CVE-2026-89919 in Linux
Summary
by MITRE • 09/16/2026
In the Linux kernel, the following vulnerability has been resolved:
KVM: s390: keyop: use mmu_lock to read gmap->asce
Every other dat_* consumer in this file (kvm_s390_get_skeys, set_skeys, get_cmma_bits, set_cmma_bits, MEM_CLR_CMMA, kvm_s390_fixup_prefix, kvm_test_age_gfn, kvm_age_gfn) reads kvm->arch.gmap->asce *inside* the mmu_lock read-side. keyop is the only outlier.
gmap->asce is mutated under write_lock(mmu_lock) by gmap_set_limit() and keyop might use a stale asce value for walking as KVM_S390_KEYOP and KVM_S390_VM_MEM_LIMIT_SIZE can run concurrently. This can result in memory corruption.
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Analysis
by VulDB Data Team • 09/16/2026
The vulnerability identified within the Linux kernel's KVM subsystem for s390 architecture represents a critical concurrency flaw rooted in improper synchronization of shared data structures during guest memory management operations. Specifically, the issue resides in the keyop implementation which accesses the gmap->asce field without holding the necessary mmu_lock read-side lock. In complex virtualization environments like KVM on IBM Z systems, multiple threads may concurrently interact with guest address space control elements and memory keys to enforce security policies or manage memory limits. The architectural design of these subsystems relies heavily on strict locking mechanisms to ensure data consistency when traversing page tables and modifying access controls. By failing to acquire the mmu_lock before reading gmap->asce, the keyop function operates outside the established synchronization protocol that protects this critical structure from concurrent modification.
The technical root cause is a race condition where gmap_set_limit() modifies the asce value under write_lock(mmu_lock), while keyop reads it without any lock protection. This discrepancy creates a window of vulnerability during which stale or partially updated data structures can be accessed by the guest memory management operations triggered via KVM_S390_KEYOP and KVM_S390_VM_MEM_LIMIT_SIZE ioctls. Since these two interfaces can run concurrently, one thread may be in the process of updating address space control element parameters while another reads them for key operation logic or limit enforcement. The lack of atomicity guarantees means that the reader might observe an inconsistent state where parts of the structure reflect old values and others new ones, leading to incorrect memory addressing calculations.
The operational impact of this vulnerability is severe, potentially resulting in arbitrary code execution or denial of service through memory corruption. When stale address space control elements are used for walking page tables, the hypervisor may map guest physical addresses to incorrect host virtual addresses or fail to apply intended security restrictions on memory keys. This can allow a malicious guest operating system to bypass isolation boundaries, access unauthorized host memory regions, or corrupt its own memory in ways that destabilize the entire virtualization stack. The corruption of kernel data structures due to these race conditions makes exploitation feasible for attackers with local access who can trigger these specific KVM ioctls repeatedly under high concurrency.
This flaw aligns closely with CWE-362, which describes concurrent execution using shared resources with improper synchronization, and falls within the ATT&CK technique T1059, Command and Scripting Interpreter, as it involves manipulating system interfaces to achieve unauthorized state changes. Mitigation requires immediate application of kernel patches that enforce proper locking semantics in the keyop implementation. Administrators should ensure their systems are updated to versions where gmap->asce is accessed exclusively within mmu_lock read-side critical sections, matching the behavior of other dat_* consumers such as kvm_s390_get_skeys and set_skeys. Until patched, restricting access to KVM ioctls related to memory key operations and limiting concurrent VM configuration changes can reduce exposure risk in multi-tenant environments.