From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-89771][MODERATE REGULAR] ring-buffer: Fix subbuf resize race with ring buffer readers [ Upstream Date: Sat, 12 Sep 2026 03:59:29 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-89771 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: 6d666f0b8b36c0765cf19fbe6de5a7ba5a73aad9 List-Id: CVE: CVE-2026-89771 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: ring-buffer: Fix subbuf resize race with ring buffer readers [ Upstream Commit: 6d666f0b8b36c0765cf19fbe6de5a7ba5a73aad9 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6d666f0b8b36c0765cf19fbe6de5a7ba5a73aad9 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89771 Analysis date: Sat, 12 Sep 2026 03:59:29 -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-2026-89771 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89771 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-2026-89771 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89771 CHECK WITH IMPACT FROM ORIG NN LOW Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-362;CWE-367;*CWE-664;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'A race in the tracing ring buffer reader path can occur because trace_buffer::subbuf_size was read without locking while ring_buffer_subbuf_order_set could resize the underlying sub buffers. A concurrent reader may calculate lengths from stale size information while the real buffer page has a different order, which can lead to invalid buffer access, kernel warning, or crash. For the CVSS the PR:L is used in the paranoid score because some deployments may delegate tracing or tracefs access to a non root service or tracing group, even though normal buffer resizing is administrative. The issue is not network reachable and requires local interaction with tracing control plane operations. Impact is at least local denial of service via kernel crash or lockup. In worst case it may allow limited confidentiality or integrity impact if stale size metadata is used to access data past the real sub buffer boundary.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3) SKIP CVE-2026-89771 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES NO NO unknown MAYBE TRACE RACE LINUS INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=3 PrimitiveClass=CLASS_C RealUAF=NO HostAdminRequired=UNKNOWN NamespaceScopedPrivilege=NO ImportantCandidate=NO ManualReview=RECOMMENDED AutoDowngradeAllowed=YES 1. ActionableScore * Raw conservative score: 3 * Conservative score: 3 * Raw paranoid score: 3 * Paranoid score: 3 * Final recommended bucket: **Borderline / manual review recommended**:: No floor or cap was applied. 2. Signal breakdown Positive signals: * +1 Weak / indirect corruption candidate. `trace_buffer::subbuf_size` was read locklessly while the backing sub-buffer order could be changed, so readers could compute sizes from stale metadata. * +1 Memory layout invariant restoration. The patch restores consistency between the actual buffer page order and the size or capacity used by reader paths. * +1 Reliable kernel crash / strong DoS concern. The stale size race can plausibly lead to invalid buffer access, warning, crash, or lockup in tracing ring-buffer reader paths. Non-awarded signals: * +0 Real UAF not awarded. The supplied patch text establishes a race on size metadata, not an object lifetime ending while a stale reference is reused. * +0 Concrete repeated release/refcount not awarded. No first release plus second lifetime mutation on the same dangling reference is shown. * +0 Structured stale-object leverage not awarded. The issue is stale sizing metadata, not a demonstrated stale active object consumed after reuse. * +0 Controlled reclaim not demonstrated. * +0 Arbitrary or directed kernel write not demonstrated. * +0 Confidentiality and integrity signals not awarded. Limited C/I appears possible only as a CVSS-paranoid interpretation of stale size use, but no disclosure path or unauthorized write primitive is established. * +0 Extra Availability not awarded. The same crash or lockup is already counted once. * +0 Remote reachability not established. * -0 Race penalty not applied. The bug is a race, but the evidence does not prove a narrow one-shot or externally uncontrollable timing window. 3. Reachability analysis The trigger requires local interaction with the tracing ring-buffer control plane: one path resizes sub-buffers through `ring_buffer_subbuf_order_set()`, while another path reads through `ring_buffer_read_start()` or `ring_buffer_read_page()`. Ordinary unprivileged reachability is not established. Tracing and tracefs control operations are commonly restricted to root, `CAP_SYS_ADMIN`, or specifically delegated tracing permissions, but the supplied input does not positively prove host-admin-equivalent privilege for the complete trigger chain. Therefore `HostAdminRequired=UNKNOWN`. No namespace-scoped complete trigger is established. This is not a network namespace or container-local networking operation. `NamespaceScopedPrivilege=NO`. There is no network packet-processing path and no remote trigger. Prior tracing setup or access to tracefs may be required. No special hardware is required. Race repeatability is plausible for a local actor that can repeatedly resize and read tracing buffers, but below-host-admin access is unresolved. 4. Primitive and severity interpretation The exact primitive is a race between lockless reader-side use of global sub-buffer size metadata and concurrent resizing of the underlying ring-buffer pages. This is not a demonstrated UAF. It is not a concrete double release. It is not a controlled overwrite. It is best classified as `CLASS_C`: invalid-state or stale-metadata behavior with likely DoS impact. The demonstrated consequence is local kernel warning, invalid access, crash, or lockup. The speculative consequence is limited out-of-bounds style behavior from stale size calculations, but the supplied evidence does not establish controlled readback, attacker-directed write, controlled reclaim, or privilege escalation. `ImportantCandidate=NO` because no Important-grade primitive is established. `AutoDowngradeAllowed=YES` because the issue appears to be a constrained local race with DoS-style impact and no unresolved UAF, repeated refcount mutation, authorization bypass, COW violation, or controlled corruption primitive. Manual review is still recommended because stale size metadata in a kernel ring-buffer reader path can be subtle. 5. One-sentence report phrase A local race in the tracing ring-buffer reader path can use stale sub-buffer size metadata during concurrent resize, leading to invalid access or kernel DoS, but no concrete UAF, controlled corruption, or privilege-escalation primitive is established. 6. Manual review recommendation MANUAL CHECK RECOMMENDED Reason: the patch touches kernel tracing buffer sizing and reader synchronization, and stale size metadata may affect memory access boundaries, but the supplied evidence supports DoS-oriented behavior rather than a confirmed stronger primitive. 7. Call-site confidence Call-site confidence: medium ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: ring-buffer: Fix subbuf resize race with ring buffer readers [ Upstream Commit: 6d666f0b8b36c0765cf19fbe6de5a7ba5a73aad9 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6d666f0b8b36c0765cf19fbe6de5a7ba5a73aad9 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=6d666f0b8b36c0765cf19fbe6de5a7ba5a73aad9 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-89771 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89771 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:H/PR:H/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H The Best / paranoid CVSS score: 5.8 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).