From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-89958][LOW] s390/vfio-ap: Fix dereference matrix_mdev->kvm without checking for NULL Date: Wed, 16 Sep 2026 07:35:51 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-89958 X-AL-KERNEL-Priority: LOW X-AL-KERNEL-Severity: LOW X-AL-KERNEL-Base-Severity: LOW X-AL-KERNEL-KPANIC: NO X-AL-KERNEL-ActionableScore: 0 X-AL-KERNEL-ActionableScore-Lower: unknown X-AL-KERNEL-Commit: 5e932abfcce6189d067fffb0eb8214443bc04374 List-Id: CVE: CVE-2026-89958 Priority: LOW AL-KERNEL base severity: LOW KPANIC flag: NO Patch: s390/vfio-ap: Fix dereference matrix_mdev->kvm without checking for NULL Commit: 5e932abfcce6189d067fffb0eb8214443bc04374 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5e932abfcce6189d067fffb0eb8214443bc04374 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89958 Analysis date: Wed, 16 Sep 2026 07:35:51 -0400 ActionableScore: 0 ActionableScore lower bound: unknown Actionable bucket: Ordinary Moderate Manual review required: NO Summary: No one-sentence report phrase was found. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-89958 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89958 The original announcement does not normally provide a security severity estimate, CVSS assessment, or enough information to determine whether the reported kernel bug represents a practically relevant security issue. This report was generated by AL-KERNEL, an AI-assisted Linux kernel vulnerability analysis system developed by Alexander Larkin. It combines an autonomous classifier with LLM-assisted technical analysis and a separate ActionableScore mechanism. The purpose of this report is to prioritize Linux kernel CVEs before manual review, identify cases that require prompt investigation, and support automatic closure of issues that are unlikely to have meaningful security impact. Published priority for this report: LOW Manual review required: NO A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-89958 LOW https://www.kernelcve.org/list/?q=CVE-2026-89958 CHECK WITH IMPACT FROM ORIG NN LOW Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';**CWE-476;CWE-703;CWE-754;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A NULL pointer dereference in s390 vfio ap can occur because AP bus config change and scan complete callbacks may take matrix_mdev kvm lock when matrix_mdev kvm is NULL. This state is possible when a vfio ap mediated device exists but is not associated with a KVM guest or the KVM pointer has not been set. For the CVSS the PR:L is used in the paranoid interpretation because some deployments may delegate limited vfio or mediated device management to service accounts, container root, or virtualization management processes that are less than full host administrators. The issue is not network reachable and requires local control over the relevant vfio ap or AP configuration path. Impact is denial of service via kernel crash only, because the patch context indicates a NULL dereference rather than UAF, OOB access, arbitrary write, or another memory corruption primitive.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 0) YES LOCK NULLPTR HARDWARE LINUS DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=0 PrimitiveClass=CLASS_C RealUAF=NO HostAdminRequired=YES NamespaceScopedPrivilege=NO ImportantCandidate=NO ManualReview=NO AutoDowngradeAllowed=YES 1. ActionableScore * Raw conservative score: -2 * Conservative score: 0 * Raw paranoid score: -1 * Paranoid score: 0 * Applied conservative floor/cap: lower bound clamp -2 -> 0 * Applied paranoid floor/cap: lower bound clamp -1 -> 0 * Final recommended bucket: **Ordinary Moderate**:: 2. Signal breakdown Positive signals: * +1 Reliable kernel crash / strong DoS. The patch and commit state that taking matrix_mdev->kvm->lock when matrix_mdev->kvm is NULL can trigger a NULL pointer dereference. * No additional Availability point awarded. The only supported availability effect is the same kernel NULL dereference crash. Negative signals: * -2 Host-admin-equivalent control. The vulnerable callbacks are tied to host AP bus scan and host AP configuration changes in s390 vfio-ap, which are host virtualization and device-management operations. * -1 conservative only for rare and difficult-to-reach subsystem. s390 vfio-ap requires specialized s390 AP crypto virtualization infrastructure and is not ordinarily reachable by a generic local user. Important non-awarded signals: * RealUAF not awarded. The bug is a NULL pointer dereference, not stale post-lifetime object use. * Controlled reclaim not demonstrated. * Arbitrary write not demonstrated. * Confidentiality and integrity impact not awarded. * Structured stale-object leverage not awarded because no stale object reuse or post-free field consumption is shown. * Concrete repeated release/refcount not awarded. There is no first put/free followed by a second lifetime mutation on the same dangling reference. * Remote reachability not established. * KVM guest-to-host exploitation leverage not awarded. The code is related to KVM/vfio-ap state, but the supplied evidence does not show a guest-controlled host corruption primitive. 