From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-74594][MODERATE 7.0] sched/psi: Shut down rtpoll_timer in psi_cgroup_free() Date: Sat, 22 Aug 2026 12:22:58 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-74594 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: 7 X-AL-KERNEL-ActionableScore-Lower: 4 X-AL-KERNEL-Commit: 1e5ca82eee59caca6988f9d6e859786aab8a5fa0 List-Id: CVE: CVE-2026-74594 Priority: MODERATE 7.0 AL-KERNEL base severity: MODERATE KPANIC flag: YES Patch: sched/psi: Shut down rtpoll_timer in psi_cgroup_free() Commit: 1e5ca82eee59caca6988f9d6e859786aab8a5fa0 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1e5ca82eee59caca6988f9d6e859786aab8a5fa0 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74594 Analysis date: Sat, 22 Aug 2026 12:22:58 -0400 ActionableScore: 7 ActionableScore lower bound: 4 Actionable bucket: Strong Important candidate / Actionable Moderate at minimum Manual review required: YES Summary: A race in sched/psi can rearm `rtpoll_timer` after the last PSI trigger is destroyed, allowing `poll_timer_fn()` to run on freed cgroup PSI memory and causing at least a local kernel crash with UAF based privilege escalation concerns. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-74594 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74594 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-2026-74594 MODERATE CHECK WITH IMPACT FROM ORIG NN 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:H/I:H/A:H';*CWE-416;CWE-362;CWE-667;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in sched/psi can leave rtpoll_timer() armed after the last PSI trigger is destroyed and the owning cgroup PSI state is freed. psi_schedule_rtpoll_work() can run locklessly from the scheduler hotpath and call mod_timer() after psi_trigger_destroy() has called timer_delete(), so poll_timer_fn() may later run on freed memory. For the CVSS the PR:L is chosen because a local user or delegated service with access to PSI triggers and cgroup lifecycle operations may be able to exercise the vulnerable path without full host root. The issue is not network reachable and requires local control plane activity around PSI trigger teardown and cgroup removal. Impact is at least local denial of service via kernel crash, and the explicit use-after-free makes limited or full confidentiality and integrity impact plausible enough for manual review.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 6) YES INIT RACE LEAK UAF TIMER LINUS KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=7 ActionableScoreLower=4 ## 1. ActionableScore * Conservative score: 4 * Paranoid score: 7 * Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**:: ## 2. Signal breakdown Conservative signals: * Local unprivileged or low privileged trigger: +1. The path requires local PSI trigger and cgroup lifecycle activity, and may be reachable by delegated cgroup users or service accounts. * Memory corruption, weak primitive: +1. The patch explicitly describes `poll_timer_fn()` running on freed memory, but no controlled reclaim or arbitrary write primitive is shown. * Real lifetime corruption: +1. This is a timer lifetime bug where `rtpoll_timer` can be rearmed after trigger destruction and survive until after the PSI cgroup memory is freed. * Reliable kernel crash / strong DoS: +1. A timer callback executing on freed memory is a credible kernel crash path. * Broad/default/common subsystem: +1. PSI and cgroup scheduler accounting are common kernel infrastructure on modern distributions. * Hard or unreliable race / special timing required: -1. The bug depends on a race between lockless scheduler hotpath scheduling and trigger destruction. Paranoid additional interpretation: * Generic memory corruption, strong corruption primitive: +2 instead of weak +1. The bug is an explicit UAF involving an asynchronous timer callback into freed cgroup PSI state. * Weak LPE concern: +1. The stale timer callback and waitqueue access on freed memory make privilege escalation worth manual review, although no concrete exploit primitive is demonstrated. * Availability impact realistic: +1. The expected failure mode is host wide kernel crash or severe instability, not merely a local process failure. ## 3. Reachability analysis The issue is not network reachable. It requires local activity involving PSI triggers and cgroup teardown. In a strict host configuration, creating and destroying the relevant PSI triggers and cgroups may require administrative control over cgroups, but delegated cgroup setups, containers, service managers, or workload orchestration can