CVE-2026-89927 in Linux
Summary
by MITRE • 09/17/2026
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---
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Analysis
by VulDB Data Team • 09/17/2026
The Linux kernel contains a critical vulnerability within the KVM hypervisor implementation for x86 architectures, specifically affecting the emulation of Microsoft Hyper-V synthetic timers. This flaw allows both userspace applications and guest virtual machines to trigger an integer overflow when programming timer deadlines, leading to severe system instability including CPU livelocks and RCU stalls. The vulnerability stems from how the kernel calculates absolute expiration times for these hardware-simulated timers based on input values provided via Model Specific Registers (MSRs). When a malicious or buggy entity provides a count value close to the maximum unsigned 64-bit integer limit, the arithmetic operation used to convert this period into an absolute monotonic deadline results in an overflow. This miscalculation causes the system to interpret the timer's expiration time as being far in the future rather than immediately past, fundamentally breaking the expected lifecycle of the timer interrupt handling mechanism.
Hyper-V exposes four synthetic timers per virtual CPU through specific MSRs that allow configuration of either absolute expiration times or periodic intervals measured in 100-nanosecond ticks. These timers can be programmed by the guest operating system using write operations to these registers, as well as by the host hypervisor via KVM ioctls. The core logic resides in the stimer_start function, which translates the configured count into an absolute deadline and arms a high-resolution timer. However, due to insufficient bounds checking on the input value before arithmetic conversion, values near U64_MAX cause the resulting deadline calculation to wrap around or produce erroneous large positive integers. This error propagates through the interrupt handling chain, creating a scenario where the system believes the timer is not yet expired despite being logically past its intended trigger point for years into the future.
The operational impact of this vulnerability manifests as a CPU livelock that starves critical kernel threads responsible for maintaining RCU grace periods. When the overflowed deadline causes the hrtimer to fire immediately, it raises a request flag indicating an outstanding synthetic timer interrupt. Upon subsequent guest entry attempts via KVM_RUN, the vCPU enters a tight loop where it processes this pending request but fails to disable the timer because the comparison against the erroneously large expiration time always evaluates as false. Consequently, the system re-arms the timer immediately after processing it, which fires again instantly due to the same overflow condition. This creates an infinite execution cycle that consumes 100% of a CPU core without yielding control to other tasks. Under real-time scheduling policies such as SCHED_FIFO, this behavior prevents RCU preemption kthreads from acquiring necessary CPU time, leading to detected stalls and potential system-wide out-of-memory conditions due to the inability to reclaim memory associated with stalled grace periods.
From a threat modeling perspective, this vulnerability aligns with CWE-190 Integer Overflow or Wraparound, as the root cause is the failure to validate input ranges before performing arithmetic operations that exceed variable capacity. It also maps to ATT&CK technique T1499 Endpoint Denial of Service, specifically under sub-techniques involving resource exhaustion through infinite loops or CPU saturation attacks. The attack vector allows for local privilege escalation in terms of impact severity because a guest user can degrade the stability of the host system and other co-resident virtual machines by inducing these stalls. This represents a significant breach of isolation guarantees provided by virtualization platforms, as one compromised or misbehaving VM can effectively deny service to the entire physical host infrastructure.
Mitigation strategies primarily involve applying kernel patches that enforce strict bounds checking on timer configuration values before they are processed into deadline calculations. Developers must ensure that any input derived from guest MSRs is validated against realistic maximum thresholds well below U64_MAX to prevent arithmetic overflow during conversion operations. Additionally, implementing defensive coding practices such as using saturating arithmetic functions or explicit range checks prior to ktime_add_ns calls can neutralize this class of vulnerabilities. System administrators should prioritize updating their Linux kernels and hypervisor components to versions that include these fixes. For environments running real-time workloads, it is advisable to monitor for RCU stall warnings in system logs as an indicator of potential exploitation attempts or residual instability from unpatched systems. Regular security audits focusing on virtualization layer inputs are recommended to identify similar logic flaws in other emulated hardware components.