* [CVE-2026-99178][LOW] x86/kvm: Don't consume async #PF flags on a kernel #PF in NMI context
@ 2026-10-29 3:57 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-29 3:57 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-99178
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: x86/kvm: Don't consume async #PF flags on a kernel #PF in NMI context
Commit: b77e9622bbade9812e2090c66d25c372f515e921
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b77e9622bbade9812e2090c66d25c372f515e921
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99178
Analysis date: Wed, 28 Oct 2026 23:57:55 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO
Summary:
A nested guest kernel page fault in NMI context can consume pending KVM async-PF flags and reliably panic the guest under a specialized compound workload.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99178 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99178
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: LOW
Manual review required: NO
A detailed explanation of the methodology and priority rules is included
at the end of this message.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-99178 LOW https://www.kernelcve.org/list/?q=CVE-2026-99178 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT LOW AEGIS_AS=1 LEGACY=LOW RESULT=SAME (final reconciliation agrees; DECISION: both LOW) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-362;CWE-670;CWE-754;BEST CVSS score: '4.7';DESCR 'A race occurs when an NMI interrupts async page fault entry before the outer handler consumes its per CPU reason flags and the NMI handler then causes a kernel mode page fault. The nested fault can consume flags belonging to the interrupted fault and panic the guest kernel after treating the event as an async page fault in an interrupt disabled region. For the CVSS the PR:L value reflects that a local guest process may help create the perf user call chain and invalid user stack conditions, although system perf restrictions may require additional permission. AC:H reflects the narrow timing window and the need for KVM async PF delivery, nonresident guest memory, an NMI, and a nested page fault. The demonstrated impact is guest denial of service only, with no evidence of UAF, OOB access, information disclosure, arbitrary write, host compromise, or privilege escalation. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 1 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. No severity changing production threshold or specialist override was recorded. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. The independent weaker Aegis shadow evaluated CVSS 4.7 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was LOW. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1) MAYBE OOB VIRT INIT RACE LEAK DMAorINTERRUPT HARDWARE - - checked
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ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw base score: 1.
* Conservative score: 1; no floor or cap applied.
* Raw paranoid score: 1.
* Paranoid score: 1; no additional technically supported primitive.
* Final recommended bucket: **Ordinary Moderate**::
2. Signal breakdown
* +1 Reliable kernel crash/strong DoS: the nested kernel-mode page fault consumes the interrupted async-PF flags and reaches an explicit panic; guest panics occurred in 3/3 tests.
* +0 Hard-race penalty withheld: the interrupt window is narrow, but the supplied workload triggered it consistently, so difficult or unreliable timing is not established.
* +0 No extra availability point: guest panic is the sole demonstrated availability effect.
* +0 No memory-corruption, UAF, OOB write, authorization bypass, disclosure, or non-crash integrity primitive is shown.
* +0 No host-impact or guest-to-host leverage: all demonstrated effects occur inside the guest kernel.
* NEW52/B5-R and NEW53/B3-PENDING do not apply; there is no heap write or shared-backing write.
3. Reachability analysis
The demonstrated setup is compound. A guest process supplies an invalid user-stack walk condition, while perf must generate NMIs and the host/VMM must provide on-demand guest memory with KVM async page faults. The evidence does not establish that an ordinary guest user can enable perf sampling or independently arrange the complete environment. It also does not prove that host-administrator powers are mandatory, so host privilege remains unknown. No namespace-scoped capability path, special hardware requirement, or host compromise is shown.
4. Primitive and severity reading
The primitive is incorrect consumption and clearing of per-CPU async-PF reason flags by a nested kernel page fault in NMI context. This is an invalid-state/race condition ending in an explicit guest panic, classified CLASS_C. It is neither stale heap storage nor an active-object UAF. No later privileged consumer uses attacker-corrupted structured state for a non-crash operation: the nested handler panics immediately. Important-grade exploit leverage and host-boundary effects are unsupported, so automatic downgrade is allowed.
