From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-64374][MODERATE REGULAR] sched/rt: Have RT_PUSH_IPI be default off for non PREEMPT_RT Date: Sat, 25 Jul 2026 10:50:40 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-64374 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: 1 X-AL-KERNEL-Commit: b99f04ae3d200d2f8844aa29145bd18eccbeecde List-Id: CVE: CVE-2026-64374 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: sched/rt: Have RT_PUSH_IPI be default off for non PREEMPT_RT Commit: b99f04ae3d200d2f8844aa29145bd18eccbeecde Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b99f04ae3d200d2f8844aa29145bd18eccbeecde Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64374 Analysis date: Sat, 25 Jul 2026 10:50:40 -0400 ActionableScore: 3 ActionableScore lower bound: 1 Actionable bucket: Borderline, manual review recommended Manual review required: YES Summary: RT_PUSH_IPI on non PREEMPT_RT kernels can cause a CPU livelock when heavy softirq activity overlaps with RT task wakeups and migration IPIs, leading to a local availability-only DoS without evidence of memory corruption or privilege escalation. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-64374 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64374 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-2026-64374 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-400;*CWE-835;CWE-667;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.7';DESCR 'RT_PUSH_IPI can cause a livelock on non PREEMPT_RT kernels when heavy softirq activity overlaps with repeated RT task wakeups and RT migration IPIs. A target CPU can remain marked overloaded while softirq execution delays scheduling, causing other CPUs to keep sending IPIs and further slowing progress. For the CVSS the PR:L is used for the paranoid score because some deployments delegate realtime scheduling capability to local users, containers, or service accounts through RLIMIT_RTPRIO or CAP_SYS_NICE like privileges. The issue is not directly network reachable, although heavy network traffic can contribute to the softirq condition. Impact is denial of service through CPU livelock and severe scheduling latency, with no evidence of memory corruption, information disclosure, or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 2) SKIP CVE-2026-64374 UNKNOWN SKIP No affected files built, so skip this CVE NO - - unknown MAYBE HARDWARE LINUS INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=3 ActionableScoreLower=1 ## 1. ActionableScore * Conservative score: 1 * Paranoid score: 3 * Final recommended bucket: **Borderline, manual review recommended** ## 2. Signal breakdown Conservative signals: * Reliable kernel crash / strong DoS: +1. The commit describes a CPU livelock where repeated RT migration IPIs can prevent progress. * Availability impact realistic: +1. The observed impact is severe scheduling latency or CPU livelock under heavy softirq and RT wakeup load. * Broad/default/common subsystem: +1. This is scheduler RT migration logic, and the affected RT_PUSH_IPI feature was default enabled on non PREEMPT_RT before the patch. * Hard or unreliable race / special timing required: -1. Triggering requires a specific combination of heavy networking softirq load, queued RT tasks, large CPU systems, and timing. * Requires admin/root/CAP_* in typical deployments: -1 to -2. Reliable triggering usually requires RT scheduling privileges such as CAP_SYS_NICE or configured RLIMIT_RTPRIO. Paranoid interpretation: * Local unprivileged trigger: +1. Some systems delegate realtime scheduling to non-root users or service accounts via limits or capabilities. * Reliable kernel crash / strong DoS: +1. * Availability impact realistic: +1. * Broad/default/common subsystem: +1. * Hard timing / workload dependency: -1. No memory corruption, UAF, OOB write, information leak, or privilege escalation primitive is supported by the patch. ## 3. Reachability analysis The bug is locally influenced through realtime scheduling behavior plus heavy softirq activity, especially networking traffic. It is not directly network reachable, because packets alone do not appear sufficient without local RT task wakeups or a workload that creates the RT scheduling pressure. Typical exploitation requires privileges to create or run RT tasks, commonly CAP_SYS_NICE or an administrator configured realtime limit. Containers or service accounts may reduce the practical privilege requirement if RT scheduling is delegated. The path was default enabled for non PREEMPT_RT kernels affected by RT_PUSH_IPI, but realistic triggering depends on large systems, heavy networking, and RT workloads. Call-site confidence: medium. The commit explains the scheduler behavior and observed livelock, but the patch is a feature-default change rather than a direct memory-safety fix. ## 4. Severity interpretation This behaves like a DoS-only scheduler livelock, not an Important-class memory corruption issue. The realistic impact is availability loss through CPU livelock or severe latency. There is no evidence of confidentiality impact, integrity impact, privilege escalation, stale pointer reuse, or attacker-controlled memory corruption. This is more than an ordinary cosmetic scheduler fix because it affected real workloads and can livelock CPUs, but it remains below Actionable