From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-89683][MODERATE 7.0] nfsd: fix dentry ref leak on V4ROOT export filehandle lookup Date: Fri, 11 Sep 2026 23:02:43 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-89683 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: 3 X-AL-KERNEL-Commit: d493cf57f4c4dae5396cd6ad41e5c209d0615a81 List-Id: CVE: CVE-2026-89683 Priority: MODERATE 7.0 AL-KERNEL base severity: MODERATE KPANIC flag: YES Patch: nfsd: fix dentry ref leak on V4ROOT export filehandle lookup Commit: d493cf57f4c4dae5396cd6ad41e5c209d0615a81 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d493cf57f4c4dae5396cd6ad41e5c209d0615a81 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89683 Analysis date: Fri, 11 Sep 2026 23:02:43 -0400 ActionableScore: 4 ActionableScore lower bound: 3 Actionable bucket: Borderline / manual review recommended Manual review required: YES Summary: No one-sentence report phrase was found. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-89683 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89683 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-2026-89683 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89683 CHECK WITH IMPACT FROM ORIG NN MODERATE Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';*CWE-401;*CWE-772;**CWE-400;Other CVSS 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '7.5';DESCR 'A dentry reference leak in nfsd_set_fh_dentry can occur when an NFSv3 filehandle lookup reaches a V4ROOT export path. The dentry reference returned by exportfs_decode_fh_raw is not released before the function exits through the V4ROOT out path, so each matching request can leak one reference. For the CVSS the PR:N is used because a remote NFS client generally does not need local privileges on the target host beyond network reachability to the NFS service. The issue is network reachable when nfsd and the affected export configuration are exposed to the attacker. Impact is primarily denial of service through sustained resource exhaustion, not a demonstrated information leak or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4) SKIP CVE-2026-89683 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES NO NO unknown MAYBE DISK SIMPLEFIX LEAK NETWORK KPANIC KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=4 ActionableScoreLower=3 PrimitiveClass=CLASS_C RealUAF=NO HostAdminRequired=NO NamespaceScopedPrivilege=NO ImportantCandidate=NO ManualReview=RECOMMENDED AutoDowngradeAllowed=YES 1. ActionableScore * Raw conservative score: 3 * Conservative score: 3. No floor or cap applied. * Raw paranoid score: 4 * Paranoid score: 4. No floor or cap applied. * Final recommended bucket: **Borderline / manual review recommended**:: 2. Signal breakdown Conservative positive signals: * +2 Remote reachable / directly network-triggerable. The commit states that a crafted NFSv3 filehandle targeting a V4ROOT export fsid triggers the leak on every request. * +1 Concrete persistent kernel resource/reference leak. exportfs_decode_fh_raw() returns a dentry reference, but the NFSEXP_V4ROOT branches go to out without dput(dentry), and fhp->fh_dentry is not assigned so fh_put() cannot compensate. Paranoid additional signal: * +1 Sustained resource exhaustion plausibility. Because each matching request can leak a dentry reference, repeated remote requests can plausibly accumulate unreclaimed kernel references and cause availability impact over time. Negative signals: * No host-admin penalty. The attacker is a remote NFS client and does not need host-admin-equivalent privilege on the target once the NFS service and affected export are reachable. * No rare-hardware or special-configuration penalty. V4ROOT export configuration is a deployment condition, but the supplied evidence does not justify treating it as rare enough for a scoring penalty. Important non-awarded signals: * RealUAF not awarded. The bug is a missing dput() causing a reference leak, not use-after-free. * Concrete repeated release/refcount not awarded. The patch adds a missing release, but there is no second put/free on a dangling reference. * Controlled reclaim not demonstrated. * Arbitrary write not demonstrated. * Confidentiality and integrity not awarded. The established primitive is resource/reference leakage, not disclosure or unauthorized modification. * Reliable immediate kernel crash not awarded. The supported impact is sustained resource exhaustion, not a single-request panic. * Extra availability not double-counted beyond the paranoid sustained-exhaustion point. * ImportantCandidate not set because this is a pure resource leak without memory corruption, authorization bypass, ownership violation, or unresolved lifetime corruption leverage. 3. Reachability analysis The trigger is network-facing when nfsd is running and an affected V4ROOT export is reachable by the attacker. The commit explicitly describes a crafted NFSv3 filehandle targeting the V4ROOT export fsid as sufficient to trigger the leak on every request. The attacker model is a remote NFS client with network access to the service. Host administrator privileges on the victim are not required for the request path itself. Prior host setup exists only in the sense that an administrator must have configured and exposed nfsd with the affected export layout, but that is normal service deployment rather than an attacker privilege requirement. No namespace-scoped privilege is involved in the supplied evidence. No special hardware is required. No race or timing condition is required. Repetition is cheap once the service and export are reachable. 4. Primitive and severity interpretation The exact primitive is a persistent dentry reference leak. exportfs_decode_fh_raw() obtains a dentry reference, but the V4ROOT early-exit paths fail to drop it. Since fhp->fh_dentry is not assigned, normal filehandle cleanup cannot release the reference later. This is not active structured UAF and not fixed stale storage. It is a resource/reference lifetime management bug. There is no cleanup stale dereference and no second lifetime/refcount mutation through a potentially dangling object. PrimitiveClass=CLASS_C because the demonstrated behavior is resource exhaustion / DoS-oriented without an independently useful corruption primitive. The demonstrated consequence is accumulation of leaked dentry references. The speculative consequence is host availability degradation or eventual resource exhaustion under sustained remote requests. There is no patch-supported path to privilege escalation, arbitrary read/write, or information disclosure. ImportantCandidate=NO because the bug class and patch support a resource leak only. AutoDowngradeAllowed=YES because no Class A or meaningful Class B primitive remains unresolved. Operational severity is borderline to ordinary Moderate, with manual review recommended mainly due to network reachability. 5. One-sentence report phrase A crafted NFSv3 filehandle against a V4ROOT export can make nfsd leak a dentry reference on each request, allowing a remote client with network access to the NFS service to cause sustained resource exhaustion and denial of service. 6. Manual review recommendation MANUAL CHECK RECOMMENDED Reason: the issue is network reachable in affected nfsd deployments, but the established primitive is a persistent reference leak without UAF, arbitrary write, authorization bypass, or demonstrated privilege escalation. 7. Call-site confidence Call-site confidence: high ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: nfsd: fix dentry ref leak on V4ROOT export filehandle lookup Commit: d493cf57f4c4dae5396cd6ad41e5c209d0615a81 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d493cf57f4c4dae5396cd6ad41e5c209d0615a81 Commit description: nfsd_set_fh_dentry() leaks the dentry reference from exportfs_decode_fh_raw() when the NFS3_FHSIZE or NFS_FHSIZE switch cases detect NFSEXP_V4ROOT and goto out. The out: label calls exp_put() but never dput(dentry), and fhp->fh_dentry was never assigned so fh_put() cannot compensate. A crafted NFSv3 filehandle targeting a V4ROOT export's fsid triggers the leak on every request. Fixes: ef7f6c4 ("nfsd: move V4ROOT version check to nfsd_set_fh_dentry()") Cc: stable@vger.kernel.org Assisted-by: kres:claude-opus-4-7 Signed-off-by: Jeff Layton Link: https://patch.msgid.link/20260531-nfsd-testing-v1-4-7bfa481b0540@kernel.org Signed-off-by: Chuck Lever Signed-off-by: Greg Kroah-Hartman Changed files: fs/nfsd/nfsfh.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=d493cf57f4c4dae5396cd6ad41e5c209d0615a81 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-89683 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89683 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:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS score: 7.5 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: 3 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).