CVE-2026-23198 in Linux
Summary
by MITRE • 02/14/2026
In the Linux kernel, the following vulnerability has been resolved:
KVM: Don't clobber irqfd routing type when deassigning irqfd
When deassigning a KVM_IRQFD, don't clobber the irqfd's copy of the IRQ's routing entry as doing so breaks kvm_arch_irq_bypass_del_producer() on x86 and arm64, which explicitly look for KVM_IRQ_ROUTING_MSI. Instead, to handle a concurrent routing update, verify that the irqfd is still active before consuming the routing information. As evidenced by the x86 and arm64 bugs, and another bug in kvm_arch_update_irqfd_routing() (see below), clobbering the entry type without notifying arch code is surprising and error prone.
As a bonus, checking that the irqfd is active provides a convenient location for documenting _why_ KVM must not consume the routing entry for an irqfd that is in the process of being deassigned: once the irqfd is deleted from the list (which happens *before* the eventfd is detached), it will no longer receive updates via kvm_irq_routing_update(), and so KVM could deliver an event using stale routing information (relative to KVM_SET_GSI_ROUTING returning to userspace).
As an even better bonus, explicitly checking for the irqfd being active fixes a similar bug to the one the clobbering is trying to prevent: if an irqfd is deactivated, and then its routing is changed, kvm_irq_routing_update() won't invoke kvm_arch_update_irqfd_routing() (because the irqfd isn't in the list). And so if the irqfd is in bypass mode, IRQs will continue to be posted using the old routing information.
As for kvm_arch_irq_bypass_del_producer(), clobbering the routing type results in KVM incorrectly keeping the IRQ in bypass mode, which is especially problematic on AMD as KVM tracks IRQs that are being posted to a vCPU in a list whose lifetime is tied to the irqfd.
Without the help of KASAN to detect use-after-free, the most common sympton on AMD is a NULL pointer deref in amd_iommu_update_ga() due to the memory for irqfd structure being re-allocated and zeroed, resulting in irqfd->irq_bypass_data being NULL when read by avic_update_iommu_vcpu_affinity():
BUG: kernel NULL pointer dereference, address: 0000000000000018 #PF: supervisor read access in kernel mode #PF: error_code(0x0000) - not-present page PGD 40cf2b9067 P4D 40cf2b9067 PUD 408362a067 PMD 0 Oops: Oops: 0000 [#1] SMP
CPU: 6 UID: 0 PID: 40383 Comm: vfio_irq_test Tainted: G U W O 6.19.0-smp--5dddc257e6b2-irqfd #31 NONE Tainted: [U]=USER, [W]=WARN, [O]=OOT_MODULE
Hardware name: Google, Inc. Arcadia_IT_80/Arcadia_IT_80, BIOS 34.78.2-0 09/05/2025 RIP: 0010:amd_iommu_update_ga+0x19/0xe0 Call Trace: <TASK> avic_update_iommu_vcpu_affinity+0x3d/0x90 [kvm_amd]
__avic_vcpu_load+0xf4/0x130 [kvm_amd]
kvm_arch_vcpu_load+0x89/0x210 [kvm]
vcpu_load+0x30/0x40 [kvm]
kvm_arch_vcpu_ioctl_run+0x45/0x620 [kvm]
kvm_vcpu_ioctl+0x571/0x6a0 [kvm]
__se_sys_ioctl+0x6d/0xb0 do_syscall_64+0x6f/0x9d0 entry_SYSCALL_64_after_hwframe+0x4b/0x53 RIP: 0033:0x46893b </TASK> ---[ end trace 0000000000000000 ]---
If AVIC is inhibited when the irfd is deassigned, the bug will manifest as list corruption, e.g. on the next irqfd assignment.
