* [CVE-2026-98813][MODERATE 7.0] bpf: Hold map BTF for the memory allocator destructor record [ Upstream
@ 2026-10-19 6:45 21% AL-KERNEL
0 siblings, 0 replies; 1+ results
From: AL-KERNEL @ 2026-10-19 6:45 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-98813
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: bpf: Hold map BTF for the memory allocator destructor record [ Upstream
Commit: b506f5699e145196ee29d76cf1ca1cbb2202c055
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b506f5699e145196ee29d76cf1ca1cbb2202c055
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98813
Analysis date: Mon, 19 Oct 2026 02:45:17 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES
Summary:
A deferred BPF map destructor can dereference map-owned BTF metadata after map teardown releases its final lifetime reference, causing a slab use-after-free and a possible local kernel crash, while stronger exploitation and below-host-admin reachability are not established.
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ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98813 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98813
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-2026-98813 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-98813 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=2 LEGACY=MODERATE_KPANIC RESULT=SAME V39_AEGIS_ONE_STEP_UP=MODERATE->MODERATE_KPANIC (final reconciliation agrees; DECISION: both 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-416;CWE-825;*CWE-664;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A use after free can occur when BPF map teardown releases the map BTF while a deferred memory allocator worker still uses a duplicated destructor record containing pointers into that BTF. The stale BTF pointer is later dereferenced by btf_is_kernel, btf_find_struct_meta, or btf_record_free while special fields from non-preallocated hash maps or resizable hash maps are being destroyed. The demonstrated primitive is a KASAN detected slab use after free read and the immediate impact is at least a local denial of service through a kernel crash. Reuse of the freed BTF allocation may also expose stale type metadata to later field destruction, creating a plausible but unproven path to broader memory corruption or privilege escalation. For the CVSS the PR:L value is used because triggering requires a local process with access to BPF map and BTF operations, including environments where BPF capabilities are delegated without granting unrestricted host administration. The issue is not network reachable. The paranoid score uses AC:H and CIA:HHH because reliable exploitation beyond a crash would require controlling object lifetime and memory reuse, and this possibility requires manual source review. Automated impact decision path. The original SimpleNet classifier selected MODERATE. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 3 with recommendation Borderline / manual review recommended. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were final reconciliation stage MODERATE to MODERATE_KPANIC because the cross model evidence reconciliation rule was applied. 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. Exact triggered rule markers were V39_AEGIS_ONE_STEP_UP and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 2 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 MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3) MAYBE READ BPF UAF LINUS KPANIC V39_AEGIS_ONE_STEP_UP MANUAL_REVIEW_REQUIRED - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=3
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 3
* Paranoid score: 3
* No floor or cap changed the score. The BPF privileged control-plane exception was checked, and REAL UAF FLOOR 5 was not applicable because the stale use is confined to deferred teardown with no established reclaim, repeated refcount mutation, or attacker-controlled post-free contents.
* Final recommended bucket: **Borderline / manual review recommended**::
2. Signal breakdown
Positive signals:
* +2 Real UAF. Map teardown can drop the final map BTF reference while the deferred allocator destructor record still contains pointers into that BTF. The workqueue later dereferences the freed allocation through `btf_is_kernel()`, `btf_find_struct_meta()`, or `btf_record_free()`.
* +1 Realistic kernel crash or strong DoS. KASAN demonstrates a slab use-after-free read in the deferred worker. Without KASAN, stale BTF or metadata access can plausibly produce an Oops or kernel crash.
Negative signals:
* No negative privilege penalty. BPF access is privilege-gated in many products, but the supplied evidence does not establish whether the complete trigger requires host-admin-equivalent privilege, a reduced capability, or an enabled unprivileged BPF interface.
* No race penalty. RCU and deferred work participate in the lifetime sequence, but the commit states that freeing a sufficiently populated map is enough to enter the deferred path. A narrow and externally uncontrollable timing window is not established.
* No niche-subsystem or hardware penalty. No special hardware is required.
Material non-awarded signals:
* Controlled reclaim or attacker-shaped replacement of the freed BTF allocation is not demonstrated.
* Structured stale-object leverage is not awarded. The stale BTF metadata is consumed during element and destructor cleanup, but meaningful attacker control over post-free contents or replacement state is not established.
* Attacker-influenced repeatable lifetime manipulation is not awarded. Userspace can create and destroy maps, but the evidence only establishes teardown-driven reproduction, not exploitation-relevant control over stale reuse.
