[CVE-2026-99057][LOW] xen/netfront: don't leak the skb when xennet_fill_frags() fails

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-99057
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: xen/netfront: don't leak the skb when xennet_fill_frags() fails
Commit: 5051df031144753d3e18862514969146b601d089
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5051df031144753d3e18862514969146b601d089
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99057
Analysis date: Mon, 26 Oct 2026 23:40:21 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
A malicious Xen backend can repeatedly submit oversized response chains to leak guest skbs and fragment pages, potentially exhausting guest memory.

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ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-99057 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99057

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.

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AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-99057	LOW	https://www.kernelcve.org/list/?q=CVE-2026-99057 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=LOW RESULT=DIFF TEMP_DISTANCE_MERGE_SKIP=AEGIS_NO_POSITIVE_SEVERITY_EVIDENCE (final Legacy and Aegis differ; FINAL: LOW, Aegis: MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';*CWE-401;*CWE-772;Other CVSS 'AV:A/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.7';DESCR 'xen netfront leaks the dequeued skb and its attached fragment pages when xennet_fill_frags fails because an oversized backend response chain exceeds the skb fragment capacity. A backend that controls response slot counts can repeat the failing path and accumulate unreleased guest memory. For the CVSS the PR:L value reflects that reliable triggering requires prior control of the Xen backend domain or backend service rather than ordinary network access to the guest. The demonstrated primitive is persistent resource retention with possible guest denial of service after sustained abuse. There is no evidence of UAF, OOB access, information disclosure, or memory corruption. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 5.7 and the production ActionableScore returned 2 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were ActionableScore thresholds stage MODERATE_KPANIC to LOW because the ActionableScore threshold rule was applied. final reconciliation stage LOW to LOW because KP specialist did not establish a qualifying panic and 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 DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 and DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 and KPANIC_UNSUPPORTED_REMOVED and V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED. The independent weaker Aegis shadow evaluated CVSS 3.9 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	MAYBE	SIMPLEFIX LEAK ERRORPATH NETWORK SKB LINUS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 KPANIC_UNSUPPORTED_REMOVED V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED	-	-	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=2
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: 2.
* Conservative score: 2; no floor or cap applied.
* Raw paranoid score: 2.
* Paranoid score: 2; no additional technically supported primitive.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Concrete persistent kernel resource/ref leak: on `xennet_fill_frags()` failure, the dequeued skb and its attached fragment pages are omitted from both `tmpq` and error cleanup. Requeuing the skb restores cleanup ownership.
* +1 Strong DoS: a malicious Xen backend controls response-chain slot counts and can repeatedly exceed skb fragment capacity, accumulating unreleased guest memory.
* No extra resource-exhaustion point: the guest-memory DoS is the same consequence already counted above.
* No UAF, OOB access/write, stale-object use, shared-backing write, authorization bypass, disclosure, or integrity primitive is shown.
* NEW52/B5-R does not apply: there is no retry-accumulated heap OOB write or later consumption of corrupted state.
* No remote-network bonus: ordinary packets do not control Xen backend response descriptors.

3. Reachability analysis

The relevant actor controls the Xen backend domain or backend service and can choose response-chain lengths. Ordinary network users and ordinary guest-local users are not shown able to trigger the leak. Whether backend control is equivalent to host administration, delegated to a less-privileged driver domain, or requires host UID 0 is not established. No namespace capability path, special hardware, race, or timing requirement is shown.

4. Primitive and severity reading

The exact primitive is persistent loss of an skb and its fragment-page references during error cleanup. It is a resource-retention/DoS defect, not memory corruption or post-lifetime use. Repetition can pressure or exhaust guest memory, but no host compromise, cross-guest effect, or useful corruption primitive is evidenced. Therefore this is CLASS_C, not an Important candidate, and automatic Moderate treatment remains appropriate.

5. One-sentence report phrase

A malicious Xen backend can repeatedly submit oversized response chains to leak guest skbs and fragment pages, potentially exhausting guest memory.

6. Manual review advice

NO MANUAL CHECK NEEDED

The patch and call path establish a bounded resource-leak class without unresolved corruption or authorization leverage.

7. Call-site confidence

Call-site confidence: high

======================================================================
UPSTREAM PATCH SUMMARY
======================================================================

Patch: xen/netfront: don't leak the skb when xennet_fill_frags() fails
Commit: 5051df031144753d3e18862514969146b601d089
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5051df031144753d3e18862514969146b601d089

Commit description:

When a response chain has more slots than fit in the skb's frags,
xennet_fill_frags() returns an error and xennet_poll() jumps to its
error path.  That path moves what's left on tmpq to errq to be freed,
but the skb being filled was already dequeued from tmpq, so it's never
freed.  Each chain that overflows leaks the skb and the pages attached
to it as frags, and the backend decides how many slots it sends.

Put the skb back on tmpq before taking the error path, like the
xennet_set_skb_gso() failure just above it does.

Fixes: ad4f15d ("xen/netfront: don't bug in case of too many frags")
Cc: stable@vger.kernel.org
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://patch.msgid.link/20261007-b4-xen-netfront-fill-frags-leak-v1-1-a8a01ff9cd52@toxicpanda.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/net/xen-netfront.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=5051df031144753d3e18862514969146b601d089

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DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-99057 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99057

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:L/PR:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:A/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.7

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: 2

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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).