CVE-2026-89927
Full vulnerability record from the local CVENow database.
Description
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). 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. ktime_add_ns(ktime_now, 100 * (stimer->exp_time - time_now)) 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: 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---
View original CVE JSON
{
"id": "CVE-2026-89927",
"sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"published": "2026-09-16T11:17:01.900",
"lastModified": "2026-10-19T09:18:38.270",
"vulnStatus": "Received",
"cveTags": [],
"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---"
}
],
"affected": [
{
"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"affectedData": [
{
"vendor": "Linux",
"product": "Linux",
"defaultStatus": "unaffected",
"programFiles": [
"arch/x86/kvm/hyperv.c"
],
"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
"versions": [
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"lessThan": "a4665762388750e08df99baabe5fce2a21d1423e",
"versionType": "git",
"status": "affected"
},
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"versionType": "git",
"status": "affected"
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},
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}
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},
{
"vendor": "Linux",
"product": "Linux",
"defaultStatus": "affected",
"programFiles": [
"arch/x86/kvm/hyperv.c"
],
"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
"versions": [
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"version": "4.5",
"status": "affected"
},
{
"version": "0",
"lessThan": "4.5",
"versionType": "semver",
"status": "unaffected"
},
{
"version": "5.10.270",
"lessThanOrEqual": "5.10.*",
"versionType": "semver",
"status": "unaffected"
},
{
"version": "5.15.221",
"lessThanOrEqual": "5.15.*",
"versionType": "semver",
"status": "unaffected"
},
{
"version": "6.1.188",
"lessThanOrEqual": "6.1.*",
"versionType": "semver",
"status": "unaffected"
},
{
"version": "6.6.157",
"lessThanOrEqual": "6.6.*",
"versionType": "semver",
"status": "unaffected"
},
{
"version": "6.12.110",
"lessThanOrEqual": "6.12.*",
"versionType": "semver",
"status": "unaffected"
},
{
"version": "6.18.51",
"lessThanOrEqual": "6.18.*",
"versionType": "semver",
"status": "unaffected"
},
{
"version": "7.2.5",
"lessThanOrEqual": "7.2.*",
"versionType": "semver",
"status": "unaffected"
},
{
"version": "7.3",
"lessThanOrEqual": "*",
"versionType": "original_commit_for_fix",
"status": "unaffected"
}
]
}
]
}
],
"metrics": {
"cvssMetricV31": [
{
"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"type": "Secondary",
"cvssData": {
"version": "3.1",
"vectorString": "CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H",
"baseScore": 7.1,
"baseSeverity": "HIGH",
"attackVector": "LOCAL",
"attackComplexity": "LOW",
"privilegesRequired": "NONE",
"userInteraction": "NONE",
"scope": "CHANGED",
"confidentialityImpact": "NONE",
"integrityImpact": "NONE",
"availabilityImpact": "HIGH"
},
"exploitabilityScore": 2.5,
"impactScore": 4.0
}
]
},
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"url": "https://git.kernel.org/stable/c/0ca49fbd2883cd53d32d85b50feef17fa04d0fbf",
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