* [CVE-2026-89927][MODERATE 7.0] KVM: x86: hyper-v: Clamp stimer deadline to avoid livelock
@ 2026-09-16 16:10 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-16 16:10 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89927
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: x86: hyper-v: Clamp stimer deadline to avoid livelock
Commit: a4665762388750e08df99baabe5fce2a21d1423e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a4665762388750e08df99baabe5fce2a21d1423e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89927
Analysis date: Wed, 16 Sep 2026 12:10:13 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89927 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89927
The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.
This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.
The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.
Published priority for this report: MODERATE 7.0
Manual review required: YES
A detailed explanation of the methodology and priority rules is included
at the end of this message.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-89927 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89927 CHECK WITH IMPACT FROM ORIG NN MODERATE Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-190;CWE-682;*CWE-835;**CWE-400;BEST CVSS score: '5.5';DESCR 'A KVM x86 Hyper-V synthetic timer can be programmed with a very large COUNT value so that conversion from 100ns ticks to an absolute hrtimer deadline overflows. This may arm the timer in the past, make it fire immediately, raise KVM_REQ_HV_STIMER again, and keep vcpu_enter_guest from making forward progress. The observable impact is CPU livelock, RCU stall, and denial of service on the host. For the CVSS the PR:L is selected because a local process with KVM access or privileged code inside a guest can program the Hyper-V synthetic timer, but no host administrator privileges are normally required in common KVM deployments. The issue is not network reachable and there is no evidence of a memory corruption primitive, information disclosure, or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate (with actual score 5) MAYBE LOCK DANGER VIRT TIMER HARDWARE LINUS SYZBOT KPANIC INCREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN5 - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 5
* Conservative score: 5
* Raw paranoid score: 5
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate**::
No floor or cap changed the score.
2. Signal breakdown
Positive signals:
* +1 Reliable host DoS: the patch and trace establish CPU livelock, RCU stall, and possible host availability failure.
* +1 Below-host-admin reachability: the timer can be programmed by guest WRMSR or by a local VMM process with KVM access, not necessarily by host administrator.
* +2 Confirmed KVM guest-to-host availability impact: supplied evidence explicitly states that guest-programmable Hyper-V synthetic timers can prevent host CPU progress.
* +1 Practical repeatable trigger condition: a very large COUNT value near U64_MAX deterministically causes arithmetic overflow in deadline computation and repeated immediate hrtimer firing.
Negative or non-awarded signals:
* 0 Real UAF not awarded: no stale object or post-lifetime access is established.
* 0 CLASS_A or CLASS_B corruption not awarded: no controlled reclaim, arbitrary write, refcount corruption, rmap stale pointer, or memory corruption primitive is shown.
* 0 Confidentiality and integrity not awarded: the evidence supports DoS only.
* 0 Extra availability not awarded: the same RCU stall and livelock are already counted as host DoS.
* 0 Remote network reachability not awarded: the issue is reachable through KVM guest or VMM interfaces, not directly via network traffic.
* 0 Strong guest-to-host exploitation leverage not awarded: the guest-to-host effect is host DoS, not demonstrated host memory corruption or guest escape.
3. Reachability analysis
The evaluated attacker can be either malicious guest code or a local host VMM process with access to the affected KVM VM. The guest path is explicitly supported because Hyper-V synthetic timers are guest-programmable through WRMSR to HV_X64_MSR_STIMERi_CONFIG and HV_X64_MSR_STIMERi_COUNT. The local host userspace path is also supported through KVM_SET_MSRS.
Host administrator privilege is not positively required for the complete trigger. A guest can program the timer if Hyper-V SynIC timers are exposed by the VMM. A local process with KVM access can also program the MSR state. This is not namespace-scoped and does not depend on CAP_NET_ADMIN or user namespace behavior.
The issue requires KVM with Hyper-V synthetic timer emulation enabled for the guest. That is a configuration prerequisite, but not rare hardware. No physical device, firmware compromise, or network exposure is required. The trigger is not a race. It is an arithmetic overflow and livelock sequence that can be repeated by programming a large timer count.
For KVM roles:
* Malicious guest: can program the Hyper-V synthetic timer MSRs if the feature is exposed.
* Local VMM or /dev/kvm process: can program the same state via KVM_SET_MSRS.
