CVE-2026-98715 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
KVM: SVM: Preserve TLB control (i.e. pending TLB flush) on failed VMRUN
Don't reset the VMCB's TLB control back to "do nothing" on a failed VMRUN, as empirical testing shows that the CPU performs the requested TLB flush if and only if VMRUN is successful, i.e. clearing TLB control on a failed VMRUN effectively drops a TLB flush.
Explicitly track the need to flush all ASIDs on a per-CPU basis, as the ASID reuse condition is tied to the pCPU, not to the vCPU. As a bonus, this also obviates the need to avoid clobbering FLUSH_ALL_ASID with TLB_CONTROL_FLUSH_ASID, e.g. in svm_flush_tlb_asid().
Deliberately don't bother saving/restoring the "old" tlb_ctl on failure, in quotes because it's not exactly the old tlb_ctl, it's the tlb_ctl from after pre_svm_run(), but before updating tlb_ctl for flush_all_asids. If VMRUN fails and TLB_CONTROL_FLUSH_ALL_ASID is forced, then the next successful run of the VMCB *may* unnecessarily flush all ASIDs, which strictly speaking could result in noisy neighbor issues. However, the fact that new_asid() is already guest-triggerable, because of KVM's flawed behavior of clearing the ASID on emulated INIT, means that a guest can already trigger a flush of all ASIDs at roughly the same rate. And once KVM stops clobbering the ASID on emulated INIT, *or* assigns a static ASID to each vCPU, this flaw goes away.
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Analysis
by VulDB Data Team • 10/19/2026
The vulnerability addressed in this Linux kernel update pertains to the Kernel-based Virtual Machine (KVM) subsystem for AMD Secure Virtual Machine (SVM) architecture, specifically involving the handling of Translation Lookaside Buffer (TLB) control states during virtual machine execution failures. The core technical flaw lies in how KVM manages pending TLB flushes when a VMRUN instruction fails. Under normal operation, the CPU performs requested TLB flushes only if the VMRUN instruction completes successfully. However, prior to this fix, KVM would reset the Virtual Machine Control Block (VMCB) TLB control field back to a "do nothing" state even upon failure of the VMRUN command. This behavior effectively discards pending TLB flush requests because the hardware does not execute them on failed runs, leading to stale entries remaining in the guest's TLB. This discrepancy between software expectations and hardware behavior creates a consistency issue where memory mappings that should have been invalidated remain accessible, potentially allowing access to freed or remapped pages that were intended to be isolated from the virtual machine context.
The operational impact of this flaw is significant for system stability and security isolation within virtualized environments. By dropping TLB flushes on failed VMRUN attempts, KVM fails to maintain proper memory protection boundaries between different guest operating systems running on the same physical host. This can lead to data leakage or corruption if a guest accesses memory regions that have been reallocated to another process or guest after a mapping change was supposed to take effect but was not properly flushed due to the VMRUN failure. Furthermore, the issue is compounded by how Address Space Identifiers (ASIDs) are managed. The ASID reuse condition is tied to the physical CPU rather than the virtual CPU, meaning that improper handling of TLB control can affect multiple guests sharing the same core. This mismanagement undermines the fundamental security principle of isolation in multi-tenant cloud environments where distinct tenants rely on strict memory separation enforced by hardware-assisted virtualization features.
To resolve this issue, the fix implements a more robust tracking mechanism for pending flushes that persists across VMRUN failures. Instead of resetting TLB control immediately upon failure, KVM now explicitly tracks the need to flush all ASIDs on a per-physical CPU basis. This ensures that if a VMRUN fails but a flush was requested, the state is preserved and executed during the next successful execution context or handled appropriately by the scheduler logic. The patch also removes unnecessary complexity regarding clobbering FLUSH_ALL_ASID with TLB_CONTROL_FLUSH_ASID in functions like svm_flush_tlb_asid(), simplifying the code path while maintaining correctness. Although there is a minor side effect where forcing TLFLUSH_ALL_ASID on failure might cause an extra flush of all ASIDs during subsequent successful runs, this is deemed acceptable because guests can already trigger similar global flushes through other mechanisms such as emulated INIT events. This trade-off prioritizes security and consistency over marginal performance optimization in edge cases involving frequent VMRUN failures.
From a cybersecurity perspective, this vulnerability aligns with CWE-20: Improper Input Validation and CWE-754: Improper Check for Unusual or Exceptional Conditions, as the software failed to correctly handle an exceptional condition (VMRUN failure) leading to incorrect state management. In terms of MITRE ATT&CK framework, this relates to techniques involving memory corruption or unauthorized access via virtualization escape vectors if exploited in conjunction with other flaws. The mitigation involves applying kernel updates that include these SVM TLB control fixes. Administrators should ensure their hypervisors are patched to the latest stable release containing KVM improvements for AMD processors. Additionally, monitoring for unusual patterns of VMRUN failures could serve as an indicator of potential exploitation attempts targeting this specific state management flaw in virtualized infrastructure environments relying on hardware-assisted memory isolation.