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 p…

Overview

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---

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.