3. Reachability analysis The trigger is associated with vfio-ap AP bus configuration callbacks, specifically vfio_ap_on_cfg_changed and vfio_ap_on_scan_complete. These callbacks are invoked around host AP bus scan/configuration changes, not by ordinary packet input or an unprivileged syscall path. The complete trigger appears to require host-level control over s390 AP/vfio-ap configuration or a host virtualization management path that can create or maintain a vfio-ap mediated device in a state where matrix_mdev->kvm has not been set and then cause AP configuration callbacks to run. That is host-admin-equivalent in the supplied context. Namespace-scoped privilege is not established. There is no indication that an attacker-created namespace can perform the complete vfio-ap and AP bus configuration sequence. Prior host setup exists in practice because vfio-ap and AP crypto virtualization must be configured on an s390 host. Special hardware/platform dependency is material: this is s390 AP crypto/vfio-ap infrastructure. No race repeatability issue is required for scoring because the described failure is an invalid NULL dereference when the relevant state exists. 4. Primitive and severity interpretation The exact primitive is a NULL pointer dereference of matrix_mdev->kvm when code attempts to lock matrix_mdev->kvm->lock without first checking matrix_mdev->kvm. This is not an active structured UAF. The matrix_mdev object itself is described as protected by matrix_dev->guests_lock during the callbacks, and the problem is that its kvm pointer can be NULL. The stale-object and repeated-release rules do not apply. PrimitiveClass is CLASS_C because the demonstrated behavior is crash/DoS from invalid dereference without an independently useful corruption primitive. Demonstrated consequence: local host-side kernel crash/DoS in a specialized privileged device-management path. Speculative consequence: no supported path to privilege escalation, guest-to-host escape, arbitrary read/write, controlled reclaim, callback control, or information disclosure. ImportantCandidate is NO because the patch supports NULL dereference only and does not leave meaningful unresolved memory-corruption or boundary-bypass leverage. AutoDowngradeAllowed is YES because this is a constrained, host-admin-gated, specialized NULL dereference with DoS-only impact. Likely operational severity: ordinary Moderate or Low-like depending on product exposure and whether vfio-ap is used. 5. One-sentence report phrase A NULL pointer dereference in s390 vfio-ap AP configuration callbacks can crash the host kernel when a mediated device has no associated KVM pointer, but triggering appears limited to privileged host AP/vfio-ap management paths and no UAF, corruption, or privilege escalation primitive is established. 6. Manual review recommendation NO MANUAL CHECK NEEDED Reason: the patch establishes a straightforward NULL dereference with DoS-only impact in a specialized host-admin-level configuration path, with no UAF, repeated lifetime mutation, authorization bypass, or meaningful corruption primitive. 7. Call-site confidence Call-site confidence: high ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: s390/vfio-ap: Fix dereference matrix_mdev->kvm without checking for NULL Commit: 5e932abfcce6189d067fffb0eb8214443bc04374 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5e932abfcce6189d067fffb0eb8214443bc04374 Commit description: The ap_driver structure has two fields which are function pointers to callbacks: * .on_config_changed: called at the start of the AP bus scan function to notify the device driver that the host AP configuration has changed and the associated AP devices will be added or removed accordingly. This gives the implementor a chance to evaluate the configuration changes and respond to them before the associated devices are added or removed. * .on_scan_complete: Called at the end of the AP bus scan function to notify the device driver that the host AP configuration has changed and the AP devices have been added or removed accordingly. This gives the implementor the opportunity to respond to the changes after the associated devices are added or removed. These two callbacks are implemented in the vfio_ap device driver via the vfio_ap_on_cfg_changed and vfio_ap_on_scan_complete functions respectively. Within the call stack of these two callback functions the matrix_mdev->kvm->lock mutex is taken without checking whether matrix_mdev->kvm is NULL or not. If matrix_mdev->kvm has never been set, trying to take the lock will trigger a NULL pointer dereference. This patch adds checks for matrix_mdev->kvm == NULL before taking the matrix_mdev->kvm->lock mutex. Note that the matrix_mdev->kvm->lock mutex taken in the vfio_ap_mdev_hot_plug_config function is moved to the calling function along with the matrix_dev->mdevs_lock which is needed there to access the fields of the matrix_mdev. It makes little sense to make the change the check for matrix_mdev->kvm there before taking the kvm->lock mutex only to have to move it out via another patch, so it is done in this patch. It is important to make note of the following: 1. The matrix_dev->guests_lock is acquired at the start of both callback functions. This ensures that matrix_mdev will not be removed via the vfio_ap_mdev_remove function because it too takes matrix_dev_guests_lock before removing the object; so, matrix_mdev will be available for the duration of the callback functions. 2. The matrix_dev->mdevs_lock mutex must be taken in order to access fields within the matrix_mdev structure 3. matrix_mdev->kvm->lock mutex must be taken before the matrix_dev->mdevs_lock to prevent a lockdep splat. 4: The kvm->lock must be held while plugging the guest's AP configuration into its SIE state description via the vfio_ap_mdev_update_guest_apcb function. 