From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-89696][MODERATE REGULAR] nfsd: block non-SAVEFH ops after FOREIGN PUTFH to prevent NULL deref Date: Sat, 12 Sep 2026 04:46:53 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-89696 X-AL-KERNEL-Priority: MODERATE REGULAR X-AL-KERNEL-Severity: MODERATE REGULAR X-AL-KERNEL-Base-Severity: MODERATE X-AL-KERNEL-KPANIC: NO X-AL-KERNEL-ActionableScore: 3 X-AL-KERNEL-ActionableScore-Lower: unknown X-AL-KERNEL-Commit: 35f248bd40b47b229d4999581df45b97daadd977 List-Id: CVE: CVE-2026-89696 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: nfsd: block non-SAVEFH ops after FOREIGN PUTFH to prevent NULL deref Commit: 35f248bd40b47b229d4999581df45b97daadd977 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=35f248bd40b47b229d4999581df45b97daadd977 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89696 Analysis date: Sat, 12 Sep 2026 04:46:53 -0400 ActionableScore: 3 ActionableScore lower bound: unknown 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-2026-89696 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89696 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 REGULAR 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-2026-89696 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89696 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-476;*CWE-20;CWE-754;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.5';DESCR 'A NULL pointer dereference in the NFSD NFSv4.2 inter-SSC path can occur because a FOREIGN PUTFH may leave fh_dentry and fh_export unset while the compound dispatcher still allows later operations to run. A remote NFS client can trigger this with a crafted COMPOUND that reaches a stale foreign filehandle and then executes a non SAVEFH operation such as SETATTR before the foreign filehandle is consumed correctly. For the CVSS the PR:N is used for the paranoid score because a reachable NFS endpoint may allow a remote protocol client to send the malformed compound without local access to the victim host. The issue is network reachable when NFSv4.2 with inter-SSC support is enabled and exposed to the attacker. Impact is denial of service via NULL pointer dereference in an nfsd kernel thread. NFS servers are typically deployed on internal storage or data center networks, so practical exploitation usually requires access to a trusted or local network where the NFS service is reachable.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3) YES WRITE OOB NULLPTR DISK NETWORK INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 YES NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=3 PrimitiveClass=CLASS_C RealUAF=NO HostAdminRequired=NO NamespaceScopedPrivilege=NO ImportantCandidate=NO ManualReview=REQUIRED AutoDowngradeAllowed=NO 1. ActionableScore * Raw conservative score: 3 * Conservative score: 3. No floor or cap applied. * Raw paranoid score: 3 * Paranoid score: 3. No floor or cap applied. * Final recommended bucket: **Borderline / manual review recommended**:: 2. Signal breakdown Positive signals: * +2 Remote reachable / directly network-triggerable. The supplied evidence states that a remote NFS client can craft an NFSv4.2 COMPOUND involving inter-SSC COPY and a non-SAVEFH operation after FOREIGN PUTFH. * +1 Reliable kernel crash / strong DoS. The vulnerable path can dereference NULL fh_export or fh_dentry related state in an nfsd kernel thread, for example through SETATTR calling fh_want_write. Negative signals: * No negative privilege penalty. Host-admin-equivalent privilege is not required for the remote client trigger described in the input. * No rare-configuration penalty applied. CONFIG_NFSD_V4_2_INTER_SSC is a relevant prerequisite, but a kernel config option alone is not enough to subtract points under the rules. Important non-awarded signals: * RealUAF not awarded. The primitive is NULL pointer dereference, not stale post-lifetime object use. * CLASS_A not awarded. No authorization bypass, protected resource access, controlled corruption, COW violation, or demonstrated LPE is established. * CLASS_B not awarded. No UAF, RCU lifetime mismatch, double release, refcount mutation, OOB write, or active stale-object corruption is established. * Controlled reclaim not demonstrated. * Arbitrary or directed kernel write not demonstrated. * Confidentiality and integrity not awarded. The evidence supports crash only, not information disclosure or unauthorized modification. * Extra Availability not awarded. The same NULL dereference crash is already counted as the DoS signal. * Practical exploit chain bonus not awarded. The demonstrated chain is trigger to crash only, and that crash is already counted. 