* [CVE-2026-98719][IMPORTANT] KVM: SEV: Nullify "have run CPUs" mask pointer when freeing it
@ 2026-10-24 9:24 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-24 9:24 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-98719
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: KVM: SEV: Nullify "have run CPUs" mask pointer when freeing it
Commit: b1de3ebf59507e0edacaa5cfa2225bc1869e1b63
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b1de3ebf59507e0edacaa5cfa2225bc1869e1b63
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98719
Analysis date: Sat, 24 Oct 2026 05:24:02 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
A failed SEV initialization can leave a freed off-stack CPU-mask pointer in VM state, causing a second heap free during VM destruction.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98719 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98719
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: IMPORTANT
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-2026-98719 IMPORTANT https://www.kernelcve.org/list/?q=CVE-2026-98719 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=IMPORTANT RESULT=DIFF INDEPENDENT_LEGACY=IMPORTANT INDEPENDENT_RESULT=DIFF TEMP_KPANIC_MERGE=IMPORTANT_WINS_DISAGREEMENT PRE_MERGE_LEGACY=IMPORTANT (final Legacy and Aegis differ; FINAL: IMPORTANT, Aegis: MODERATE_KPANIC) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-415;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'KVM SEV can retain a stale have_run_cpus pointer when __sev_guest_init fails after allocating the CPU mask and frees it through the error path. Later VM destruction can free the same mask again when CONFIG_CPUMASK_OFFSTACK is enabled. This is a kernel heap double free with a direct crash risk and a plausible but unproven path to allocator corruption or local privilege escalation. For the CVSS the PR:L value reflects that an attacker needs local access to KVM and the ability to create and initialize an SEV VM. AC:H reflects the need to reach a later SEV initialization failure and potentially control heap reuse. The issue is not directly network reachable and requires SEV capable hardware plus an affected kernel configuration. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 5 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to IMPORTANT because the CVSS score, vector or bug class threshold rule was applied. final reconciliation stage IMPORTANT to IMPORTANT because KP specialist did not establish a qualifying panic. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist produced a review artifact and its decision was processed by the applicable downgrade and preservation rules. Exact triggered rule markers were INCREASED_TO_HIGH_BASED_ON_GUESSCVSS and TEMP_KPANIC_MERGE_IMPORTANT_WINS and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 4 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is IMPORTANT.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) MAYBE DANGER VIRT INIT NOMEMCHK ERRORPATH UAF HARDWARE INCREASED_TO_HIGH_BASED_ON_GUESSCVSS TEMP_KPANIC_MERGE_IMPORTANT_WINS KPANIC_UNSUPPORTED_REMOVED - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw base score: 3.
* Conservative score: 5. REAL UAF FLOOR 5 applies.
* Raw paranoid score: 4.
* Paranoid score: 5. The same floor applies; no unsupported escalation above it.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
* +2 Real lifetime violation: `e_free` frees the off-stack `have_run_cpus` allocation without clearing `sev->have_run_cpus`.
* +1 Concrete repeated release: the exact stale field survives and `sev_vm_destroy()` later passes it to `free_cpumask_var()` again. The first free can end the allocation’s lifetime; the second is a real lifetime/allocator mutation.
* +1 Reliable crash/strong DoS: a heap double-free plausibly faults or corrupts allocator state.
* -1 Reduced/delegated privilege: supplied evidence requires local KVM access and permission to create and initialize an SEV VM, but not host-administrator-equivalent power.
* Paranoid +1: the second free acts on already-freed heap storage and can mutate allocator metadata. This is unknown post-lifetime corruption, not proof of controlled reclaim or arbitrary write.
* FLOOR 5 applies because the first free can destroy the allocation, the exact pointer remains, and a specific second free occurs.
* Not awarded: controlled reclaim, chosen adjacent victim, arbitrary write, confidentiality/integrity impact, LPE, guest-only reachability, or extra availability for the same crash.
3. Reachability analysis
The supported actor is a local VMM process with `/dev/kvm` access that can create an SEV VM, drive `__sev_guest_init()` into a post-allocation failure, and later destroy the VM. This is not shown as a malicious-guest-only trigger. SEV-capable hardware and `CONFIG_CPUMASK_OFFSTACK` are required, but neither supports a rare-hardware penalty. No timing race is shown. User-namespace or namespace-scoped completion of the trigger is not established.
4. Primitive and severity reading
This is fixed stale heap storage, but not merely a stale cleanup dereference: it is a specific second free through a dangling pointer. The fix centralizes freeing and zeroes the pointer, preventing `sev_vm_destroy()` from freeing it again. The proven primitive is CLASS_B heap double-free with crash and allocator-corruption potential. Controlled exploitation remains unproven, so it is not CLASS_A, but the concrete repeated lifetime mutation requires Important-vs-Moderate review and prohibits automatic downgrade.
5. One-sentence report phrase
A failed SEV initialization can leave a freed off-stack CPU-mask pointer in VM state, causing a second heap free during VM destruction.
6. Manual review advice
MANUAL CHECK REQUIRED
The exact double-free is proven, while post-free allocator leverage remains materially unresolved.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: KVM: SEV: Nullify "have run CPUs" mask pointer when freeing it
Commit: b1de3ebf59507e0edacaa5cfa2225bc1869e1b63
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b1de3ebf59507e0edacaa5cfa2225bc1869e1b63
Commit description:
Nullify have_run_cpus when freeing the mask, particularly in the error path
of __sev_guest_init(), so that KVM doesn't have to subtly use sev->active
to track whether or not the mask has been freed. As pointed out by Sashiko,
blindly freeing the mask in sev_vm_destroy() results in a double-free if
the mask is freed if __sev_guest_init() fails.
Throw the logic in a helper as nullifying the pointer is frustratingly
difficult and weird due to have_run_cpus being a single-entry array when
CPUMASK_OFFSTACK=n. Deliberately don't use CPUMASK_VAR_NULL, as it's not
directly assignable when the cpumask is on-stack, e.g. requires using a
local variable and a memcpy(), which is beyond ridiculous. Furthermore,
while clearing the on-stack bitmask is an unnecessary and arguably unwanted
side effect, KVM absolutely relies on '0' being the "null" value given that
the struct is zero-allocated.
Opportunistically add an alloc() helper to pair with free(); there are just
enough call sites to make doing so worthwhile.
Fixes: 12c1f6e ("KVM: SEV: Free have_run_cpus during VM destruction even if VM is no longer SEV")
Cc: [email protected]
Reported-by: Sashiko Bot <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>
Message-ID: <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
arch/x86/kvm/svm/sev.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=b1de3ebf59507e0edacaa5cfa2225bc1869e1b63
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98719 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98719
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: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
The Best / paranoid CVSS score: 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: 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: IMPORTANT
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): IMPORTANT
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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