CVE-2026-89927
In the Linux kernel, the following vulnerability has been resolved:
KVM: x86: hyper-v: Clamp stimer deadline to avoid livelock
Fix an issue where userspace or the guest can program an Hyper-V synthetic timer to have a deadline in the past via integer overflow, preventing the CPU from making progress and triggering an RCU stall.
Hyper-V's SynIC exposes 4 per-vCPU synthetic timers to the guest, which are emulated by KVM. Each is programmed through the HV_X64_MSR_STIMERi_CONFIG and HV_X64_MSR_STIMERi_COUNT MSRs. Depending on CONFIG, COUNT represents either the absolute expiration time or the period of a periodic timer, both expressed in 100ns ticks. These timers may be set both by the guest (WRMSR) and the host (KVM_SET_MSRS).
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When the timer is enabled, stimer_start() translates COUNT to an absolute monotonic deadline and arms an hrtimer. If COUNT is set to a value close to U64_MAX, the deadline calculation can overflow.
This can result in a CPU livelock. stimer_start() arms the timer via hrtimer_start() with a deadline in the past, which causes it to immediately fire. The stimer callback then raises KVM_RQ_HV_STIMER, with the intention of causing KVM to deliver a synthetic interrupt on the next vCPU guest enter.
Then, once userspace issues KVM_RUN, vcpu_enter_guest() consumes the request, calling kvm_hv_process_stimers(). This would normally disable the timer via stimer_expiration() once the deadline is in the past. However, the deadline comparison is done between the KVM reference counter and stime->exp_time, which is a big value close to U64_MAX, so this never happens for a few thousand years.
kvm_hv_process_timers() then re-arms the timer via stimer_start(), since it was not disabled, which again fires immediately. Before entering the guest, kvm_vcpu_exit_request() checks kvm_request_pending(), which returns true due to the newly raised KVM_REQ_HV_STIMER. Then vcpu_enter_guest() aborts the guest entry, returning early into vcpu_run(), which loops back again into vcpu_enter_guest(), restarting the cycle.
Since there are no manual yields in this loop, a task with SCHED_FIFO may starve RCU grace-period kthreads, which exposes the stalls found by syzcaller:
Detalles técnicos trazas, registros y código del informe original
ktime_add_ns(ktime_now, 100 * (stimer->exp_time - time_now))
rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
rcu: (detected by 1, t=10502 jiffies, g=14269, q=1142 ncpus=2)
rcu: All QSes seen, last rcu_preempt kthread activity 10500 (4294965239-4294954739), jiffies_till_next_fqs=1, root ->qsmask 0x0
rcu: rcu_preempt kthread starved for 10500 jiffies! g14269 f0x2 RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=0
rcu: Unless rcu_preempt kthread gets sufficient CPU time, OOM is now expected behavior.
( ... )
Call Trace:
<IRQ>
__run_hrtimer kernel/time/hrtimer.c:1773 [inline]
__hrtimer_run_queues+0x408/0xc30 kernel/time/hrtimer.c:1841
hrtimer_interrupt+0x45b/0xaa0 kernel/time/hrtimer.c:1903
local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1045 [inline]
__sysvec_apic_timer_interrupt+0x102/0x3e0 arch/x86/kernel/apic/apic.c:1062
instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1056 [inline]
sysvec_apic_timer_interrupt+0xa1/0xc0 arch/x86/kernel/apic/apic.c:1056
</IRQ>
<TASK>
asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:697
RIP: 0010:__raw_spin_unlock_irqrestore include/linux/spinlock_api_smp.h:152 [inline]
RIP: 0010:_raw_spin_unlock_irqrestore+0xa8/0x110 kernel/locking/spinlock.c:194
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
RSP: 0018:ffffc900040a7320 EFLAGS: 00000206
RAX: 5de15cb931505900 RBX: 0000000000000a06 RCX: 5de15cb931505900
RDX: 0000000000000007 RSI: ffffffff8daa9dc3 RDI: 0000000000000001
RBP: ffffc900040a73b0 R08: ffffffff8fc3d0
---truncated---CVSS
- Versión: 3.1
- Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H
- Puntuación base: 7.1
Probabilidad de explotación (EPSS)
- Probabilidad de explotación en los próximos 30 días: 0.19%
- Percentil entre todas las CVEs puntuadas: 7
- Fecha de la puntuación: 5/10/2026
EPSS (Exploit Prediction Scoring System, de FIRST) estima la probabilidad de que una vulnerabilidad sea explotada en 30 días. Complementa a CVSS (impacto) y a CISA KEV (explotación confirmada).
🎯 Técnicas ATT&CK
Cómo se explota esta vulnerabilidad y qué consigue el atacante, en el lenguaje de MITRE ATT&CK.
- Explotación
T1068Exploitation for Privilege Escalationprivilege escalation85 % - Impacto principal
T1499.004Application or System Exploitationimpact90 % - Impacto secundario
T1529System Shutdown/Rebootimpact75 %
Vulnerabilidad local de desbordamiento entero en KVM/Hyper-V que permite a atacante local (AV:L/PR:N) causar DoS por livelock/RCU stall sin interacción del usuario, forzando reinicio del sistema.
Inferido por nuestro agente de análisis a partir de la descripción oficial, el vector CVSS y la CWE, y comprobado por un supervisor. Puede contener errores.
🛡️ Mitigaciones ATT&CK que cubren estas técnicas
Tecnologías afectadas (1)
⚠ Inferidas por IA a partir de la descripción — NVD aún no ha analizado esta CVE; no son CPE verificados.
Referencias
- 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
- https://git.kernel.org/stable/c/8e19ded84336891646b31375720717635f0fdd90
- https://git.kernel.org/stable/c/a4665762388750e08df99baabe5fce2a21d1423e
- https://git.kernel.org/stable/c/bdb732ebee545b7e3bee7060efc754a8d99818b9
JSON original (NVD)
Mostrar
{
"id": "CVE-2026-89927",
"cveTags": [],
"metrics": {
"cvssMetricV31": [
{
"type": "Secondary",
"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"cvssData": {
"scope": "CHANGED",
"version": "3.1",
"baseScore": 7.1,
"attackVector": "LOCAL",
"baseSeverity": "HIGH",
"vectorString": "CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H",
"integrityImpact": "NONE",
"userInteraction": "NONE",
"attackComplexity": "LOW",
"availabilityImpact": "HIGH",
"privilegesRequired": "NONE",
"confidentialityImpact": "NONE"
},
"impactScore": 4,
"exploitabilityScore": 2.5
}
]
},
"affected": [
{
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"affectedData": [
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"programFiles": [
"arch/x86/kvm/hyperv.c"
],
"defaultStatus": "unaffected"
},
{
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"vendor": "Linux",
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"versions": [
{
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"version": "4.5"
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{
"status": "unaffected",
"version": "0",
"lessThan": "4.5",
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{
"status": "unaffected",
"version": "5.10.270",
"versionType": "semver",
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},
{
"status": "unaffected",
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{
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{
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"version": "6.12.110",
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{
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"version": "6.18.51",
"versionType": "semver",
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{
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{
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"lessThanOrEqual": "*"
}
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"programFiles": [
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],
"defaultStatus": "affected"
}
]
}
],
"published": "2026-09-16T11:17:01.900",
"references": [
{
"url": "https://git.kernel.org/stable/c/0ca49fbd2883cd53d32d85b50feef17fa04d0fbf",
"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
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"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
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{
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"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
}
],
"vulnStatus": "Received",
"descriptions": [
{
"lang": "en",
"value": "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---"
}
],
"lastModified": "2026-09-16T15:18:18.483",
"sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
}