From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-74658][MODERATE REGULAR] futex: Prevent robust futex exit race some more Date: Sat, 22 Aug 2026 18:20:04 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-74658 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: 2 X-AL-KERNEL-Commit: 83b0f71d5a313a765754acab51d2ecc5de76e0b9 List-Id: CVE: CVE-2026-74658 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: futex: Prevent robust futex exit race some more Commit: 83b0f71d5a313a765754acab51d2ecc5de76e0b9 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=83b0f71d5a313a765754acab51d2ecc5de76e0b9 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74658 Analysis date: Sat, 22 Aug 2026 18:20:04 -0400 ActionableScore: 3 ActionableScore lower bound: 2 Actionable bucket: Ordinary Moderate / Low-like, with borderline manual-review only for critical futex users Manual review required: NO Summary: A robust futex exit race can lose a one shot wakeup when a woken waiter exits before acquiring the futex or rearming FUTEX_WAITERS, allowing remaining waiters to sleep indefinitely and causing a local denial of service. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-74658 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74658 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: NO A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-74658 MODERATE 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: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 robust futex exit handling can lose a one shot wakeup when a woken waiter dies before it can acquire the futex or re arm FUTEX_WAITERS. A third thread may acquire the futex through the uncontended fast path and later unlock it without seeing FUTEX_WAITERS, leaving other waiters sleeping forever behind a free futex. For the CVSS the PR:L is used because reliable triggering requires local code execution in a process that can participate in or share the affected futex state. The issue is not directly network reachable and is triggered through local futex synchronization behavior rather than packet traffic. Impact is denial of service through a hang of affected threads or services. No confidentiality or integrity impact is assigned because the patch indicates a lost wakeup logic race and does not show a memory corruption primitive.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like, with borderline manual-review only for critical futex users (with actual score 3) YES LOCK RACE LINUS DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=3 ActionableScoreLower=2 ## 1. ActionableScore * Conservative score: 2 * Paranoid score: 3 * Final recommended bucket: **Ordinary Moderate / Low-like, with borderline manual-review only for critical futex users** ## 2. Signal breakdown Conservative signals: * Local unprivileged trigger: +1 Robust futexes are reachable by local unprivileged processes using ordinary futex synchronization. * Availability impact realistic: +1 The bug can leave waiters sleeping forever, causing a local application or service hang. * Broad/default/common subsystem: +1 Futexes are a core Linux synchronization primitive used broadly. * Hard or unreliable race / special timing required: -1 The scenario requires a specific interleaving: one waiter is woken, another task acquires the futex through the fast path, and the woken waiter exits before rearming FUTEX_WAITERS. Paranoid adjustment: * Paranoid score adds practical service-impact concern: +1 If the affected robust futex is inside a critical multi-threaded service, the lost wakeup can produce a persistent service-level DoS. This does not imply kernel memory corruption or privilege escalation. Not counted: * No generic memory corruption. The patch fixes a lost wakeup and FUTEX_WAITERS consistency issue, not UAF, OOB, double-free, refcount misuse, or type confusion. * No privilege escalation plausible signal. There is no supported primitive for controlled overwrite, stale object reuse, callback control, or credential manipulation. * No remote reachability. This is a local synchronization path, not packet-triggered. * No confidentiality or integrity impact. The demonstrated consequence is waiters sleeping forever. ## 3. Reachability analysis A local unprivileged process can create robust futex users and attempt to trigger the interleaving with multiple threads and task exit. Containers and namespaces do not fundamentally remove futex reachability, but the impact is normally confined to processes or services sharing the affected futex state. The path is default-enabled because futex support is core kernel functionality. Realistic exploitation requires local code execution and timing control, but no elevated capability is required. ## 4. Severity interpretation This behaves like an ordinary Moderate or Low-like local DoS rather than an Important-class kernel vulnerability. The issue is a race and lost-wakeup logic flaw that can hang waiters, but it does not show memory corruption, kernel crash, privilege escalation, or cross-boundary data access. The paranoid score is slightly higher only because hangs in critical services can be operationally significant. ## 5. One-sentence report phrase A robust futex exit race can lose a one shot wakeup when a woken waiter exits before acquiring the futex or rearming FUTEX_WAITERS, allowing remaining waiters to sleep indefinitely and causing a local denial of service. ## 6. Manual review recommendation NO MANUAL CHECK NEEDED This can usually be auto-closed for urgent security triage because the patch supports local DoS only, with no memory corruption or privilege escalation primitive. Manual review is only useful if the product relies on robust futexes inside critical host services where service deadlock has elevated operational impact. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: futex: Prevent robust futex exit race some more Commit: 83b0f71d5a313a765754acab51d2ecc5de76e0b9 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=83b0f71d5a313a765754acab51d2ecc5de76e0b9 Commit description: A robust futex unlock stores 0 over the whole futex value - wiping FUTEX_WAITERS - and wakes a single waiter. That wakeup is a one-shot notification: the protocol relies on its recipient to either acquire the futex (and eventually unlock while aware of the remaining contention) or re-arm FUTEX_WAITERS before sleeping again. If the woken waiter is killed before it can do either, the kernel must jump in and wake the next task down the line. This is a known complication of the futex protocol with a previous partial fix in commit ca16d5b ("futex: Prevent robust futex exit race"). Unfortunately, that fix is insufficient. If a third task re-acquired the futex through the uncontended fast path in the meantime, the notification is lost: robust exit processing sees that it is owned by another task and does nothing, while the new owner sees no FUTEX_WAITERS when it unlocks and wakes nobody. The remaining waiters sleep forever behind a free futex: A owns the futex, B and C sleep in FUTEX_WAIT uval == A | FUTEX_WAITERS A robust unlock: store 0, FUTEX_WAKE(1) wakes B uval == 0 D fast path acquire: cmpxchg(0 -> D) uval == D, no FUTEX_WAITERS B killed before acting on the wakeup B exit walk, pending op: owner D != B -> no action D unlock: no FUTEX_WAITERS -> no wake C sleeps forever This is clearly a shortcoming in the implementation, which fails to keep the FUTEX_WAITERS bit consistent. Work around this by augmenting the robust list exit processing to also perform the extra wakeup if the futex word is owned by another thread but FUTEX_WAITERS is not set. This does not fix the problem of a non-contended take over/release and free sequence, which has been discussed for years and has been addressed by commit 3ca9595 ("futex: Add support for unlocking robust futexes") and subsequent changes, but failed to take the problem described above into account. A more complete solution which is based on the in kernel unlock of contended robust futexes has been discussed in the context of this change and should show up in mainline sooner than later. [ tglx: Amend change log slightly and fixup coding style ] Fixes: ca16d5b ("futex: Prevent robust futex exit race") Signed-off-by: Keno Fischer Signed-off-by: Thomas Gleixner Signed-off-by: Ingo Molnar Signed-off-by: Thomas Gleixner Assisted-by: ClaudeCode:claude-fable-5 tla+ Cc: stable@vger.kernel.org Link: https://patch.msgid.link/20260730194705.38981-1-keno@juliacomputing.com Signed-off-by: Sasha Levin Changed files: kernel/futex/core.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=83b0f71d5a313a765754acab51d2ecc5de76e0b9 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-74658 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74658 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: 2 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).