5. One-sentence report phrase
A nested guest kernel page fault in NMI context can consume pending KVM async-PF flags and reliably panic the guest under a specialized compound workload.
6. Manual review advice
NO MANUAL CHECK NEEDED
The patch and tests support guest-only denial of service without a corruption, authorization, or guest-to-host security primitive.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: x86/kvm: Don't consume async #PF flags on a kernel #PF in NMI context
Commit: b77e9622bbade9812e2090c66d25c372f515e921
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b77e9622bbade9812e2090c66d25c372f515e921
Commit description:
An async #PF "page not present" event arrives through the #PF vector,
and its reason sits in the per-CPU apf_reason.flags until
__kvm_handle_async_pf() reads and clears it. If an NMI arrives after
the event is injected and before the #PF handler reads the flags, and
the NMI handler itself takes a #PF, that fault reads and clears the
flags belonging to the #PF it interrupted. It then finds interrupts
disabled in the NMI context it interrupted, and panics:
Kernel panic - not syncing: Host injected async #PF in interrupt
disabled region
Call Trace:
<NMI>
panic+0x52/0x60
__kvm_handle_async_pf+0xb0/0xb0
exc_page_fault+0x71/0xb0
asm_exc_page_fault+0x27/0x30
RIP: 0010:__get_user_nocheck_8+0x6/0x20
perf_callchain_user+0x147/0x1a0
get_perf_callchain+0x12d/0x1c0
...
perf_event_nmi_handler+0x26/0x50
exc_nmi+0xe8/0x110
RIP: 0010:asm_exc_page_fault+0x0/0x30
</NMI>
The last RIP line shows the NMI landing on the first instruction of
the #PF entry. perf with user call chains takes such a fault: the
sampling NMI walks the user stack with __get_user(), which faults when
a saved frame pointer is not a valid user address or the walk runs off
the top of the stack. The same window exists when KVM, running as an
L1 hypervisor with DELIVERY_AS_PF_VMEXIT, reads the flags itself after
a #PF VM-exit.
The NMI entry code saves and restores CR2 for this nesting, but not the
async #PF reason. Andy Lutomirski described the race, with perf as the
example, when async #PF delivery was reworked in 2020 [1]. Saving and
clearing the flags across the NMI, next to CR2, was discussed there but
not merged.
Since that rework, Linux has stopped setting KVM_ASYNC_PF_SEND_ALWAYS,
so the host delivers "page not present" only for user mode faults and
a kernel-mode fault in NMI context (including kernel-mode #MC and #DB)
cannot be an async page fault. In that case, return without touching
the flags so that the interrupted handler finds them intact.
Tested on v7.3-rc5 in a 4-vCPU, 2 GiB guest restored from a snapshot
whose memory is served on demand through userfaultfd, so that reading
it back takes async #PFs, while every thread samples hardware cycles
with user call chains and points its frame pointer at an unmapped page.
Without this patch the guest panicked as above in 3 of 3 runs. With it,
the guest survived 3 of 3 runs of 60 seconds, and the host counted
148,441 to 153,334 kvm_async_pf_not_present events in the first 20
seconds of each. On 6.18.50 the results were the same: the guest
panicked without the patch and survived with it.
Link: https://lore.kernel.org/all/ed71d0967113a35f670a9625a058b8e6e0b2f104.1583547991.git.luto@kernel.org/ [1]
Link: https://lore.kernel.org/all/[email protected]/ [2]
Cc: [email protected] # v5.9+
Signed-off-by: EJ Campbell <[email protected]>
Link: https://patch.msgid.link/CADkxfh-uQTca_6jB1JPDq1MsKffF_UKGQR339oQSJNcoV75GNQ@mail.gmail.com
Signed-off-by: Paolo Bonzini <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
arch/x86/kernel/kvm.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=b77e9622bbade9812e2090c66d25c372f515e921
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99178 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99178
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:H/PR:L/UI:N/S:U/C:N/I:N/A:H
The Best / paranoid CVSS score: 4.7
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: 1
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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW
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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