* [CVE-2026-98047][MODERATE 7.0] bpf: Check ancestor frames for rbtree callbacks [ Upstream
@ 2026-09-25 18:54 21% AL-KERNEL
0 siblings, 0 replies; 1+ results
From: AL-KERNEL @ 2026-09-25 18:54 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-98047
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: bpf: Check ancestor frames for rbtree callbacks [ Upstream
Commit: b545fa447e09e6844868deb2bee228827fe54d15
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b545fa447e09e6844868deb2bee228827fe54d15
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98047
Analysis date: Fri, 25 Sep 2026 14:54:55 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98047 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98047
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-2026-98047 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-98047 CHECK WITH IMPACT FROM ORIG NN LOW Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-284;*CWE-664;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H';BEST CVSS score: '7.8';DESCR 'A BPF verifier flaw allows nested static subprogram calls in an rbtree comparator callback to bypass restrictions that require the rbtree root lock and compared node to remain valid. The callback can reach a state where the node is removed and freed while native rbtree insertion retains stale parent and link pointers. Native insertion can then resume and link freed memory into the tree, creating a use after free and kernel memory corruption condition. For the CVSS the PR:L value is used for the paranoid score because BPF access may be delegated to a container root, service account, or other reduced capability user in some deployments. The base score uses PR:H because loading the required privileged BPF program normally requires elevated BPF capabilities. The issue is local and is not directly network reachable. Impact may include kernel crash, confidentiality impact, integrity impact, or local privilege escalation due to the use after free primitive. YES REQUIRES MANUAL CHECK.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) YES LOCK BPF UAF IMPROVEONLY LINUS KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK YES NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 4
* Conservative score: 5
* Raw paranoid score: 4
* Paranoid score: 5
* Applied conservative floor: 4 -> 5. The stale use is active rbtree insertion processing, not a cleanup-only dereference. The BPF privileged control-plane exception does not apply because the callback directly drives node removal and release during an active tree mutation.
* Applied paranoid floor: 4 -> 5.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
* +2 Real UAF and active lifetime violation. The comparator callback can remove and drop the node while native rbtree insertion retains raw parent and link pointers. Insertion later resumes with stale references.
* +1 Attacker-influenced active-object lifetime manipulation. The verifier bypass permits the BPF callback to unlock the tree, remove the compared node, drop it, and relock before insertion resumes.
* +1 Structured stale-object exploitation leverage. Native rbtree insertion consumes retained parent and link state after the node lifetime ends and links freed memory into the active tree. This is meaningful active-state corruption beyond an immediate poisoned-pointer fault.
* +0 Controlled reclaim not demonstrated. The patch does not establish attacker-controlled allocation reuse or replacement of the freed node.
* +0 Arbitrary or directed kernel write not demonstrated. The stale tree-link operation is concrete, but target selection and write contents are not shown to be attacker-arbitrary.
* +0 Reliable kernel crash not separately awarded. Kernel tree corruption can plausibly crash the kernel, but the supplied evidence does not establish a reliable observed crash.
* +0 Local unprivileged trigger not awarded. The complete BPF load, program type, kfunc access, and attachment privilege path is not established below host-admin-equivalent privilege.
* +0 Remote reachability not established.
* +0 Weak LPE and strong LPE bonuses not awarded. The patch establishes a real UAF and active tree corruption, but not controlled reclaim, credential manipulation, callback control, arbitrary read or write, or a demonstrated escalation chain.
3. Reachability analysis
The relevant attacker is a local process able to load a verifier-accepted BPF program using rbtree kfuncs and static subprogram calls. The program must invoke an rbtree comparator callback, call a nested static subprogram, release the required rbtree lock, remove and drop the node being compared, reacquire the lock, and allow native insertion to resume.
The supplied patch does not establish whether the complete operation is available to an ordinary unprivileged user, a delegated BPF-capable service, a container root, or only a host administrator. BPF loading and use of the relevant kfuncs are commonly privilege-gated, but the exact capability checks and namespace scope for this specific program type are not visible. Therefore HostAdminRequired and NamespaceScopedPrivilege are both UNKNOWN.
No remote trigger, special hardware dependency, firmware prerequisite, or timing race is established. This is an ordered verifier-bypass sequence rather than a narrow external race. The attacker controls the callback execution and the lifetime-changing operations once the verifier accepts the program.
4. Primitive and severity interpretation
The exact primitive is a real active-object use-after-free. Native rbtree insertion preserves raw parent and link pointers across a BPF comparator callback. The verifier flaw permits a nested static subprogram to evade callback restrictions and free the node involved in the insertion. Native insertion then resumes and links freed memory into the live rbtree.
This is an active structured-object UAF, not fixed stale storage and not a shallow destructor-only stale dereference. The stale operation performs active tree mutation after the affected node may have been freed. However, the supplied evidence does not show controlled replacement of the freed allocation, attacker-shaped reclaimed contents, arbitrary write, callback dispatch through reused memory, information disclosure, or a demonstrated privilege-escalation chain.
The appropriate classification is CLASS_B. The corruption primitive is stronger than an ordinary crash-only NULL dereference because it creates a stale tree-state mutation in an active kernel data structure. It is not CLASS_A because controlled post-free reuse or a directed corruption primitive is not established.
ImportantCandidate is YES because active rbtree corruption leaves meaningful exploitation leverage unresolved. AutoDowngradeAllowed is NO because automatically treating this as an ordinary crash-only BPF issue would discard a concrete post-lifetime tree-corruption primitive. Operationally, this is at least Actionable Moderate and requires manual analysis of the BPF rbtree implementation, node allocation model, object lifetime semantics, and potential reuse paths.
5. One-sentence report phrase
A BPF verifier bypass allows a nested rbtree comparator subprogram to free a node while native insertion retains stale tree pointers, resulting in an active kernel use-after-free and rbtree corruption primitive.
6. Manual review recommendation
MANUAL CHECK REQUIRED
The patch directly establishes that freed memory can be linked into an active rbtree after verifier restrictions are bypassed. Controlled reclaim and privilege reachability are unresolved, so the issue should not be automatically downgraded.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: bpf: Check ancestor frames for rbtree callbacks [ Upstream
Commit: b545fa447e09e6844868deb2bee228827fe54d15
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b545fa447e09e6844868deb2bee228827fe54d15
Changed files:
kernel/bpf/verifier.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=b545fa447e09e6844868deb2bee228827fe54d15
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98047 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98047
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:H/UI:N/S:U/C:H/I:H/A:H
The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
The Best / paranoid CVSS score: 7.8
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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