Moderate in the conservative case. The paranoid score reaches borderline manual-review territory mainly because RT scheduling may be delegated to non-root workloads in some environments. ## 5. One-sentence report phrase RT_PUSH_IPI on non PREEMPT_RT kernels can cause a CPU livelock when heavy softirq activity overlaps with RT task wakeups and migration IPIs, leading to a local availability-only DoS without evidence of memory corruption or privilege escalation. ## 6. Manual review recommendation MANUAL CHECK RECOMMENDED Review is recommended for products with large CPU systems, heavy networking, realtime workloads, or delegated RT scheduling. It is not a strong Important candidate because the patch supports DoS-only impact and does not show memory corruption or a privilege-escalation primitive. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: sched/rt: Have RT_PUSH_IPI be default off for non PREEMPT_RT Commit: b99f04ae3d200d2f8844aa29145bd18eccbeecde Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b99f04ae3d200d2f8844aa29145bd18eccbeecde Commit description: RT migration is done aggressively. When a CPU schedules out a high priority RT task for a lower priority task, it will look to see if there's any RT tasks that are waiting to run on another CPU that is of higher priority than the task this CPU is about to run. If it finds one, it will pull that task over to the CPU and allow it to run there instead. Normally, this pulling is done by looking at the RT overloaded mask (rto) which contains all the CPUs in the scheduler domain with RT tasks that are waiting to run due to a higher priority RT task currently running on their CPU. The CPU that is about to schedule a lower priority task will grab the rq lock of the overloaded CPU and move the RT task from that CPU's runqueue to the local one and schedule the higher priority RT task. This caused issues when a lot of CPUs would schedule a lower priority task at the same time. They would all try to grab the same runqueue lock of the CPU with the overloaded RT tasks. Only the first CPU that got in will get that task. All the others would wait until they got the runqueue lock and see there's nothing to pull and do nothing. On systems with lots of CPUs, this caused a large latency (up to 500us) which is beyond what PREEMPT_RT is to allow. The solution to that was to create an RT_PUSH_IPI logic. When any CPU wanted to pull a task, instead of grabbing the runqueue lock of the overloaded CPU, it would start by sending an IPI to the overloaded CPU, and that IPI handler would have the CPU with the waiting RT task do a push instead. Then that handler would send an IPI to the next CPU with overloaded RT tasks, and so on. Note, after the first CPU starts this process, if another CPU wanted to do a pull, it would see that the process has already begun and would only increment a counter to have the IPIs continue again. The RT_PUSH_IPI solved the latency problem with PREEMPT_RT but could cause a new issue with non PREEMPT_RT. Namely, softirqs run in a threaded context on PREEMPT_RT but they can run in an interrupt context in non-RT. If an IPI lands on a CPU that has just woken up multiple RT tasks and the current CPU is running a non RT or a low priority RT task, instead of doing a push, it would simply do a schedule on that CPU. But if a softirq was also executing on this CPU, the schedule would need to wait until the softirq finished. Until then, the CPU would still be considered overloaded as there are RT tasks still waiting to run on it. A live lock occurred on a workload that was doing heavy networking traffic on a large machine where the softirqs would run 500us out of 750us. And it would also be waking up RT tasks, causing the RT pull logic to be constantly executed. When a softirq triggered on a CPU with RT tasks queued but not running yet, and the other CPUs would see this CPU as being overloaded, they would send an IPI over to it. The CPU would notice that the waiting RT tasks are of higher priority than the currently running task and simply schedule that CPU instead. But because the softirq was executing, before it could schedule, it would receive another IPI to do the same. The amount of IPIs would slow down the currently running softirq so much that before it could return back to task context, it would execute another softirq never allowing the CPU to schedule. This live locked that CPU. As RT_PUSH_IPI was created to help PREEMPT_RT, make it default off if PREEMPT_RT is not enabled. Fixes: b6366f0 ("sched/rt: Use IPI to trigger RT task push migration instead of pulling") Closes: https://lore.kernel.org/all/20260506235716.2530720-1-tj@kernel.org/ Reported-by: Tejun Heo Signed-off-by: Steven Rostedt Signed-off-by: Peter Zijlstra (Intel) Cc: stable@vger.kernel.org Link: https://patch.msgid.link/20260515103740.25ccbed8@gandalf.local.home Signed-off-by: Greg Kroah-Hartman Changed files: kernel/sched/features.h 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=b99f04ae3d200d2f8844aa29145bd18eccbeecde ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-64374 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64374 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: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: 1 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).