From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-98921][MODERATE 7.0] ovl: fix UAF in ovl_do_mkdir() debug print [ Upstream Date: Mon, 26 Oct 2026 15:45:51 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-98921 X-AL-KERNEL-Priority: MODERATE 7.0 X-AL-KERNEL-Severity: MODERATE 7.0 X-AL-KERNEL-Base-Severity: MODERATE X-AL-KERNEL-KPANIC: YES X-AL-KERNEL-ActionableScore: 4 X-AL-KERNEL-ActionableScore-Lower: unknown X-AL-KERNEL-Commit: 719bd33ab359c539a68a278c15a97deb77565cdb List-Id: CVE: CVE-2026-98921 Priority: MODERATE 7.0 AL-KERNEL base severity: MODERATE KPANIC flag: YES Patch: ovl: fix UAF in ovl_do_mkdir() debug print [ Upstream Commit: 719bd33ab359c539a68a278c15a97deb77565cdb Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=719bd33ab359c539a68a278c15a97deb77565cdb Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98921 Analysis date: Mon, 26 Oct 2026 15:45:51 -0400 ActionableScore: 4 ActionableScore lower bound: unknown Actionable bucket: Borderline / manual review recommended Manual review required: YES Summary: OverlayFS debug logging may read a dentry after `vfs_mkdir()` released or replaced it, causing a configuration-dependent local kernel UAF and possible crash. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98921 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98921 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: MODERATE 7.0 Manual review required: YES A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-98921 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-98921 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE_KPANIC RESULT=SAME V39_AEGIS_ONE_STEP_UP=MODERATE->MODERATE_KPANIC (final reconciliation agrees; DECISION: both MODERATE_KPANIC) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:H';*CWE-416;CWE-125;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.1';DESCR 'ovl_do_mkdir used the original dentry in a pd2 debug print after vfs_mkdir even though vfs_mkdir can release the dentry on failure or replace it on success. This creates a local UAF read when CONFIG_OVERLAY_FS_DEBUG is enabled and an attacker can reach the OverlayFS mkdir path. For the CVSS the PR:L value is selected because reliable triggering requires a local process with permission to create a directory in a writable OverlayFS location. The primary impact is a kernel crash and local denial of service. The paranoid score allows limited confidentiality impact because stale dentry fields may be formatted into the kernel log, but there is no supported write primitive or integrity impact. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 6.1 and the production ActionableScore returned 4 with recommendation Borderline / manual review recommended. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were KP and FD specialist checks stage MODERATE_KPANIC to MODERATE because KP specialist did not establish a qualifying panic. final reconciliation stage MODERATE to MODERATE_KPANIC because the cross model evidence reconciliation rule was applied. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP and V39_AEGIS_ONE_STEP_UP and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 3.1 and ActionableScore 1 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4) MAYBE DANGER DISK UAF LINUS SYZBOT DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP KPANIC V39_AEGIS_ONE_STEP_UP MANUAL_REVIEW_REQUIRED - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=4 PrimitiveClass=CLASS_B RealUAF=YES HostAdminRequired=NO NamespaceScopedPrivilege=UNKNOWN ImportantCandidate=NO ManualReview=RECOMMENDED AutoDowngradeAllowed=YES RequiresHostUID0=NO AuthorizationDecisionBypass=NO ProtectedResourceImpact=NONE B4PendingReview=NO B4PendingConfirmed=NO 1. ActionableScore * Raw base score: 3. * Conservative score: 4. Real-UAF floor 4 applied: 3 -> 4. * Raw paranoid score: 3. * Paranoid score: 4. The same floor applies; no separate disclosure point is supported. * Final recommended bucket: **Borderline / manual review recommended**:: 2. Signal breakdown * +2 Real UAF: `vfs_mkdir()` may drop or replace the input dentry, after which `%pd2` dereferences the original dentry. * +1 Reliable kernel crash/DoS: formatting a freed dentry can fault while executing kernel debug output. * +1 Local unprivileged trigger: supplied evidence grounds triggering through mkdir permission on an existing writable OverlayFS location, without host-administrator powers. * -1 Rare/difficult configuration: the vulnerable read exists only with `CONFIG_OVERLAY_FS_DEBUG`, a build-time condition the attacker cannot enable. * No extra structured-object leverage: the stale dentry is only consumed by a debug formatter; no later mutation, callback, traversal operation, or ownership decision is shown. * No repeated release/refcount operation, controlled reclaim, OOB write, arbitrary write, or integrity primitive is shown. * No confidentiality point: `%pd2` may emit stale path-related data, but attacker access to the resulting kernel log or sensitive leaked content is not established. * No extra availability point because the same possible kernel fault is already counted once. 3. Reachability analysis A local process able to create a directory in an already mounted writable OverlayFS can reach the wrapper. Creating or mounting that OverlayFS may require prior privileged setup, but the supplied evidence supports a below-host-admin subsequent trigger. Namespace-scoped completion is not established. The required debug configuration is compile-time. No race or special retry sequence is required. 4. Primitive and severity reading The primitive is a real read-side UAF of an active structured `dentry`, but its stale use is narrowly confined to `%pd2` debug formatting immediately after `vfs_mkdir()`. The shown effect is a possible Oops/kernel crash; limited log disclosure remains unproven. There is no second lifetime mutation or material leverage past the stale read, so this is constrained CLASS_B rather than CLASS_A. ImportantCandidate is NO and automatic Moderate downgrade remains permissible, while manual review is recommended because the UAF is concrete. 5. One-sentence report phrase OverlayFS debug logging may read a dentry after `vfs_mkdir()` released or replaced it, causing a configuration-dependent local kernel UAF and possible crash. 6. Manual review advice MANUAL CHECK RECOMMENDED Confirm product enablement of `CONFIG_OVERLAY_FS_DEBUG`, OverlayFS exposure, and kernel-log accessibility. 7. Call-site confidence Call-site confidence: high ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: ovl: fix UAF in ovl_do_mkdir() debug print [ Upstream Commit: 719bd33ab359c539a68a278c15a97deb77565cdb Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=719bd33ab359c539a68a278c15a97deb77565cdb Changed files: fs/overlayfs/overlayfs.h 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=719bd33ab359c539a68a278c15a97deb77565cdb ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98921 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98921 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:L/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:L/I:N/A:H The Best / paranoid CVSS score: 6.1 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: 4 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: MODERATE KPANIC detected for this report: YES Published priority for this report (same as in Subject): MODERATE 7.0 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).