* Host administrator: may configure the VM initially, but host-admin privilege is not required for the vulnerable operation once the guest feature is available.
4. Primitive and severity interpretation
The exact primitive is an integer overflow in conversion of Hyper-V synthetic timer COUNT values from 100ns ticks into an hrtimer deadline. A value close to U64_MAX can wrap the computed deadline into the past. The timer then fires immediately, raises KVM_REQ_HV_STIMER, and causes vcpu_enter_guest to abort and loop without forward progress.
This is not an active structured UAF and not fixed stale storage. No concrete repeated release or refcount mutation exists. No first lifetime operation frees an object and no second lifetime operation mutates a dangling reference.
PrimitiveClass is CLASS_C because the demonstrated behavior is CPU livelock and host DoS without an independently useful memory corruption primitive. The demonstrated consequence is host availability impact through RCU stall and potential starvation. The speculative consequence would be broader host instability, but there is no evidence of information disclosure, integrity compromise, privilege escalation, arbitrary R/W, or guest escape.
ImportantCandidate is NO because no CLASS_A or strong CLASS_B primitive is established. AutoDowngradeAllowed is NO because this is still an actionable KVM guest-to-host host-availability issue with A:H style operational impact and should not be automatically closed as ordinary low-risk noise.
Likely operational severity is Actionable Moderate: important for virtualization hosts exposing Hyper-V SynIC timers, but not an Important-class memory corruption or privilege-escalation candidate based on the supplied evidence.
5. One-sentence report phrase
A KVM x86 Hyper-V synthetic timer integer overflow lets guest or VMM controlled timer values arm an hrtimer in the past, causing repeated immediate timer firing, CPU livelock, RCU stall, and host denial of service without evidence of memory corruption or privilege escalation.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: this is a KVM guest-to-host host DoS condition with strong availability impact, even though the primitive is arithmetic overflow and livelock rather than UAF or memory corruption.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: KVM: x86: hyper-v: Clamp stimer deadline to avoid livelock
Commit: a4665762388750e08df99baabe5fce2a21d1423e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a4665762388750e08df99baabe5fce2a21d1423e
Commit description:
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: ffffffff8fc3d077 R09: 1ffffffff1f87a0e
R10: dffffc0000000000 R11: fffffbfff1f87a0f R12: dffffc0000000000
R13: 0000000000000000 R14: ffff8880b8628240 R15: 1ffff92000814e64
hrtimer_start include/linux/hrtimer.h:259 [inline]
stimer_start arch/x86/kvm/hyperv.c:682 [inline]
kvm_hv_process_stimers+0xd0a/0x16a0 arch/x86/kvm/hyperv.c:893
vcpu_enter_guest arch/x86/kvm/x86.c:11193 [inline]
vcpu_run+0x2240/0x76b0 arch/x86/kvm/x86.c:11639
kvm_arch_vcpu_ioctl_run+0x1148/0x1c90 arch/x86/kvm/x86.c:11984
kvm_vcpu_ioctl+0x99a/0xed0 virt/kvm/kvm_main.c:4492
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
[Commit description truncated; see the upstream URL below]
Changed files:
kernel/time/hrtimer.c
arch/x86/kernel/apic/apic.c
arch/x86/include/asm/idtentry.h
include/linux/spinlock_api_smp.h
kernel/locking/spinlock.c
include/linux/hrtimer.h
arch/x86/kvm/hyperv.c
arch/x86/kvm/x86.c
virt/kvm/kvm_main.c
fs/ioctl.c
arch/x86/entry/syscall_64.c
Diff excerpt:
Not included in this email. See the upstream URL for the full patch.
Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a4665762388750e08df99baabe5fce2a21d1423e
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89927 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89927
The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.
The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.
This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.
The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.
Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:
Conservative CVSS vector: Not available
The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
The Best / paranoid CVSS score: 5.5
The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.
CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.
A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.
Conservative ActionableScore: unknown
Paranoid ActionableScore: 5
The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.
For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:
ActionableScore below 5 -> MODERATE REGULAR
ActionableScore 5 or more -> MODERATE 7.0
The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.
There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).
KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.
AL-KERNEL base severity for this report: MODERATE
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): MODERATE 7.0
These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).
Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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