* [CVE-2026-98571][MODERATE 7.0] net: stmmac: clear stale buf->page after recycling on skb build failure [ Upstream
@ 2026-10-19 10:41 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-19 10:41 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-98571
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net: stmmac: clear stale buf->page after recycling on skb build failure [ Upstream
Commit: 501baaeca7d128fa0a2f6953cf5873c5b0d7cf89
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=501baaeca7d128fa0a2f6953cf5873c5b0d7cf89
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98571
Analysis date: Mon, 19 Oct 2026 06:41:09 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
A recycled page remains referenced by an active stmmac RX descriptor after skb construction failure, creating duplicate page ownership and a plausible attacker-influenced DMA write into memory reassigned to another consumer.
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ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98571 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98571
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.
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AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-98571 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-98571 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE_KPANIC RESULT=SAME (final reconciliation agrees; DECISION: both MODERATE_KPANIC) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-672;*CWE-416;CWE-362;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '8.1';DESCR 'A stale page pointer remains in the stmmac RX buffer after napi_build_skb fails and the page is returned to the page pool. The following refill can program the recycled page back into an RX descriptor without obtaining ownership again. If the page pool assigns the same page to another consumer, the network device may later DMA attacker influenced packet data into memory that is already used elsewhere. This creates a plausible UAF like or duplicate ownership condition that can cause memory corruption, information exposure, a kernel crash, or potentially privilege escalation. For the CVSS the PR:N value is used because the paranoid scenario requires no account or local execution when inbound network traffic can reach the affected stmmac interface. Attack complexity is high because reliable triggering also depends on napi_build_skb failure, page reuse timing, and a system using the affected stmmac receive path. The base assessment treats exposure as adjacent network and uses limited confidentiality and integrity impact. The paranoid assessment treats routed traffic as potentially sufficient and uses high confidentiality, integrity, and availability impact because DMA may overwrite a page after ownership has changed. Automated impact decision path. The original SimpleNet classifier selected IMPORTANT. The primary LLM estimated paranoid CVSS 8.1 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 KP and FD specialist checks stage IMPORTANT to MODERATE_KPANIC because a specific evidence based protection or manual review veto was applied and FD false positive review allowed a downgrade. 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 V33_ACTIVE_STATE_BOUNDARY_CANDIDATE and IMPORTANT_CANDIDATE_MANUAL_REVIEW and KP_ONLY_DOWNGRADE_VETO and DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH. The independent weaker Aegis shadow evaluated CVSS 3.1 and ActionableScore 1 with primitive class CLASS_B and manual review RECOMMENDED. Its independently reconciled impact was MODERATE_KPANIC. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) MAYBE REMOTE SIMPLEFIX NETWORK SKB LINUS KPANIC V33_ACTIVE_STATE_BOUNDARY_CANDIDATE IMPORTANT_CANDIDATE_MANUAL_REVIEW KP_ONLY_DOWNGRADE_VETO DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw conservative score: 3
* Conservative score: 4
* Raw paranoid score: 4
* Paranoid score: 5
* Applied conservative floor: 3 -> 4 under the real lifetime violation floor.
* Applied paranoid floor: 4 -> 5 because an active RX descriptor can remain associated with a page whose page-pool ownership was relinquished, creating meaningful unresolved DMA corruption leverage.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
Conservative signals:
* +2 Real lifetime and ownership violation. `page_pool_recycle_direct()` relinquishes ownership, but the stale `buf->page` is subsequently consumed by refill and installed into an active RX descriptor.
* +1 High-value DMA and page-ownership state. The defect concerns ownership of a page exposed as a receive DMA destination, not passive metadata.
* Total: 3.
Paranoid additional signal:
* +1 Structured stale-object leverage. The descriptor can remain active after the page becomes available to the pool, and later packet DMA may write attacker-influenced bytes after reassignment to another consumer.
* Total: 4.
Not awarded:
* Direct remote reachability +2 was withheld because packets reach the RX path, but supplied evidence does not show that a remote sender can reliably force `napi_build_skb()` failure and the required page reuse.
* Reliable kernel crash +1 was withheld because no crash is demonstrated.
* Strong exploitation leverage +2 was withheld because controlled page reclaim, target selection, arbitrary write, callback control, and privilege escalation are not established.
