CVE-2026-98164 in Linux
Summary
by MITRE • 09/29/2026
In the Linux kernel, the following vulnerability has been resolved:
KVM: x86/mmu: Check write tracking in all address spaces
kvm_gfn_is_write_tracked() checks only the supplied memslot, but page tracking is per-address-space and shadow pages are shared across all address spaces. With SMM, a GFN can therefore be write-tracked in one address space and appear untracked through the other.
Check the supplied slot first, then the slot for the other address space. This ensures all callers honor write tracking regardless of the active address space. In particular, it prevents mmu_try_to_unsync_pages() from marking an upper-level shadow page unsync and eventually triggering the BUG in pte_list_remove().
[invert direction of the conditional. - Paolo]
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Analysis
by VulDB Data Team • 09/29/2026
The Linux kernel's Kernel-based Virtual Machine (KVM) subsystem contains a critical logic flaw within its memory management unit emulation code, specifically concerning how write tracking is validated across different address spaces. The vulnerability resides in the kvm_gfn_is_write_tracked function, which is responsible for determining whether a given guest physical frame number has been marked as writable by external observers or internal mechanisms that require synchronization of shadow pages. Under normal operation with standard virtual machine configurations, this check might appear sufficient because it typically examines only the memory slot associated with the currently active address space context. However, this assumption breaks down in environments utilizing System Management Mode (SMM), a privileged execution mode used by firmware for tasks such as power management and hardware initialization that operates independently of the operating system's normal privilege levels.
The core technical flaw is rooted in the architectural design where page tracking metadata is maintained per-address-space rather than globally across all address spaces, while shadow pages themselves are shared resources accessible from multiple contexts. When a guest physical frame number is write-tracked within one address space but accessed or modified through another, such as during an SMM transition, the kvm_gfn_is_write_tracked function fails to detect this tracking status because it only inspects the slot relevant to the current context. This discrepancy creates a state inconsistency where the kernel believes a page is safe to modify or unsynchronize when it is actually subject to write tracking constraints in another address space domain.
This logical error leads to severe operational instability within the virtualization layer. Specifically, the flaw allows the mmu_try_to_unsync_pages function to incorrectly mark an upper-level shadow page as unsynchronized. This incorrect state propagation eventually triggers a fatal kernel bug during the pte_list_remove operation, which attempts to manipulate page table entries that are still under active tracking protection. The result is typically a kernel panic or system crash, effectively causing a denial of service for both the host machine and any virtual machines running on it. In security terms, this represents an availability impact due to the instability introduced into the hypervisor's memory management subsystem.
From a classification perspective, this vulnerability aligns with CWE-841, Improper Enforcement of Behavioral Validation, as the code fails to properly validate the state of shared resources across different execution contexts. It also relates to CWE-362, Concurrent Execution using Shared Resource with Improper Synchronization, given that the issue arises from race-like conditions between address space transitions and memory tracking states. In terms of MITRE ATT&CK mapping, this could be leveraged in an Initial Access or Impact scenario where an attacker might attempt to crash a host system through crafted guest operations if they can trigger specific SMM interactions while managing write-tracking states, although the primary impact is stability rather than direct privilege escalation.
To mitigate this vulnerability, it is imperative that systems running affected versions of the Linux kernel are updated with the provided patch. The fix involves modifying the kvm_gfn_is_write_tracked function to perform a comprehensive check across all relevant address spaces before concluding whether write tracking is active. By checking the supplied slot first and then verifying the corresponding slot in the other address space, the code ensures that callers correctly honor write tracking constraints regardless of which address space is currently active. Administrators should apply this update promptly to restore the integrity of the KVM memory management subsystem and prevent potential system crashes triggered by SMM-related page table manipulations.