From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-99255][MODERATE REGULAR] io_uring/io-wq: don't hide a link from cancellations Date: Tue, 03 Nov 2026 22:07:14 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-99255 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: 98470f164286f7f62aaee519d4d11b9370ce6d7b List-Id: CVE: CVE-2026-99255 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: io_uring/io-wq: don't hide a link from cancellations Commit: 98470f164286f7f62aaee519d4d11b9370ce6d7b Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=98470f164286f7f62aaee519d4d11b9370ce6d7b Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99255 Analysis date: Tue, 03 Nov 2026 22:07:14 -0400 ActionableScore: 3 ActionableScore lower bound: unknown Actionable bucket: Borderline / manual review recommended Manual review required: YES Summary: A local io_uring submitter can race linked-work handoff against exit cancellation and leave its exiting task permanently blocked. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-99255 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99255 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-2026-99255 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-99255 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE AEGIS_AS=0 LEGACY=MODERATE RESULT=SAME (final reconciliation agrees; DECISION: both MODERATE) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-362;CWE-667;*CWE-833;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'A race in io-wq creates a short interval where the next linked io_uring request is neither pending nor recorded as current worker work, so exit cancellation cannot find it. If io_uring_cancel_generic enters sleep after the final request free notification during this interval, the worker can later park the linked request without generating another completion or wakeup. This can leave do_exit permanently blocked and the exiting process can no longer be terminated normally. For the CVSS the PR:L value is selected because an attacker must execute local code and submit suitable io_uring linked requests. AC:H reflects the required race window. The demonstrated primitive is a permanent task hang rather than UAF, OOB access, information disclosure, or arbitrary write. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 3 with recommendation Borderline / manual review recommended. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage LOW to MODERATE_KPANIC because the manual review requirement was applied. ActionableScore thresholds stage MODERATE_KPANIC to MODERATE because the ActionableScore threshold rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK and DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4. The independent weaker Aegis shadow evaluated CVSS 4.7 and ActionableScore 0 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3) MAYBE LOCK INIT LEAK HARDWARE TEST IO_URING INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=3 PrimitiveClass=CLASS_C RealUAF=NO HostAdminRequired=NO NamespaceScopedPrivilege=NO ImportantCandidate=NO ManualReview=RECOMMENDED AutoDowngradeAllowed=YES RequiresHostUID0=NO AuthorizationDecisionBypass=NO ProtectedResourceImpact=NONE B4PendingReview=NO B4PendingConfirmed=NO 1. ActionableScore * Raw base score: 3 (+1 local unprivileged trigger, +1 high-control request sequencing, +1 persistent task hang, +1 broadly exposed io_uring path, -1 demonstrated narrow race). * Conservative score: 3; no floor or cap applied. * Raw paranoid score: 3; no additional non-crash primitive is technically supported. * Paranoid score: 3. * Final recommended bucket: **Borderline / manual review recommended**:: 2. Signal breakdown * +1 Local unprivileged trigger: a local process can submit the linked NOP/FUTEX_WAIT sequence and exit. * +1 High-control API: the submitter controls linked-request ordering and asynchronous execution relevant to the visibility gap. * +1 Availability impact: the demonstrated result is a permanently blocked `do_exit`; the task can no longer be killed normally. * +1 Broad exposure: io_uring is broadly deployed and the affected path uses ordinary linked asynchronous requests. * -1 Hard race: evidence gives a window of up to roughly 63 microseconds and only 5–8 hangs per 10,000 rounds. * No UAF, OOB access, overwrite, stale-object consumption, disclosure, authorization bypass, or integrity primitive is shown. * No extra availability point: only the same task-exit hang is established; host-wide failure or persistent resource exhaustion is not shown. * NEW52/B5-R does not apply: there is no retry-accumulated heap OOB write or later consumption of corrupted structured state. * NEW53/B3-PENDING and NEW54/B4-PENDING do not apply. 