reduce this to a low privileged local actor. Namespaces and cgroup delegation therefore matter for practical PR assessment. The affected code is in scheduler PSI and cgroup lifetime handling, not a rare device driver. The race is timing dependent, but it is between realistic operations: scheduler hotpath `psi_schedule_rtpoll_work()` and `psi_trigger_destroy()` during final trigger removal. Exploitation conditions are local and race dependent, but the commit clearly states that the timer can remain pending after the group is freed. Call-site confidence: medium. The patch and commit provide the relevant lifetime sequence and the callback consequence, but not a full reproducer or reclaim primitive. ## 4. Severity interpretation This is stronger than an ordinary Moderate crash-only bug because the failure mode is an explicit use-after-free of cgroup PSI state by an asynchronous timer callback. Conservative handling can keep it as borderline Moderate because the race is hard and no controlled reclaim, arbitrary write, or demonstrated LPE primitive is shown. Paranoid triage should treat it as an Important candidate because local kernel UAFs involving timers and freed cgroup objects can be underestimated and may become exploitable depending on allocator reuse and timing. Theoretical memory corruption potential is real. Realistic exploitation evidence currently supports at least local DoS, while privilege escalation remains plausible but unproven. ## 5. One-sentence report phrase A race in sched/psi can rearm `rtpoll_timer` after the last PSI trigger is destroyed, allowing `poll_timer_fn()` to run on freed cgroup PSI memory and causing at least a local kernel crash with UAF based privilege escalation concerns. ## 6. Manual review recommendation MANUAL CHECK REQUIRED. Reason: explicit UAF on freed cgroup PSI memory through an asynchronous timer callback, local reachable control-plane conditions, and plausible but unproven privilege escalation potential. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: sched/psi: Shut down rtpoll_timer in psi_cgroup_free() Commit: 1e5ca82eee59caca6988f9d6e859786aab8a5fa0 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1e5ca82eee59caca6988f9d6e859786aab8a5fa0 Commit description: psi_schedule_rtpoll_work() is called locklessly from the scheduler hotpath and can race psi_trigger_destroy() taking down the last rtpoll trigger under rtpoll_trigger_lock: psi_schedule_rtpoll_work() psi_trigger_destroy() rcu_read_lock(); task = rcu_dereference(rtpoll_task); rcu_assign_pointer(rtpoll_task, NULL); timer_delete(&rtpoll_timer); mod_timer(&rtpoll_timer, ...); rcu_read_unlock(); synchronize_rcu(); kthread_stop(task_to_destroy); The group can then be freed with the re-armed timer still pending, and poll_timer_fn() runs on freed memory. 461daba ("psi: eliminate kthread_worker from psi trigger scheduling mechanism") deleted the timer synchronously after the synchronize_rcu(), which prevented this but raced trigger creation instead: the deletion could cancel the timer that a new trigger set armed during the grace period and, as creation also reinitialized the timer at the time, corrupt it. 8f91efd ("psi: Fix race between psi_trigger_create/destroy") moved the initialization into group_init() and the deletion into the locked section, trading the creation races for the window above. Neither placement in the destruction path works. A pending timer firing while the group is alive is harmless though. poll_timer_fn() just wakes the rtpoll waitqueue and doesn't re-arm itself. Bind the timer to the group's lifetime instead and shut it down in psi_cgroup_free(). Nothing can arm it by then. timer_shutdown_sync() because the timer is never armed again. Fixes: 8f91efd ("psi: Fix race between psi_trigger_create/destroy") Cc: stable@vger.kernel.org # v5.10+ Reported-by: Sashiko AI Closes: https://lore.kernel.org/all/20260711000434.36C4A1F000E9@smtp.kernel.org/ Signed-off-by: Tejun Heo Acked-by: Johannes Weiner Tested-by: Matt Fleming Acked-by: Suren Baghdasaryan Signed-off-by: Greg Kroah-Hartman Changed files: kernel/sched/psi.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=1e5ca82eee59caca6988f9d6e859786aab8a5fa0 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-74594 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74594 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:L/I:L/A:H The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H The Best / paranoid CVSS score: 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: 4 Paranoid ActionableScore: 7 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).