From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-89955][LOW] s390/vfio-ap: Fix NULL deref in status_show() during queue probe Date: Wed, 16 Sep 2026 11:11:41 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-89955 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: 2 X-AL-KERNEL-ActionableScore-Lower: unknown X-AL-KERNEL-Commit: 31fa0a8a3c337ed2d200166d6926ab23c14f8b2e List-Id: CVE: CVE-2026-89955 Priority: LOW AL-KERNEL base severity: LOW KPANIC flag: NO Patch: s390/vfio-ap: Fix NULL deref in status_show() during queue probe Commit: 31fa0a8a3c337ed2d200166d6926ab23c14f8b2e Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=31fa0a8a3c337ed2d200166d6926ab23c14f8b2e Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89955 Analysis date: Wed, 16 Sep 2026 11:11:41 -0400 ActionableScore: 2 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-2026-89955 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89955 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-2026-89955 LOW https://www.kernelcve.org/list/?q=CVE-2026-89955 CHECK WITH IMPACT FROM ORIG NN MODERATE Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-362;**CWE-476;CWE-667;BEST CVSS score: '4.7';DESCR 'A NULL pointer dereference in the s390 vfio ap sysfs status path can occur because the status attribute may become visible before queue driver data is installed during vfio_ap_mdev_probe_queue. A local user or process that reads the status attribute in this narrow race window can make status_show receive NULL from dev_get_drvdata and later dereference it through vfio_ap_mdev_for_queue, causing a kernel crash. For the CVSS the PR:L is used because the trigger is a local sysfs read and does not require full administrative privileges once the attribute is accessible. The issue is not network reachable and depends on vfio ap queue probing on s390 systems. Impact is denial of service only because the patch context indicates a NULL dereference race and does not show a UAF, OOB write, arbitrary write, or information leak primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2) YES READ NULLPTR HARDWARE LINUS DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_GUESSCVSS_AND_CIANNH NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=2 PrimitiveClass=CLASS_C RealUAF=NO HostAdminRequired=NO NamespaceScopedPrivilege=NO ImportantCandidate=NO ManualReview=RECOMMENDED AutoDowngradeAllowed=YES 1. ActionableScore * Raw conservative score: 2 * Conservative score: 2 * Raw paranoid score: 2 * Paranoid score: 2 * No floor or cap changed the score. * Final recommended bucket: **Ordinary Moderate**:: 2. Signal breakdown Positive signals: * +1 Reliable kernel crash / strong DoS: the patch and commit directly establish a NULL pointer dereference in `status_show()` via `vfio_ap_mdev_for_queue(q)` when `q == NULL`. * +1 Local below-host-admin trigger: the triggering operation is a local sysfs `status` attribute read once the attribute is visible. The supplied context supports PR:L style reachability rather than host-admin-equivalent privilege for the read side. Negative or limiting signals: * No -1 hard race penalty was applied, although the commit describes a very small window. The race is real, but the scoring rules say not to subtract merely because a race exists unless the timing is materially difficult and externally uncontrollable. Here the window is narrow, but the local read path is straightforward once the attribute is exposed. * No -1 rare subsystem penalty was applied. `s390/vfio-ap` is platform and device specific, but niche subsystem alone is not enough for a negative point. * No C/I points were awarded: the primitive is NULL dereference, not information disclosure or unauthorized modification. * No UAF points were awarded: the object is not freed and then reused. The issue is missing initialization / publication ordering of driver data. * No structured stale-object leverage was awarded: there is no stale reclaimed object, controlled replacement, callback, function pointer, or post-free structured consumption. * No concrete repeated release/refcount signal exists: there is no first lifetime-ending put/free followed by a second put/free on the same dangling reference. * No extra availability point was awarded because the kernel crash / DoS is already counted once. * No remote/network reachability was awarded. 3. Reachability analysis The reachable trigger is a local sysfs read of the `status` attribute while `vfio_ap_mdev_probe_queue()` has created the sysfs group but has not yet installed queue driver data with `dev_set_drvdata()`. The supplied context indicates that a local user or local process can perform the read once the sysfs attribute is visible, so `HostAdminRequired=NO` is used for the vulnerable read operation. The complete environmental setup still depends on an s390 system with vfio-ap/AP queue devices and queue probing activity. That affects deployment exposure, but it does not prove host-admin privilege is required for the actual vulnerable read. This is not namespace-scoped. There is no evidence that user or network namespaces are involved. There is no remote packet or network-triggered path. Race repeatability is limited by the probe window, but the bug class is still a local race against sysfs attribute publication. The attacker does not appear to control object allocation, replacement, or lifetime reuse in a way useful beyond causing the crash. 4. Primitive and severity interpretation The exact primitive is a NULL pointer dereference caused by a race between queue probe initialization and sysfs `status` reads. `sysfs_create_group()` exposes the attribute before the queue object is stored in device driver data. A concurrent `status_show()` can receive NULL from `dev_get_drvdata()` and pass it into `vfio_ap_mdev_for_queue()`, where `q->apqn` is dereferenced. This is not a real UAF. It is not a fixed stale storage case. It is not an active structured post-lifetime object reuse case. It is a Class C invalid-state / NULL-deref crash primitive. The demonstrated consequence is local denial of service via kernel crash. Stronger consequences such as privilege escalation, arbitrary write, controlled reclaim, information disclosure, or security-boundary bypass are not supported by the patch or supplied context. ImportantCandidate=NO because the evidence does not establish memory corruption or unresolved exploitation leverage beyond crash. AutoDowngradeAllowed=YES because this is a constrained NULL-deref race with DoS-only impact and no concrete corruption, repeated lifetime mutation, authorization bypass, or ownership violation. 5. One-sentence report phrase A local sysfs status read can race with s390 vfio-ap queue probing before driver data is installed, causing `status_show()` to dereference NULL and crash the kernel. 6. Manual review recommendation MANUAL CHECK RECOMMENDED Reason: this is a kernel race in a sysfs path and should be sanity-checked for permissions and device exposure, but the supplied evidence supports DoS-only NULL dereference and does not require Important-class handling. 7. Call-site confidence Call-site confidence: high ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: s390/vfio-ap: Fix NULL deref in status_show() during queue probe Commit: 31fa0a8a3c337ed2d200166d6926ab23c14f8b2e Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=31fa0a8a3c337ed2d200166d6926ab23c14f8b2e Commit description: When vfio_ap_mdev_probe_queue() creates the sysfs attribute group, the queue's driver data has not yet been set. A concurrent read of the 'status' attribute can therefore call dev_get_drvdata() and get NULL, which is then passed directly to vfio_ap_mdev_for_queue() where q->apqn is unconditionally dereferenced, causing a NULL pointer dereference. Fix this by acquiring the update locks before calling sysfs_create_group(). The status_show() function acquires guests_lock before reading the driver data, so any concurrent read will block until after dev_set_drvdata() has been called and the update locks are released. As a bonus, the APQN no longer needs to be read from the queue struct after allocation — it can be read directly from apdev before allocation and stored in a local variable, which is then assigned to q->apqn once the allocation succeeds. Fixes: 260f3ea ("s390/vfio-ap: move probe and remove callbacks to vfio_ap_ops.c") 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=31fa0a8a3c337ed2d200166d6926ab23c14f8b2e ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-89955 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89955 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: Not available The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS score: 4.7 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: 2 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).