3. Reachability analysis The demonstrated actor is an ordinary local process controlling its own io_uring requests; host UID 0, capabilities, namespace privileges, prior privileged setup, and special hardware are not required. The race is repeatable but statistically infrequent. No remote trigger or cross-user effect is established. 4. Primitive and severity reading The primitive is a cancellation-visibility race: the linked request is temporarily neither pending nor recorded as current work, so exit cancellation sleeps and misses the final wakeup. This is CLASS_C synchronization/DoS behavior, not a lifetime or memory-corruption primitive. The shown effect is an unkillable exiting task; kernel panic, host-wide denial of service, confidentiality/integrity impact, and privilege escalation remain unproven. ImportantCandidate is therefore NO, and auto-downgrade is allowed. 5. One-sentence report phrase A local io_uring submitter can race linked-work handoff against exit cancellation and leave its exiting task permanently blocked. 6. Manual review advice MANUAL CHECK RECOMMENDED Confirm whether product-specific io_uring restrictions or broader task/resource retention materially change operational impact. 7. Call-site confidence Call-site confidence: high ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: io_uring/io-wq: don't hide a link from cancellations Commit: 98470f164286f7f62aaee519d4d11b9370ce6d7b Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=98470f164286f7f62aaee519d4d11b9370ce6d7b Commit description: A task that exits cancels its tracked requests and waits until they are gone. io_uring_cancel_generic() looks everywhere a request can be, and goes to sleep if it found nothing to cancel. It is woken up each time one of the requests of the task is completed by task work or freed. When io-wq is done with a request that has a link, there is a time in which the next request of the link can't be found: io_worker_handle_work() io_assign_current_work(worker, NULL) io_wq_free_work(work) io_req_find_next() // takes the link off the request io_free_req() // task work for the task io_assign_current_work(worker, linked) io_run_task_work() cond_resched() worker->cur_work = linked It is not pending, it is not the current work of a worker, and it is not yet on the list that it is about to be parked on. If the pass that the exiting task makes because of io_free_req() falls into that time, it finds nothing and the task goes to sleep. The worker then runs the link, which IO_WQ_BIT_EXIT doesn't prevent for bounded work. If that parks the request nothing is completed or freed, and nobody wakes up the task. If it is the process that exits it can't be killed any more either. NOP, IOSQE_ASYNC | IOSQE_IO_LINK FUTEX_WAIT, FUTEX2_PRIVATE exit of the thread, or of the process, up to 63us later With four of these per round 5 to 8 of 10,000 rounds hang (4 CPUs, KVM): io_uring_cancel_generic do_exit Commit f8b632e ("io_uring: tighten task exit cancellations") is about the same gap, and makes the task look again for whatever happens before io_free_req(). Free the request once the link is the current work of the worker, or pending. Whoever is woken up by that can find the link, until it is parked where cancellations look as well. To do so split io_wq_free_work() in two. [ 6.1: io-wq calls back through struct io_wq_data, so the split is a new ->put_work callback next to ->free_work, and io_put_req_find_next() is folded into io_wq_put_work(). io_wq_put_work() keeps the link check of 6.1, without the REQ_F_CQE_SKIP condition that commit 5530a51 adds in 7.3 ] Fixes: 768134d ("io_uring: don't do flush cancel under inflight_lock") Assisted-by: LLM Signed-off-by: Josef Bacik Link: https://patch.msgid.link/20261015-b4-io-uring-fixes-v1-5-c1556f5d7bf5@toxicpanda.com Signed-off-by: Jens Axboe Co-Authored-By: Claude Fable 5.1 Signed-off-by: Greg Kroah-Hartman Changed files: io_uring/io-wq.c io_uring/io-wq.h io_uring/io_uring.c io_uring/io_uring.h io_uring/tctx.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=98470f164286f7f62aaee519d4d11b9370ce6d7b ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-99255 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99255 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:L/UI:N/S:U/C:N/I:N/A:L The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS score: 4.7 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).