CVE-2025-71104

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---