* Confidentiality and integrity bonuses were withheld because no concrete disclosure or protected-object modification is demonstrated.
* No extra availability point was awarded.
* No race penalty was applied. The stale-pointer sequence is deterministic after the allocation failure, although useful overlapping reuse has timing dependencies.
* No hardware rarity penalty was applied merely because this is an stmmac driver.
* No concrete repeated release or refcount operation exists. The first operation returns the page to the pool, but no second `put`, `free`, or refcount mutation on the same reference is shown.
* The dedicated repeated-release +1 and paranoid repeated-refcount +1 were therefore withheld.
* FLOOR 5 was applied only to the paranoid score for active DMA and page-ownership leverage, not for repeated release.
* NEW52 B5-R does not apply because no retry-accumulated heap OOB write is established.
* NEW53 B3-PENDING does not apply because this is not a demonstrated shared file-backed write.
* NEW54 B4-PENDING does not apply because there is no authorization decision.
3. Reachability analysis
Inbound traffic reaches `stmmac_rx()` on systems using an affected stmmac interface. Packet contents can influence bytes later written by RX DMA, but the complete security-relevant trigger also requires `napi_build_skb()` failure, stale-page refill, pool reassignment, and subsequent DMA.
No host credential check is involved in ordinary receive processing. However, the evidence does not establish that an external or local unprivileged actor can force the complete sequence, so `HostAdminRequired` and `RequiresHostUID0` remain `UNKNOWN` rather than being inferred as `NO`.
No namespace-specific capability path is supplied. Prior host setup must provide and activate the stmmac interface, but that normal setup alone does not establish either host-admin-only triggering or namespace-scoped triggering. There is no explicit race in the faulty control flow. Exploitation-relevant overlap depends on allocation failure and page reuse timing.
4. Primitive and severity interpretation
The exact primitive is a stale post-ownership page reference. The page is returned to `page_pool`, but the uncleared pointer causes refill to install that page into an active RX descriptor without reacquiring ownership. This is a directly implied UAF-like lifetime violation even though the page is pooled rather than immediately freed.
This is active structured state, not fixed stale storage. The stale pointer controls an operational DMA descriptor, and the recycled page can potentially be assigned to another consumer. The subsequent privileged consumer is the NIC and host RX path, which may DMA packet bytes into the page.
The demonstrated consequence is duplicate ownership and invalid descriptor programming. A later DMA overwrite of reassigned memory is technically credible but not demonstrated against a named object. Kernel crash, disclosure, privilege escalation, and arbitrary write remain unresolved consequences.
`PrimitiveClass=CLASS_B` because a concrete active ownership/lifetime violation exists, but controlled reclaim or directed corruption is not established. `ImportantCandidate=YES` and `AutoDowngradeAllowed=NO` preserve the issue for review because automatic downgrade would discard the unresolved page-reuse and DMA-write primitive. This does not prove Important severity.
Under V33, the three relevant elements are:
1. Write: RX DMA can write attacker-influenced packet data through the reinstalled descriptor.
2. Privileged consumer: the host NIC and stmmac receive path consume the descriptor.
3. Boundary: external traffic can supply packet bytes, but control over allocation failure and cross-consumer page reuse remains unresolved.
5. One-sentence report phrase
A recycled page remains referenced by an active stmmac RX descriptor after skb construction failure, creating duplicate page ownership and a plausible attacker-influenced DMA write into memory reassigned to another consumer.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Confirm whether `page_pool_recycle_direct()` can make the page available to another descriptor or consumer before refill reuses the stale pointer, and whether remotely supplied traffic can reliably combine memory pressure, reassignment, and subsequent DMA.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: net: stmmac: clear stale buf->page after recycling on skb build failure [ Upstream
Commit: 501baaeca7d128fa0a2f6953cf5873c5b0d7cf89
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=501baaeca7d128fa0a2f6953cf5873c5b0d7cf89
Changed files:
drivers/net/ethernet/stmicro/stmmac/stmmac_main.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=501baaeca7d128fa0a2f6953cf5873c5b0d7cf89
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98571 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98571
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:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H
The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
The Best / paranoid CVSS score: 8.1
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: 4
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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