CVE-2026-89727 in Linux
Summary
by MITRE • 09/11/2026
In the Linux kernel, the following vulnerability has been resolved:
KVM: arm64: GICv2: Don't WARN on out-of-range GICV_DIR INTID
vgic_v2_deactivate() passes the INTID a guest wrote to GICV_DIR straight to vgic_get_vcpu_irq(), and treats a failed lookup as a "can't happen" condition with WARN_ON_ONCE().
The guest can make it happen at will, though: for any INTID outside of the implemented SGI, PPI and SPI ranges the lookup returns NULL, since GICv2 has no LPIs. A guest running with EOImode==1 writing such an INTID to GICV_DIR triggers the WARN, and panics hosts running with panic_on_warn.
Drop the WARN and ignore failed lookups.
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Analysis
by VulDB Data Team • 09/11/2026
The vulnerability identified in the Linux kernel's KVM arm64 subsystem involves a logic error within the Generic Interrupt Controller version 2 (GICv2) emulation layer, specifically affecting how guest operating systems interact with virtualized interrupt lines. The core issue resides in the vgic_v2_deactivate function, which is responsible for handling deactivation requests from guests writing to the GICV_DIR register. When a guest writes an Interrupt Identifier (INTID) to this register, the hypervisor passes that value directly to the internal helper function vgic_get_vcpu_irq() to locate the corresponding virtual interrupt structure. The original implementation assumed that any failure in this lookup represented an impossible state or a programming error within the kernel itself. Consequently, it triggered a WARN_ON_ONCE macro upon receiving a NULL pointer from the lookup operation. This assumption fails because GICv2 architecture does not support Locality-specific Peripheral Interrupts (LPIs), and valid INTIDs are strictly confined to specific ranges for Software Generated Interrupts (SGIs), Private Peripheral Interrupts (PPIs), and Shared Peripheral Interrupts (SPIs).
From a technical perspective, the flaw allows any guest operating system with appropriate privileges or configuration settings to trigger this warning condition deliberately. If a guest is configured with End Of Interrupt mode set to one (EOImode==1) and writes an INTID that falls outside the implemented SGI, PPI, and SPI ranges into GICV_DIR, the lookup function correctly returns NULL because no such interrupt exists in the emulated hardware model. However, instead of gracefully handling this out-of-bounds access as a guest error or ignoring it, the host kernel interprets it as an internal inconsistency. This leads to the emission of a warning message on the host console and potentially causes a system panic if the host is configured with panic_on_warn enabled. This behavior effectively turns a benign configuration mismatch into a Denial of Service (DoS) vector against the entire physical server hosting multiple virtual machines, compromising availability for all tenants sharing that hardware resource.
The operational impact of this vulnerability is significant in multi-tenant cloud environments or any deployment where host stability is critical. An attacker controlling a guest VM can execute a simple write operation to trigger the warning and potentially crash the hypervisor process managing the KVM instance. This results in immediate downtime for all virtual machines running on that physical host, leading to service disruption across multiple customers or applications. The vulnerability highlights a lack of proper input validation regarding interrupt identifiers before they are processed by internal kernel structures. It also underscores the risks associated with treating guest-induced states as fatal errors rather than handling them through standard error recovery paths or logging mechanisms without escalating severity.
To mitigate this risk, the resolution involves removing the WARN_ON_ONCE call and modifying the logic to ignore failed lookups resulting from out-of-range INTIDs. This approach aligns with best practices for virtualization security by ensuring that guest misconfigurations do not propagate as host-level failures. By treating invalid interrupt identifiers as non-fatal events, the system maintains stability even when guests attempt unsupported operations. From a standards perspective, this vulnerability maps to CWE-20 Improper Input Validation and CWE-754 Improper Check for Unusual or Exceptional Conditions within the Common Weakness Enumeration framework. In terms of attack tactics, it relates to ATT&CK technique T1499 Endpoint Denial of Service, as the primary effect is rendering the host system unavailable through resource exhaustion via panic triggers rather than traditional CPU or memory consumption attacks. Administrators should ensure their kernels are updated with this patch and consider reviewing panic_on_warn settings in production environments to add an additional layer of resilience against similar future vulnerabilities that might trigger kernel warnings.