CVE-2026-98473 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
s390/vfio-ap: fix KVM GISC and page leak when queue removed from host config
Three related problems exist in the handling of KVM interrupt and page resources when a queue is removed from the host's AP configuration while assigned to a mediated device (mdev).
Problem 1: ~~~~~~~~~ AP_RESPONSE_Q_NOT_AVAIL not handled in vfio_ap_mdev_reset_queue()
When the AP bus removes a queue device whose adapter or domain has been removed from the host's AP configuration, vfio_ap_mdev_remove_queue() is called. If the queue is still in the host's AP configuration at that point, it calls vfio_ap_mdev_reset_queue(), which issues a PQAP(ZAPQ). Since the adapter is already gone from the host configuration, ap_zapq() returns AP_RESPONSE_Q_NOT_AVAIL (0x01). This response code is not handled in vfio_ap_mdev_reset_queue()'s switch statement and falls through to the default case, which issues a WARN but does not call vfio_ap_free_aqic_resources(). As a result, if IRQ handling was enabled for the queue by the guest, the KVM GISC registration and the pinned guest page holding the notification indicator byte (NIB) are both leaked.
This is fixed by adding AP_RESPONSE_Q_NOT_AVAIL to the same case as AP_RESPONSE_DECONFIGURED and AP_RESPONSE_CHECKSTOPPED in vfio_ap_mdev_reset_queue(). Like those response codes, Q_NOT_AVAIL indicates the queue is not operational and no further reset attempts are possible; the correct action is to free the IRQ resources immediately.
Problem 2: ~~~~~~~~~ AP_RESPONSE_Q_NOT_AVAIL not handled in apq_status_check()
In vfio_ap_mdev_reset_queue(), there are four cases that indicate a queue reset has not yet completed, in which case apq_reset_check() is queued to a work queue to verify completion of the reset operation. This function uses the PQAP(TAPQ) function to get the queue's status and calls apq_status_check() to verify whether the reset has completed, failed or needs to be executed again. As described in Problem #1 above, apq_reset_check() does not specifically check for AP_RESPONSE_Q_NOT_AVAIL, thereby potentially leaking KVM GISC registration and the pinned guest page holding the NIB.
This is fixed by adding a case statement for AP_RESPONSE_Q_NOT_AVAIL to apq_status_check() and returning -ENODEV for that case. The caller, apq_reset_check() will then check for this return code and call vfio_ap_free_aqic_resources() to prevent the leak.
Problem 3: ~~~~~~~~~ vfio_ap_free_aqic_resources() leaks saved_isc when kvm is NULL
vfio_ap_free_aqic_resources() guards the call to kvm_s390_gisc_unregister() with:
if (q->saved_isc != VFIO_AP_ISC_INVALID && !WARN_ON(!(q->matrix_mdev && q->matrix_mdev->kvm)))
If matrix_mdev->kvm is NULL -- which can happen when vfio_ap_mdev_unset_kvm() has already run and cleared kvm before a subsequent cleanup path reaches this function -- the WARN_ON fires and the entire block is skipped. This leaves q->saved_isc set to a non-invalid value, creating a potential double-free on any subsequent call to this function.
When kvm is NULL the KVM guest is already torn down, so kvm_s390_gisc_unregister() need not and cannot be called; however, q->saved_isc must always be cleared. Fix this by separating the kvm_s390_gisc_unregister() call from the q->saved_isc reset. The WARN_ON now guards only the genuinely impossible case of matrix_mdev being NULL. A NULL kvm is handled gracefully by skipping only the unregister call, and q->saved_isc = VFIO_AP_ISC_INVALID is set unconditionally whenever saved_isc was not already invalid.
Additionally, add an else clause to the host-config check in vfio_ap_mdev_remove_queue() to call vfio_ap_free_aqic_resources() directly when the queue is not in the host's AP configuration. This serves as a backstop: when the AP bus fires the driver .remove callback after an adapter is removed from the host config, the queue is by definition no longer addressable, so vfio_ap_mdev_reset_queue ---truncated---
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Analysis
by VulDB Data Team • 10/18/2026
The Linux kernel vulnerability in the s390 VFIO-AP subsystem involves critical resource leaks and potential double-free conditions arising from improper handling of AP response codes during queue removal. This issue manifests when an adapter or domain is removed from the host's Advanced Peripheral bus configuration while a queue remains assigned to a mediated device. The core technical flaw lies in the failure to correctly interpret specific hardware return codes, specifically AP_RESPONSE_Q_NOT_AVAIL, which indicates that the queue is no longer available for operations due to its removal from the system topology. When this response code is encountered during reset or status check routines, it falls through default cases that fail to invoke necessary cleanup functions, leading to persistent leaks of kernel resources and guest memory pages.
