CVE-2025-71104 KVM: x86: Fix VM hard lockup after prolonged inactivity with periodic HV timer
A flaw in KVM x86 periodic HV timer handling can cause host hard lockups after prolonged VM inactivity, requiring specific Intel CPU configurations.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-71104?
CVE-2025-71104 is a medium-severity denial of service vulnerability in Linux kernel, published on 14 Jan 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
KVM: x86: Fix VM hard lockup after prolonged inactivity with periodic HV timer
When advancing the target expiration for the guest's APIC timer in periodic mode, set the expiration to "now" if the target expiration is in the past (similar to what is done in update_target_expiration()). Blindly adding the period to the previous target expiration can result in KVM generating a practically unbounded number of hrtimer IRQs due to programming an expired timer over and over. In extreme scenarios, e.g. if userspace pauses/suspends a VM for an extended duration, this can even cause hard lockups in the host.
Currently, the bug only affects Intel CPUs when using the hypervisor timer (HV timer), a.k.a. the VMX preemption timer. Unlike the software timer, a.k.a. hrtimer, which KVM keeps running even on exits to userspace, the HV timer only runs while the guest is active. As a result, if the vCPU does not run for an extended duration, there will be a huge gap between the target expiration and the current time the vCPU resumes running. Because the target expiration is incremented by only one period on each timer expiration, this leads to a series of timer expirations occurring rapidly after the vCPU/VM resumes.
More critically, when the vCPU first triggers a periodic HV timer expiration after resuming, advancing the expiration by only one period will result in a target expiration in the past. As a result, the delta may be calculated as a negative value. When the delta is converted into an absolute value (tscdeadline is an unsigned u64), the resulting value can overflow what the HV timer is capable of programming. I.e. the large value will exceed the VMX Preemption Timer's maximum bit width of cpu_preemption_timer_multi + 32, and thus cause KVM to switch from the HV timer to the software timer (hrtimers).
After switching to the software timer, periodic timer expiration callbacks may be executed consecutively within a single clock interrupt handler, because hrtimers honors KVM's request for an expiration in the past and immediately re-invokes KVM's callback after reprogramming. And because the interrupt handler runs with IRQs disabled, restarting KVM's hrtimer over and over until the target expiration is advanced to "now" can result in a hard lockup.
E.g. the following hard lockup was triggered in the host when running a Windows VM (only relevant because it used the APIC timer in periodic mode) after resuming the VM from a long suspend (in the host).
NMI watchdog: Watchdog detected hard LOCKUP on cpu 45 ... RIP: 0010:advance_periodic_target_expiration+0x4d/0x80 [kvm] ... RSP: 0018:ff4f88f5d98d8ef0 EFLAGS: 00000046 RAX: fff0103f91be678e RBX: fff0103f91be678e RCX: 00843a7d9e127bcc RDX: 0000000000000002 RSI: 0052ca4003697505 RDI: ff440d5bfbdbd500 RBP: ff440d5956f99200 R08: ff2ff2a42deb6a84 R09: 000000000002a6c0 R10: 0122d794016332b3 R11: 0000000000000000 R12: ff440db1af39cfc0 R13: ff440db1af39cfc0 R14: ffffffffc0d4a560 R15: ff440db1af39d0f8 FS: 00007f04a6ffd700(0000) GS:ff440db1af380000(0000) knlGS:000000e38a3b8000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 000000d5651feff8 CR3: 000000684e038002 CR4: 0000000000773ee0 PKRU: 55555554 Call Trace: <IRQ> apic_timer_fn+0x31/0x50 [kvm] __hrtimer_run_queues+0x100/0x280 hrtimer_interrupt+0x100/0x210 ? ttwu_do_wakeup+0x19/0x160 smp_apic_timer_interrupt+0x6a/0x130 apic_timer_interrupt+0xf/0x20 </IRQ>
Moreover, if the suspend duration of the virtual machine is not long enough to trigger a hard lockup in this scenario, since commit 98c25ead5eda ("KVM: VMX: Move preemption timer <=> hrtimer dance to common x86"), KVM will continue using the software timer until the guest reprograms the APIC timer in some way. Since the periodic timer does not require frequent APIC timer register programming, the guest may continue to use the software timer in ---truncated---
Which versions are affected by CVE-2025-71104?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2025-71104?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2025-71104 being exploited?
There is no known exploitation of CVE-2025-71104 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as BELL-CVE-2025-71104, DEBIAN-CVE-2025-71104, GHSA-37vf-xcvf-jrjr, ROOT-OS-DEBIAN-11-CVE-2025-71104, ROOT-OS-DEBIAN-12-CVE-2025-71104, ROOT-OS-DEBIAN-13-CVE-2025-71104 and 3 more.