{"cve":"CVE-2026-89927","nvd":{"published":"2026-09-16T11:17:01.900","modified":"2026-10-19T09:18:38.270","status":"Received","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nKVM: x86: hyper-v: Clamp stimer deadline to avoid livelock\n\nFix an issue where userspace or the guest can program an Hyper-V\nsynthetic timer to have a deadline in the past via integer overflow,\npreventing the CPU from making progress and triggering an RCU stall.\n\nHyper-V's SynIC exposes 4 per-vCPU synthetic timers to the\nguest, which are emulated by KVM. Each is programmed through the\nHV_X64_MSR_STIMERi_CONFIG and HV_X64_MSR_STIMERi_COUNT MSRs. Depending\non CONFIG, COUNT represents either the absolute expiration time or the\nperiod of a periodic timer, both expressed in 100ns ticks. These timers\nmay be set both by the guest (WRMSR) and the host (KVM_SET_MSRS).\n\nWhen the timer is enabled, stimer_start() translates COUNT to an\nabsolute monotonic deadline and arms an hrtimer. If COUNT is set to a\nvalue close to U64_MAX, the deadline calculation can overflow.\n\n    ktime_add_ns(ktime_now, 100 * (stimer->exp_time - time_now))\n\nThis can result in a CPU livelock. stimer_start() arms the timer\nvia hrtimer_start() with a deadline in the past, which causes it to\nimmediately fire. The stimer callback then raises KVM_RQ_HV_STIMER, with\nthe intention of causing KVM to deliver a synthetic interrupt on the\nnext vCPU guest enter.\n\nThen, once userspace issues KVM_RUN, vcpu_enter_guest() consumes the\nrequest, calling kvm_hv_process_stimers(). This would normally disable\nthe timer via stimer_expiration() once the deadline is in the past.\nHowever, the deadline comparison is done between the KVM reference\ncounter and stime->exp_time, which is a big value close to U64_MAX, so\nthis never happens for a few thousand years.\n\nkvm_hv_process_timers() then re-arms the timer via stimer_start(), since\nit was not disabled, which again fires immediately. Before entering\nthe guest, kvm_vcpu_exit_request() checks kvm_request_pending(),\nwhich returns true due to the newly raised KVM_REQ_HV_STIMER. Then\nvcpu_enter_guest() aborts the guest entry, returning early into\nvcpu_run(), which loops back again into vcpu_enter_guest(), restarting\nthe cycle.\n\nSince there are no manual yields in this loop, a task with SCHED_FIFO\nmay starve RCU grace-period kthreads, which exposes the stalls found\nby syzcaller:\n\n    rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:\n    rcu:    (detected by 1, t=10502 jiffies, g=14269, q=1142 ncpus=2)\n    rcu: All QSes seen, last rcu_preempt kthread activity 10500 (4294965239-4294954739), jiffies_till_next_fqs=1, root ->qsmask 0x0\n    rcu: rcu_preempt kthread starved for 10500 jiffies! g14269 f0x2 RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=0\n    rcu:    Unless rcu_preempt kthread gets sufficient CPU time, OOM is now expected behavior.\n        ( ... )\n    Call Trace:\n     <IRQ>\n     __run_hrtimer kernel/time/hrtimer.c:1773 [inline]\n     __hrtimer_run_queues+0x408/0xc30 kernel/time/hrtimer.c:1841\n     hrtimer_interrupt+0x45b/0xaa0 kernel/time/hrtimer.c:1903\n     local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1045 [inline]\n     __sysvec_apic_timer_interrupt+0x102/0x3e0 arch/x86/kernel/apic/apic.c:1062\n     instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1056 [inline]\n     sysvec_apic_timer_interrupt+0xa1/0xc0 arch/x86/kernel/apic/apic.c:1056\n     </IRQ>\n     <TASK>\n     asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:697\n    RIP: 0010:__raw_spin_unlock_irqrestore include/linux/spinlock_api_smp.h:152 [inline]\n    RIP: 0010:_raw_spin_unlock_irqrestore+0xa8/0x110 kernel/locking/spinlock.c:194\n    Code: 74 05 e8 0b f4 5f f6 48 c7 44 24 20 00 00 00 00 9c 8f 44 24 20 f6 44 24 21 02 75 4f f7 c3 00 02 00 00 74 01 fb bf 01 00 00 00 <e8> 23 6b 27 f6 65 8b 05 7c 60 5a 07 85 c0 74 40 48 c7 04 24 0e 36\n    RSP: 0018:ffffc900040a7320 EFLAGS: 00000206\n    RAX: 5de15cb931505900 RBX: 0000000000000a06 RCX: 5de15cb931505900\n    RDX: 0000000000000007 RSI: ffffffff8daa9dc3 RDI: 0000000000000001\n    RBP: ffffc900040a73b0 R08: ffffffff8fc3d0\n---truncated---","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H","score":7.1},"cwes":[],"references":["https://git.kernel.org/stable/c/0ca49fbd2883cd53d32d85b50feef17fa04d0fbf","https://git.kernel.org/stable/c/3097582b73a8ed1cd6f6790fa78706f4a79b5a49","https://git.kernel.org/stable/c/61954727ee08f026f5e1c9ee69e1b404a68f2f7a","https://git.kernel.org/stable/c/6a8ba9213cce613455b1502ee0fd178656bf617b","https://git.kernel.org/stable/c/8aa467fe757d8cb2278e98d7fbf44fc05eb0dbf8"],"products":[],"source":"nvd","synced_at":"2026-10-19T22:54:41.405518556+00:00"},"epss":{"cve":"CVE-2026-89927","score":0.0018,"percentile":0.07552,"date":"2026-10-25"},"kev":null,"osv":[],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}