5. The vfio_ap_mdev_update_guest_apcb checks matrix_mdev->kvm to verify it is not NULL before doing the hot plug of the guest's AP configuration. Fixes: eeb386a ("s390/vfio-ap: handle config changed and scan complete notification") Cc: stable@vger.kernel.org Signed-off-by: Anthony Krowiak Reviewed-by: Matthew Rosato Signed-off-by: Christian Borntraeger Signed-off-by: Greg Kroah-Hartman Changed files: drivers/s390/crypto/vfio_ap_ops.c Diff excerpt: Not included in this email. See the upstream URL for the full patch. Full patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5e932abfcce6189d067fffb0eb8214443bc04374 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-89958 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89958 The original CVE announcement normally does not include a security-level estimate. In particular, it may not contain a CVSS assessment, an impact level, or enough information to determine whether the reported bug is a practically relevant security issue. One purpose of this parallel CVE list is to provide that missing technical and prioritization information. The original goal of the AL-KERNEL project was to prioritize Linux kernel CVE analysis automatically before manual review. The system can also help identify non-security issues that may be suitable for automatic closure. This report was generated by AL-KERNEL, an AI-assisted Linux kernel vulnerability analysis system developed by Alexander Larkin. The first analysis stage combines an autonomous classifier with additional LLM-based analysis. The autonomous classifier runs locally on a CPU and is based on a backpropagation neural network. Together, these mechanisms produce a technical vulnerability description, identify likely weakness types, estimate CVSS severity, and provide input for ActionableScore. Two CVSS estimates are retained because incomplete kernel vulnerability information often permits more than one defensible interpretation: Conservative CVSS vector: AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS score: 5.5 The conservative vector represents a lower-impact interpretation. The Best/paranoid vector intentionally represents a plausible upper-bound interpretation and should not automatically be treated as demonstrated real-world impact. CVSS may also need to be adjusted for a particular Linux deployment, because actual reachability, privileges, enabled kernel configuration, hardware, namespaces, exposed device nodes, and other environmental conditions can differ significantly between systems. A separate ActionableScore mechanism evaluates practical remediation urgency. Its analysis may include reachability, attack prerequisites, subsystem exposure, memory-corruption characteristics, denial-of-service reliability, and possible confidentiality, integrity, or privilege-escalation impact. Conservative ActionableScore: unknown Paranoid ActionableScore: 0 The final base severity is taken directly from the second tab-separated field of the AL-KERNEL classification result. ActionableScore does not replace or independently override that final AL-KERNEL decision, and if ActionableScore adjusted impact level of ALKERNEL, then you would see self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7. For an AL-KERNEL result of MODERATE, this report uses the following additional presentation split: ActionableScore below 5 -> MODERATE REGULAR ActionableScore 5 or more -> MODERATE 7.0 The distinction between MODERATE REGULAR and MODERATE 7.0 makes it possible to identify Moderate issues that should receive manual analysis and fixes before lower-priority MODERATE REGULAR issues. In many cases, MODERATE REGULAR fixes may wait for a later rebase or routine update. There is one override in which MODERATE REGULAR becomes MODERATE 7.0 even when the ActionableScore is below 5. When the AL-KERNEL result contains the KPANIC flag, a MODERATE result is always presented as MODERATE 7.0. The KPANIC flag selected with few regexps without usage of AI at all, so it helps to detect cases when Kernel Crash happens and similar (to filter False-Negative results from the LLM usage). KPANIC indicates that a reliable kernel crash, kernel panic, or similarly serious kernel availability impact was identified by the classification workflow. AL-KERNEL base severity for this report: LOW KPANIC detected for this report: NO Published priority for this report (same as in Subject): LOW These results are intended to support engineering triage. They are machine-generated estimates, and cases marked for manual review should be validated by a human security engineer before final disposition. For more info read docs linked from here: https://kernelcve.org/ (and you can submit you own patch there to generate such a report for non-existant CVE-id yet). Note that in many cases this AI tool selects higher severity, than real is (means you can expect Importants instead of Moderate 7.0 or Moderates 7.0 instead of regular Moderates). If you see such cases, please use reply email interface to add additional manual analyses info to this particular CVE. And please, please, let me know when you see Lows instead of Importants or Important instead of Low (because particular for such cases I need to tune this AI tool to make it better for this one and next similar). My contact email for such notifications is alexanjelausa@gmail.com (and both send reply to CVE record itself too and see "reply" button below for howto reply).