CVE-2025-21816 (GCVE-0-2025-21816)
Vulnerability from cvelistv5
Published
2025-02-27 20:04
Modified
2025-11-03 17:32
Severity ?
Summary
In the Linux kernel, the following vulnerability has been resolved: hrtimers: Force migrate away hrtimers queued after CPUHP_AP_HRTIMERS_DYING hrtimers are migrated away from the dying CPU to any online target at the CPUHP_AP_HRTIMERS_DYING stage in order not to delay bandwidth timers handling tasks involved in the CPU hotplug forward progress. However wakeups can still be performed by the outgoing CPU after CPUHP_AP_HRTIMERS_DYING. Those can result again in bandwidth timers being armed. Depending on several considerations (crystal ball power management based election, earliest timer already enqueued, timer migration enabled or not), the target may eventually be the current CPU even if offline. If that happens, the timer is eventually ignored. The most notable example is RCU which had to deal with each and every of those wake-ups by deferring them to an online CPU, along with related workarounds: _ e787644caf76 (rcu: Defer RCU kthreads wakeup when CPU is dying) _ 9139f93209d1 (rcu/nocb: Fix RT throttling hrtimer armed from offline CPU) _ f7345ccc62a4 (rcu/nocb: Fix rcuog wake-up from offline softirq) The problem isn't confined to RCU though as the stop machine kthread (which runs CPUHP_AP_HRTIMERS_DYING) reports its completion at the end of its work through cpu_stop_signal_done() and performs a wake up that eventually arms the deadline server timer: WARNING: CPU: 94 PID: 588 at kernel/time/hrtimer.c:1086 hrtimer_start_range_ns+0x289/0x2d0 CPU: 94 UID: 0 PID: 588 Comm: migration/94 Not tainted Stopper: multi_cpu_stop+0x0/0x120 <- stop_machine_cpuslocked+0x66/0xc0 RIP: 0010:hrtimer_start_range_ns+0x289/0x2d0 Call Trace: <TASK> start_dl_timer enqueue_dl_entity dl_server_start enqueue_task_fair enqueue_task ttwu_do_activate try_to_wake_up complete cpu_stopper_thread Instead of providing yet another bandaid to work around the situation, fix it in the hrtimers infrastructure instead: always migrate away a timer to an online target whenever it is enqueued from an offline CPU. This will also allow to revert all the above RCU disgraceful hacks.
References
Impacted products
Vendor Product Version
Linux Linux Version: 75b5016ce325f1ef9c63e5398a1064cf8a7a7354
Version: 53f408cad05bb987af860af22f4151e5a18e6ee8
Version: 5c0930ccaad5a74d74e8b18b648c5eb21ed2fe94
Version: 5c0930ccaad5a74d74e8b18b648c5eb21ed2fe94
Version: 5c0930ccaad5a74d74e8b18b648c5eb21ed2fe94
Version: 9a2fc41acb69dd4e2a58d0c04346c3333c2341fc
Version: 54d0d83a53508d687fd4a225f8aa1f18559562d0
Version: 7f4c89400d2997939f6971c7981cc780a219e36b
Version: 6fcbcc6c8e52650749692c7613cbe71bf601670d
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          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nhrtimers: Force migrate away hrtimers queued after CPUHP_AP_HRTIMERS_DYING\n\nhrtimers are migrated away from the dying CPU to any online target at\nthe CPUHP_AP_HRTIMERS_DYING stage in order not to delay bandwidth timers\nhandling tasks involved in the CPU hotplug forward progress.\n\nHowever wakeups can still be performed by the outgoing CPU after\nCPUHP_AP_HRTIMERS_DYING. Those can result again in bandwidth timers being\narmed. Depending on several considerations (crystal ball power management\nbased election, earliest timer already enqueued, timer migration enabled or\nnot), the target may eventually be the current CPU even if offline. If that\nhappens, the timer is eventually ignored.\n\nThe most notable example is RCU which had to deal with each and every of\nthose wake-ups by deferring them to an online CPU, along with related\nworkarounds:\n\n_ e787644caf76 (rcu: Defer RCU kthreads wakeup when CPU is dying)\n_ 9139f93209d1 (rcu/nocb: Fix RT throttling hrtimer armed from offline CPU)\n_ f7345ccc62a4 (rcu/nocb: Fix rcuog wake-up from offline softirq)\n\nThe problem isn\u0027t confined to RCU though as the stop machine kthread\n(which runs CPUHP_AP_HRTIMERS_DYING) reports its completion at the end\nof its work through cpu_stop_signal_done() and performs a wake up that\neventually arms the deadline server timer:\n\n   WARNING: CPU: 94 PID: 588 at kernel/time/hrtimer.c:1086 hrtimer_start_range_ns+0x289/0x2d0\n   CPU: 94 UID: 0 PID: 588 Comm: migration/94 Not tainted\n   Stopper: multi_cpu_stop+0x0/0x120 \u003c- stop_machine_cpuslocked+0x66/0xc0\n   RIP: 0010:hrtimer_start_range_ns+0x289/0x2d0\n   Call Trace:\n   \u003cTASK\u003e\n     start_dl_timer\n     enqueue_dl_entity\n     dl_server_start\n     enqueue_task_fair\n     enqueue_task\n     ttwu_do_activate\n     try_to_wake_up\n     complete\n     cpu_stopper_thread\n\nInstead of providing yet another bandaid to work around the situation, fix\nit in the hrtimers infrastructure instead: always migrate away a timer to\nan online target whenever it is enqueued from an offline CPU.\n\nThis will also allow to revert all the above RCU disgraceful hacks."
        }
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        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
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          "url": "https://git.kernel.org/stable/c/82ac6adbbb2aad14548a71d5e2e37f4964a15e38"
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          "url": "https://git.kernel.org/stable/c/63815bef47ec25f5a125019ca480882481ee1553"
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          "url": "https://git.kernel.org/stable/c/53dac345395c0d2493cbc2f4c85fe38aef5b63f5"
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      "title": "hrtimers: Force migrate away hrtimers queued after CPUHP_AP_HRTIMERS_DYING",
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    "datePublished": "2025-02-27T20:04:15.356Z",
    "dateReserved": "2024-12-29T08:45:45.774Z",
    "dateUpdated": "2025-11-03T17:32:08.230Z",
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