The first major component of this vulnerability occurs within the vfio_ap_mdev_reset_queue function. Upon receiving an AP_RESPONSE_Q_NOT_AVAIL response from a PQAP ZAPQ instruction, the code previously ignored the error in its switch statement, proceeding only to issue a warning without freeing associated resources. Consequently, if interrupt handling was active for that queue, both the KVM Global Interrupt State Controller registration and the pinned guest page containing the Notification Indicator Byte remain allocated but orphaned. This represents a classic resource leak where system memory and interrupt vectors are retained indefinitely despite the underlying hardware device being inaccessible. The fix involves explicitly adding AP_RESPONSE_Q_NOT_AVAIL to the case block that handles deconfigured or checkstopped queues, ensuring immediate invocation of vfio_ap_free_aqic_resources to reclaim these assets.
A second related flaw exists in the apq_status_check function, which is responsible for verifying the completion of queue reset operations via a work queue mechanism. Similar to the previous issue, this routine failed to recognize AP_RESPONSE_Q_NOT_AVAIL as a terminal state indicating that further resets are impossible. Without explicit handling, the status check would not return an error code sufficient to trigger resource cleanup in its caller, apq_reset_check. By adding a specific case for AP_RESPONSE_Q_NOT_AVAIL that returns -ENODEV, the system now correctly identifies this condition and propagates it up the call stack. This ensures that vfio_ap_free_aqic_resources is called appropriately, preventing the leakage of KVM GISC registrations and pinned guest pages during asynchronous reset verification processes.
The third vulnerability involves a logic error in vfio_ap_free_aqic_resources regarding the handling of NULL pointers when freeing saved interrupt state controller identifiers. The original code guarded the call to kvm_s390_gisc_unregister with a condition that also checked for the validity of matrix_mdev and its associated KVM structure. If kvm was already null, indicating prior teardown by vfio_ap_mdev_unset_kvm, the WARN_ON macro would trigger and skip the entire block. This left q->saved_isc set to a non-invalid value even though the unregister call did not occur. Such state inconsistency creates a risk of double-free vulnerabilities if the function is invoked again later in the cleanup sequence. The remediation separates the unregistration logic from the state reset, ensuring that saved_isc is always cleared regardless of whether kvm exists, thereby maintaining consistent internal state and preventing potential memory corruption or crashes on subsequent calls.
From an industry standards perspective, this vulnerability aligns with CWE-401 Missing Release of Memory after Effective Lifetime, as resources are not freed when they become unreachable due to hardware removal. It also relates to CWE-835 Loop without Progress if the status check logic were to enter a retry loop indefinitely, although in practice it primarily results in resource exhaustion over time. In terms of ATT&CK mapping, this falls under T1496 Resource Hijacking or more accurately T1072 Software Deployment Tools for persistence mechanisms involving kernel modules, though strictly speaking it is an unintentional denial of service via resource depletion rather than malicious exploitation. The lack of proper error handling in low-level hardware interaction code highlights the importance of rigorous state management in virtualization drivers where host and guest lifecycles are decoupled.
The operational impact of these flaws includes gradual degradation of system stability due to memory leaks, particularly on systems with high rates of adapter or domain reconfiguration. Over time, the accumulation of pinned guest pages can lead to significant RAM consumption pressure, potentially triggering out-of-memory conditions for other workloads. Furthermore, the potential double-free scenario poses a severe security risk, as it could allow an attacker with access to mediated devices to trigger kernel panics or potentially achieve arbitrary code execution through heap corruption if they can manipulate the timing of queue removals relative to KVM teardown operations.
Mitigation strategies involve applying the upstream Linux kernel patches that address these specific logic errors in the s390 vfio-ap driver. Administrators should ensure their systems are updated with versions containing fixes for AP_RESPONSE_Q_NOT_AVAIL handling and improved NULL pointer checks in resource freeing routines. For environments where immediate patching is not feasible, limiting dynamic reconfiguration of AP adapters or domains while mediated devices remain active can reduce the exposure window. Additionally, monitoring system memory usage and kernel logs for warnings related to vfio-ap operations can help detect instances where these leaks are occurring before they lead to service disruption.