CVE-2026-98470 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
KVM: arm64: Don't WARN on an unknown VM ioctl in protected mode
kvm_pkvm_ioctl_allowed() WARNs when kvm_get_cap_for_kvm_ioctl() doesn't find the ioctl number in vm_ioctl_caps[], and kvm_arch_vm_ioctl() calls
it for every number the generic code doesn't handle, so ioctl(vm_fd, 0xdeadbeef) from userspace taints a pKVM host and panics it under panic_on_warn. The lookup is fed userspace input: return false, and userspace gets the -EINVAL kvm_arch_vm_ioctl() returns for that number on a host without pKVM.
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Analysis
by VulDB Data Team • 10/18/2026
The Linux kernel's KVM subsystem for ARM64 architectures contains a critical flaw in its handling of virtual machine control interface requests when running under protected mode (pKVM). This vulnerability arises from an improper validation mechanism within the kvm_arch_vm_ioctl function, which serves as the entry point for various ioctl commands issued by userspace applications to manage virtual machines. In standard configurations without pKVM enabled, unknown or unsupported ioctl numbers are gracefully rejected with a -EINVAL error code, allowing userspace programs to handle such errors appropriately without disrupting system stability. However, in protected mode environments where enhanced security isolation is enforced via the PSCI-based hypervisor, this behavior diverges significantly due to an overly aggressive warning mechanism embedded within kvm_pkvm_ioctl_allowed.
The core technical flaw lies in how unknown ioctl numbers are processed when they do not match any entry in the vm_ioctl_caps lookup table. Instead of simply returning a failure status as done in non-pKVM configurations, the code invokes WARN_ON or similar diagnostic macros that generate kernel warnings and potentially trigger panic_on_warn conditions if configured to do so. This means that a userspace process sending an arbitrary ioctl command with an invalid file descriptor number, such as 0xdeadbeef, can cause the host system to emit warning logs and subsequently crash under strict panic-on-warn settings. The issue is exacerbated by the fact that this lookup table check occurs after generic KVM code has already determined it cannot handle the specific ioctl, passing control down to architecture-specific handlers like kvm_arch_vm_ioctl without sufficient prior validation of whether the operation should even be attempted in protected mode contexts.
From a security perspective, this vulnerability represents an improper input validation flaw where untrusted userspace data directly influences kernel execution paths leading to denial-of-service conditions. The attack vector is straightforward: any local user with access to create or interact with KVM virtual machines can send crafted ioctl requests containing invalid command numbers. While the immediate impact appears limited to system instability rather than privilege escalation, the consequences are severe in production environments where pKVM hosts are deployed for sensitive workloads requiring high availability and integrity guarantees. A successful exploitation results in a complete host crash, disrupting all running virtual machines and potentially affecting other tenants sharing the same physical hardware resources. This aligns with CWE-20 Improper Input Validation as the root cause, since the system fails to adequately sanitize or validate incoming ioctl identifiers before processing them through security-critical code paths.
The operational impact extends beyond simple service disruption because pKVM environments are typically deployed in multi-tenant cloud infrastructure scenarios where isolation guarantees are paramount. A crash caused by a single malicious tenant can lead to cascading failures across multiple virtual machines hosted on the same physical server, violating availability commitments and potentially exposing data through side-channel effects during system recovery or reboot cycles. Furthermore, repeated exploitation attempts could be used for resource exhaustion attacks if combined with other techniques that consume CPU time while waiting for kernel panic handling procedures to complete. The vulnerability also highlights a broader architectural concern regarding how security features interact with error handling mechanisms; the assumption that all unknown ioctls should trigger warnings in protected mode ignores the reality that userspace applications may legitimately probe system capabilities or contain legacy code paths sending obsolete commands, making such aggressive failure modes counterproductive to operational stability.
Mitigation strategies must address both immediate remediation and long-term architectural improvements. The primary fix involves modifying kvm_pkvm_ioctl_allowed to return false for unknown ioctl numbers rather than invoking warning macros that can trigger panic conditions. This restores parity with non-pKVM behavior where unsupported commands are simply rejected without side effects. Administrators running pKVM hosts should ensure their kernel configurations do not enable panic_on_warn as a temporary countermeasure, though this is only a partial mitigation since it does not prevent the underlying validation flaw from existing in the codebase. Long-term solutions require implementing more robust ioctl dispatching logic that validates command numbers against allowed lists before any architecture-specific processing occurs, ensuring consistent behavior across all kernel configurations regardless of whether pKVM is enabled or disabled.
This vulnerability underscores the importance of rigorous testing for security features during development and integration phases. The interaction between KVM's generic ioctl handling layer and ARM64-specific extensions created an unexpected attack surface that was not adequately considered when designing protected mode enforcement mechanisms. Future developments in virtualization security should prioritize fail-safe defaults where unknown operations are rejected silently rather than triggering diagnostic routines that compromise system stability. Industry standards such as CWE-20 provide essential guidance for identifying these validation gaps, while ATT&CK techniques related to resource exhaustion and denial-of-service highlight the potential operational risks when kernel-level input validation is insufficiently hardened against adversarial inputs from unprivileged userspace processes seeking to disrupt critical infrastructure components.