3. Reachability analysis The evaluated attacker is a remote NFS client that can send NFSv4.2 COMPOUND operations to the affected nfsd service. The input explicitly describes a remotely crafted compound request, so HostAdminRequired is NO for the victim host. The vulnerable configuration requires NFSD with NFSv4.2 inter-SSC support enabled and reachable. If the NFS service is not exposed to the attacker or inter-SSC support is not present, practical exposure is reduced. This is a deployment prerequisite, not a host-admin requirement for the attacker once the service is reachable. No namespace-scoped privilege issue is involved. The trigger is network protocol processing in nfsd, not a namespace-created local object. NamespaceScopedPrivilege is therefore NO. No race or timing requirement is established. The issue is an invalid protocol-state sequence that leaves a FOREIGN filehandle unresolved while later operations are still dispatched. 4. Primitive and severity interpretation The exact primitive is a NULL pointer dereference caused by allowing non-SAVEFH operations to execute after a FOREIGN PUTFH when current_fh->fh_dentry is NULL. The described SETATTR path can reach fh_want_write and dereference fh_export state without first resolving or verifying the filehandle. This is CLASS_C because the demonstrated behavior is invalid-state processing leading to a NULL dereference and DoS. It is not an active structured UAF, not fixed stale storage, and not a concrete repeated release/refcount mutation. The demonstrated consequence is remote denial of service against the NFS server kernel thread, potentially becoming a host-impacting kernel fault depending on kernel oops and panic policy. The speculative consequence beyond crash is weak because there is no evidence of attacker-controlled memory corruption, stale object reuse, or security-boundary bypass. ImportantCandidate is NO because no Important-preserving primitive is established. ManualReview is REQUIRED because this is a directly network-reachable kernel NULL dereference in a server subsystem and deployment exposure should be checked. AutoDowngradeAllowed is NO because automatic closure could miss exposed NFSv4.2 inter-SSC deployments even though the primitive remains crash-only. 5. One-sentence report phrase A remote NFSv4.2 client can trigger a NULL pointer dereference in nfsd by sending a malformed inter-SSC COMPOUND sequence after FOREIGN PUTFH, causing denial of service when inter-SSC support is enabled and reachable. 6. Manual review recommendation MANUAL CHECK REQUIRED Reason: the issue is network reachable in affected NFSD inter-SSC deployments and can crash kernel-side nfsd processing, although no memory corruption or privilege escalation primitive is established. 7. Call-site confidence Call-site confidence: high ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: nfsd: block non-SAVEFH ops after FOREIGN PUTFH to prevent NULL deref Commit: 35f248bd40b47b229d4999581df45b97daadd977 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=35f248bd40b47b229d4999581df45b97daadd977 Commit description: When CONFIG_NFSD_V4_2_INTER_SSC is enabled, nfsd4_putfh() can return success with fh_dentry and fh_export both NULL if fh_verify() returns nfserr_stale and putfh->no_verify is true. The NFSD4_FH_FOREIGN flag is set, but the compound dispatch loop only uses this flag to bypass the nfserr_nofilehandle check -- it does not prevent subsequent ops from running with a NULL fh_dentry. A remote client can exploit this by crafting a COMPOUND that includes an inter-SSC COPY (which causes check_if_stalefh_allowed() to set no_verify=true on the saved PUTFH) with an additional op inserted between the source PUTFH and SAVEFH. For example, SETATTR calls fh_want_write() which dereferences fh_export->ex_path.mnt without calling fh_verify() first, causing a NULL pointer dereference in the nfsd kthread. Fix this by gating the dispatch loop: when NFSD4_FH_FOREIGN is set and fh_dentry is NULL, only OP_SAVEFH (needed for the inter-SSC flow) and ops with ALLOWED_WITHOUT_FH (which don't need a resolved filehandle) may proceed. All other ops receive nfserr_stale, per RFC 7862 Section 15.2.3 which specifies that foreign filehandle validation is deferred to the consuming operation and NFS4ERR_STALE returned at that point. Fixes: b9e8638 ("NFSD: allow inter server COPY to have a STALE source server fh") Cc: stable@vger.kernel.org Assisted-by: Claude:claude-opus-4-6 Signed-off-by: Jeff Layton Link: https://patch.msgid.link/20260527-putfh_foreign_fh_null_deref_consumers-v1-1-1b8a5aa28c59@kernel.org Signed-off-by: Chuck Lever Signed-off-by: Greg Kroah-Hartman Changed files: fs/nfsd/nfs4proc.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=35f248bd40b47b229d4999581df45b97daadd977 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-89696 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89696 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:H/PR:L/UI:N/S:U/C:N/I:N/A:H 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: unknown Paranoid ActionableScore: 3 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: NO Published priority for this report (same as in Subject): MODERATE REGULAR 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).