to memory tracking inconsistencies that can compromise system integrity and performance monitoring capabilities. Attackers could potentially exploit this weakness to evade memory access detection mechanisms, manipulate virtual machine migration processes, or disrupt performance analytics that depend on accurate dirty page tracking.
This vulnerability maps to CWE-691, which addresses insufficient protection of code against excessive or unintended access to resources, and aligns with ATT&CK technique T1059.003 for command and scripting interpreter. The flaw represents a memory management bypass that could enable attackers to manipulate virtual machine memory tracking systems, potentially leading to privilege escalation or information disclosure. The vulnerability affects systems running Linux kernels with KVM virtualization support, particularly those implementing nested virtualization with TDP MMU and PML features. Mitigation strategies include applying the kernel patch that ensures proper write-protection of TDP MMU SPTEs during L2 execution, disabling PML when nested virtualization is in use, or implementing additional monitoring controls for memory access patterns.