From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-89695][MODERATE 7.0] nfsd: cap decoded POSIX ACL count to bound sort cost Date: Sat, 12 Sep 2026 04:26:25 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-89695 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: ea14d71d6ecb925673761bcf79f781f7dc9042cc List-Id: CVE: CVE-2026-89695 Priority: MODERATE 7.0 AL-KERNEL base severity: MODERATE KPANIC flag: YES Patch: nfsd: cap decoded POSIX ACL count to bound sort cost Commit: ea14d71d6ecb925673761bcf79f781f7dc9042cc Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ea14d71d6ecb925673761bcf79f781f7dc9042cc Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89695 Analysis date: Sat, 12 Sep 2026 04:26:25 -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-2026-89695 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89695 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-2026-89695 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89695 CHECK WITH IMPACT FROM ORIG NN LOW 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-400;CWE-770;CWE-407;Other CVSS 'AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.5';DESCR 'nfsd4_decode_posixacl accepted a client chosen POSIX ACL entry count from the wire without enforcing NFS_ACL_MAX_ENTRIES. The count is used for allocation and then passed to sort_pacl_range which performs O(n2) sorting, so a crafted NFSv4 compound request can force excessive CPU work in the server processing path. For the CVSS the PR:N is used for the paranoid score because an exposed nfsd endpoint may process the malformed request from a network client before any meaningful authorization decision limits the decode cost. The base score uses PR:L because many real deployments require access to an exported filesystem or valid NFS client credentials. The issue is network reachable when NFSv4 POSIX ACL support is enabled and reachable by the attacker. Impact is denial of service through CPU exhaustion or request processing stalls. No concrete memory corruption primitive is indicated by the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4) SKIP CVE-2026-89695 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES NO NO unknown MAYBE DISK NETWORK HARDWARE 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 * Raw paranoid score: 4 * Paranoid score: 4 * Final recommended bucket: **Borderline / manual review recommended**:: No floor or cap changed the conservative score. No floor or cap changed the paranoid score. 2. Signal breakdown Conservative positive signals: * +2 Remote reachable / directly network-triggerable: the decoded POSIX ACL count is read from NFS XDR data supplied by a network client to nfsd. * +1 Reliable strong DoS: a client-controlled count can drive O(n2) CPU cost in sort_pacl_range during compound request processing. Conservative negative signals: * No negative privilege penalty: host-admin-equivalent privilege is not required for the remote client side of the trigger. * No rare-subsystem penalty: NFS server exposure depends on deployment, but the supplied evidence does not establish a genuinely rare or difficult-to-reach configuration. Paranoid additional signal: * +1 Additional resource-pressure concern: the uncapped count is also passed to posix_acl_alloc before validation, so large requests may combine CPU amplification with allocation pressure. This is still resource exhaustion, not memory corruption. Non-awarded signals: * RealUAF not awarded: no use-after-free or stale lifetime object use is established. * Controlled reclaim not demonstrated. * Arbitrary write not demonstrated. * Confidentiality and integrity not awarded: no disclosure, protected-object read, unauthorized write, or ownership violation is established. * Important-grade exploitation leverage not awarded: this is algorithmic complexity and allocation pressure, not a kernel memory corruption primitive. * Extra availability beyond the paranoid allocation-pressure point is not awarded. * Concrete resource/reference leak not awarded: the patch prevents excessive allocation but does not show a persistent leaked reference. * Security-boundary bypass not awarded: no authorization ALLOW-vs-DENY change is shown. 