list_add corruption. next->prev should be prev (ffff8d474d5cd588), but was 0000000000000000. (next=ffff8d8658f86530). ------------[ cut here ]------------
kernel BUG at lib/list_debug.c:31! Oops: invalid opcode: 0000 [#1] SMP
CPU: 128 UID: 0 PID: 80818 Comm: vfio_irq_test Tainted: G U W O 6.19.0-smp--f19dc4d680ba-irqfd #28 NONE Tainted: [U]=USER, [W]=WARN, [O]=OOT_MODULE
Hardware name: Google, Inc. Arcadia_IT_80/Arcadia_IT_80, BIOS 34.78.2-0 09/05/2025 RIP: 0010:__list_add_valid_or_report+0x97/0xc0 Call Trace: <TASK> avic_pi_update_irte+0x28e/0x2b0 [kvm_amd]
kvm_pi_update_irte+0xbf/0x190 [kvm]
kvm_arch_irq_bypass_add_producer+0x72/0x90 [kvm]
irq_bypass_register_consumer+0xcd/0x170 [irqbypa
---truncated---
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Analysis
by VulDB Data Team • 05/05/2026
The vulnerability described in CVE-2026-23198 resides within the Linux kernel's KVM subsystem, specifically impacting how irqfd routing entries are managed during deassignment operations. This flaw manifests in a critical race condition where the irqfd's routing type is inadvertently clobbered when an irqfd is being removed from the system. The improper handling of routing information leads to a cascade of issues, particularly on x86 and arm64 architectures, where the function kvm_arch_irq_bypass_del_producer() expects to find a KVM_IRQ_ROUTING_MSI entry. When this expectation is violated due to clobbering, the system fails to properly manage interrupt bypass mechanisms, resulting in incorrect IRQ handling and potential system instability.
The root cause of this vulnerability stems from a flawed approach to managing concurrent routing updates during irqfd deassignment. The original implementation attempted to clobber the irqfd's routing entry type without proper synchronization, which breaks architectural assumptions made by the underlying KVM implementations. This issue is particularly severe on AMD platforms where KVM maintains IRQ tracking lists tied to irqfd lifetimes, making the clobbering operation especially problematic. The vulnerability directly maps to CWE-362, which describes a race condition where a resource is accessed concurrently, and the improper handling of such access leads to incorrect behavior. The issue also aligns with ATT&CK technique T1059.003, which involves the execution of system commands, as the improper routing handling can lead to unexpected kernel behavior and potential privilege escalation paths.
The operational impact of this vulnerability is significant, particularly in virtualized environments using KVM with AMD processors. The most common manifestation is a NULL pointer dereference in the amd_iommu_update_ga() function, where the irqfd structure memory has been reallocated and zeroed, leaving irqfd->irq_bypass_data as NULL when accessed by avic_update_iommu_vcpu_affinity(). This results in kernel oops and system crashes, severely impacting system stability. Additionally, the bug can cause list corruption when AVIC is inhibited during irqfd deassignment, leading to further kernel panics and potential data loss. The vulnerability affects the integrity of interrupt routing management, which can cause virtual machines to lose proper interrupt handling, potentially leading to guest operating system instability and complete system hangs. The race condition also impacts the proper functioning of VFIO-based interrupt handling, which is critical for high-performance I/O virtualization scenarios.
Mitigation strategies for this vulnerability involve implementing proper synchronization mechanisms to verify that irqfd entries remain active before consuming routing information, thereby preventing the clobbering of routing types. The fix requires modifying the irqfd deassignment logic to check for active irqfd status before processing routing updates, ensuring that routing information is only consumed when the irqfd is still valid. Organizations should prioritize updating their kernel versions to include the patched implementation, as the vulnerability affects core virtualization functionality. System administrators should monitor for kernel oops messages and NULL pointer dereferences, particularly in systems using AMD processors with KVM virtualization. Additionally, implementing proper kernel lockdown mechanisms and restricting privileged access to KVM interfaces can help minimize exploitation risks. The vulnerability highlights the importance of careful synchronization in kernel-level code and the need for comprehensive testing of concurrent access patterns in virtualization subsystems, particularly when dealing with interrupt routing mechanisms that are critical for system stability and security.