* Strong or weak LPE leverage is not awarded merely because the stale object contains type metadata and pointers.
* Arbitrary or directed kernel read or write is not demonstrated.
* Confidentiality and integrity points are not awarded. The confirmed primitive is an internal stale read without a disclosure path or controlled modification of protected state.
* No extra Availability point is awarded because it would duplicate the same potential kernel crash.
* Remote reachability is not established.
* No concrete repeated release or refcount mutation exists. The map teardown can drop the BTF lifetime reference, but the later vulnerable operations read or traverse stale BTF state rather than performing a second `btf_put()` on the same dangling reference.
* Paranoid repeated-refcount leverage is therefore withheld.
* FLOOR 5 is not applied.
3. Reachability analysis
The trigger requires creation of BTF and an affected non-preallocated or resizable BPF hash map whose elements contain special fields such as local-type kptrs, list heads, or rb roots. The map must retain enough elements for bulk teardown to leave RCU callbacks in flight, after which map and associated program teardown can release the BTF before deferred allocator cleanup runs.
The patch does not establish that an ordinary unprivileged user can complete BTF loading, map creation, population, and teardown. Contemporary products commonly restrict BPF operations using `CAP_BPF`, `CAP_SYS_ADMIN`, related capability checks, and the `unprivileged_bpf_disabled` sysctl. However, the supplied evidence does not identify the evaluated product policy or exact capability checks for the complete sequence. Consequently, host-admin requirement and below-host-admin reachability remain unknown rather than being assumed either way.
No BPF program attachment, XDP attachment, netdevice control, or `CAP_NET_ADMIN` operation is shown as necessary. Namespace-scoped privilege is therefore not established. There is no network trigger, prior device setup, boot-mode dependency, or special hardware requirement.
The lifecycle appears reproducible through map population and teardown rather than through a clearly narrow race. This supports the UAF finding but does not independently establish controlled heap reuse.
4. Primitive and severity interpretation
The exact primitive is a slab use-after-free read of map-owned BTF state. A duplicated destructor record retains borrowed pointers into the map BTF, but it does not hold a reference that keeps the BTF alive until deferred allocator cleanup completes.
The stale allocation is an active structured object because its BTF and struct metadata are consulted while special fields are destroyed. Nevertheless, the demonstrated uses occur in a deferred teardown and destructor context. The supplied evidence does not show attacker-controlled reclaim, attacker-shaped post-free BTF contents, callback dispatch, a directed write, disclosure, or a security-boundary bypass.
This is category A from the cleanup distinction: an ordinary stale cleanup dereference. It is not category B repeated-lifetime corruption because no concrete second release or refcount mutation through the dangling BTF reference is present.
PrimitiveClass is CLASS_B because a real asynchronous lifetime violation and post-free dereference are directly demonstrated. It is not CLASS_A because no controlled replacement, arbitrary access, refcount takeover, or practical privilege-escalation primitive is established.
The demonstrated consequence is a KASAN-detected UAF with plausible kernel crash or DoS. Broader memory corruption or privilege escalation would require additional control over reclaim and stale metadata consumption that is not established by the patch.
ImportantCandidate is NO because this is a privilege-gated BPF teardown UAF with crash as the supported consequence and no meaningful independent reuse primitive. AutoDowngradeAllowed is YES because no unresolved repeated lifetime mutation, controlled corruption, authorization bypass, or cross-boundary primitive remains. Manual review is still recommended because the stale BTF metadata participates in nontrivial object-field destruction and product-specific BPF privilege exposure is unresolved.
5. One-sentence report phrase
A deferred BPF map destructor can dereference map-owned BTF metadata after map teardown releases its final lifetime reference, causing a slab use-after-free and a possible local kernel crash, while stronger exploitation and below-host-admin reachability are not established.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
Confirm the product-specific BPF privilege policy and inspect whether reclaimed BTF or struct metadata can influence `bpf_obj_free_fields()` beyond an immediate cleanup fault.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: bpf: Hold map BTF for the memory allocator destructor record [ Upstream
Commit: b506f5699e145196ee29d76cf1ca1cbb2202c055
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b506f5699e145196ee29d76cf1ca1cbb2202c055
Changed files:
kernel/bpf/hashtab.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=b506f5699e145196ee29d76cf1ca1cbb2202c055
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98813 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98813
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:L/PR:L/UI:N/S:U/C:N/I:N/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: 3
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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