3. Reachability analysis The trigger is a network-supplied NFSv4 POSIX ACL decode path in nfsd. A remote NFS client that can reach the server and submit the relevant compound operation can influence the ACL entry count. The conservative interpretation assumes the attacker may need valid NFS client access or credentials for the relevant export, but not host administrator privilege. HostAdminRequired=NO because the attacker is a remote client, not a local host administrator. NamespaceScopedPrivilege=NO because namespace-scoped local capabilities are not material to this network server path. There is prior privileged setup in the operational sense that an administrator must run nfsd and expose NFS service, but this is normal service deployment rather than a privilege requirement for the attacker. No special hardware, race, timing window, or local-only interface is required. The vulnerability is not public Internet by default in many environments, but NFS is commonly deployed on internal storage or data-center networks. 4. Primitive and severity interpretation The exact primitive is attacker-controlled algorithmic complexity plus possible allocation pressure: nfsd4_decode_posixacl accepts an uncapped u32 count from the wire, allocates based on it, and then passes it into an O(n2) sort. PrimitiveClass=CLASS_C because the demonstrated behavior is DoS/resource exhaustion without an independently useful corruption, lifetime, ownership, authorization, or semantic-reinterpretation primitive. The demonstrated consequence is denial of service through excessive CPU consumption and request processing stalls. The paranoid consequence is stronger availability impact if allocation pressure combines with the O(n2) sorting cost. No practical privilege escalation, arbitrary kernel read/write, information disclosure, or security-boundary bypass is supported by the supplied patch. ImportantCandidate=NO because there is no memory corruption, UAF, repeated lifetime mutation, COW ownership violation, or authorization bypass requiring Important-vs-Moderate exploitation review. AutoDowngradeAllowed=YES because the unresolved risk is resource exhaustion only, although manual review is still recommended due to network reachability. 5. One-sentence report phrase A network client can supply an excessive POSIX ACL entry count to nfsd4_decode_posixacl, causing unbounded O(n2) sorting work and possible allocation pressure in the NFS server compound path, resulting in denial of service. 6. Manual review recommendation MANUAL CHECK RECOMMENDED Reason: the issue is network reachable in nfsd and may produce high availability impact on exposed NFS servers, but the patch supports resource exhaustion only and does not establish memory corruption or privilege escalation. 7. Call-site confidence Call-site confidence: high ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: nfsd: cap decoded POSIX ACL count to bound sort cost Commit: ea14d71d6ecb925673761bcf79f781f7dc9042cc Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ea14d71d6ecb925673761bcf79f781f7dc9042cc Commit description: nfsd4_decode_posixacl() reads a u32 entry count off the wire and passes it straight to posix_acl_alloc() and sort_pacl_range(). The latter is an O(n^2) bubble sort, so a client-chosen count drives unbounded CPU in the server's compound processing path. nfsd4_decode_posixacl() xdr_stream_decode_u32(&count) /* uncapped u32 */ posix_acl_alloc(count, GFP_KERNEL) sort_pacl_range(*acl, 0, count - 1) /* O(n^2) bubble sort */ The encoder side in the same file already rejects ACLs whose a_count exceeds NFS_ACL_MAX_ENTRIES, but the decoder introduced in commit 5fc51df ("NFSD: Add support for XDR decoding POSIX draft ACLs") omitted the symmetric check. Fix by rejecting a wire count greater than NFS_ACL_MAX_ENTRIES with nfserr_inval, before any allocation, so the sort is bounded by NFS_ACL_MAX_ENTRIES^2 comparisons. While we're in here, also fix the nfserr_resource return if posix_acl_alloc() fails. That's not a legal error code for v4.1+. Change it to return nfserr_jukebox as that's more appropriate for memory allocation failures. Fixes: 5fc51df ("NFSD: Add support for XDR decoding POSIX draft ACLs") Cc: stable@vger.kernel.org Assisted-by: kres:claude-opus-4-7 Reported-by: Chris Mason Signed-off-by: Chris Mason Link: https://patch.msgid.link/20260530-nfsd-fixes-v2-8-f27e8eb4d974@kernel.org Signed-off-by: Chuck Lever Signed-off-by: Greg Kroah-Hartman Changed files: fs/nfsd/nfs4xdr.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=ea14d71d6ecb925673761bcf79f781f7dc9042cc ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-89695 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89695 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: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: 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).