* [CVE-2026-100080][IMPORTANT] xen-netback: Fix UAF when reading statistics data
@ 2026-11-07 2:11 21% AL-KERNEL
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From: AL-KERNEL @ 2026-11-07 2:11 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-100080
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: xen-netback: Fix UAF when reading statistics data
Commit: 4223b33be49ee09acb6f1d26beb4a36dd3b8d63a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4223b33be49ee09acb6f1d26beb4a36dd3b8d63a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-100080
Analysis date: Fri, 06 Nov 2026 22:11:47 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
A Xen guest can induce queue teardown without an RCU grace period, allowing a concurrent host sysfs statistics reader to access freed or repurposed netback queue memory.
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ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-100080 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-100080
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-2026-100080 IMPORTANT https://www.kernelcve.org/list/?q=CVE-2026-100080 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=2 LEGACY=IMPORTANT RESULT=DIFF INDEPENDENT_LEGACY=IMPORTANT INDEPENDENT_RESULT=DIFF TEMP_KPANIC_MERGE=IMPORTANT_HARD_VETO 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:A/AC:H/PR:L/UI:N/S:C/C:H/I:N/A:H';*CWE-416;CWE-362;CWE-367;Other CVSS 'AV:A/AC:H/PR:L/UI:R/S:C/C:L/I:N/A:H';BEST CVSS score: '7.7';DESCR 'An RCU synchronization race in xen netback can free queue statistics while a host sysfs reader still holds an RCU protected reference, producing a read side UAF. A malicious or faulty Xen guest can supply illegal Xenstore data to force queue teardown during frontend connection, but exploitation also requires a concurrent host read of the affected statistics, so AC:H is appropriate. For the CVSS the PR:L value reflects control of a guest frontend and its Xenstore data rather than host administrative access. The demonstrated primitive is a UAF read that can expose reused kernel data or crash the backend or host, while no write primitive supports integrity impact. Automated impact decision path. The original SimpleNet classifier selected IMPORTANT. The primary LLM estimated paranoid CVSS 7.7 and the production ActionableScore returned 6 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were final reconciliation stage IMPORTANT to IMPORTANT because a specific evidence based protection or manual review veto was applied and 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 TEMP_KPANIC_MERGE_IMPORTANT_VETO and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 2.1 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 IMPORTANT.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6) MAYBE LEAK UAF NETWORK LINUS TEMP_KPANIC_MERGE_IMPORTANT_VETO KPANIC_UNSUPPORTED_REMOVED - - checked
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ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw base score: 4.
* Conservative score: 5. Concrete RCU referenced-object lifetime calibration raises 4 to 5.
* Raw paranoid score: 5.
* Paranoid score: 6. Active structured RCU state with explicitly possible reuse and incompletely characterized reader behavior supports the one-point RCU calibration.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
* +2 real UAF: queue/statistics storage is freed while a sysfs RCU reader may still access it.
* +1 concrete RCU lifetime mismatch: the queue remains observable to readers, but teardown frees its data before a grace period; `synchronize_net()` restores that ordering.
* +1 plausible host crash/DoS from dereferencing freed or repurposed queue memory.
* Paranoid +1: the commit explicitly permits access after storage is “in new use,” and the reader consumes structured statistics fields rather than a guaranteed poison fault.
* No race penalty: concurrency is required, but the evidence does not establish an exceptionally narrow or one-shot race.
* No confidentiality point: sysfs provides a read-back path, but disclosure of protected data to an attacker is not established.
* No write, controlled reclaim, attacker-shaped replacement, callback use, integrity impact, or LPE is shown.
* No extra availability point; it would duplicate the same possible crash.
3. Reachability analysis
A malicious Xen guest can provide illegal Xenstore data and induce backend connection-error teardown. A separate concurrent host sysfs statistics read is required; the guest’s ability to induce that read is not shown. Thus this is a compound guest-plus-host-reader trigger. The reader’s UID/capability requirements are not supplied, so effective host-admin and host-UID-0 requirements remain unknown. Linux namespace capabilities are not involved. No special physical hardware is required beyond an active Xen netback configuration.
4. Primitive and severity reading
This is an active structured-object read UAF, not fixed stale storage: queue data can be freed and potentially reused while an RCU reader consumes statistics fields. It is broader than a destructor-only stale dereference, but no second release/refcount mutation exists. The shown effects are stale/reused-memory reads and possible host failure; useful disclosure or guest-controlled host corruption remains unproven.
CLASS_B is appropriate. ImportantCandidate is YES because guest-influenced teardown crosses into host-kernel lifetime state and the behavior of reads from reused storage remains materially unknown. Auto-downgrade is disallowed pending review, but this is not proof of host escape or Important severity.
5. One-sentence report phrase
A Xen guest can induce queue teardown without an RCU grace period, allowing a concurrent host sysfs statistics reader to access freed or repurposed netback queue memory.
6. Manual review advice
MANUAL CHECK REQUIRED
Review the host-reader permissions, repeatability, reused-object field exposure, and whether the guest can induce or synchronize the sysfs read.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: xen-netback: Fix UAF when reading statistics data
Commit: 4223b33be49ee09acb6f1d26beb4a36dd3b8d63a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4223b33be49ee09acb6f1d26beb4a36dd3b8d63a
Commit description:
The network statistic data in the driver is protected by RCU.
Unfortunately when tearing down the queues after an error during
connecting to the frontend (e.g. due to illegal Xenstore data filled
in by the guest), the statistic data is being freed without
synchronizing the RCU state. In case the data is being read via sysfs
in this phase, access to already freed memory or memory being in new
use is possible.
Fix that by calling synchronize_net() before freeing the queue data.
This is XSA-526 / CVE-2026-100080
Reported-by: Yehyeong Lee <[email protected]>
Signed-off-by: Juergen Gross <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
drivers/net/xen-netback/xenbus.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=4223b33be49ee09acb6f1d26beb4a36dd3b8d63a
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-100080 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-100080
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:L/UI:R/S:C/C:L/I:N/A:H
The Best / paranoid CVSS vector: AV:A/AC:H/PR:L/UI:N/S:C/C:H/I:N/A:H
The Best / paranoid CVSS score: 7.7
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: 5
Paranoid ActionableScore: 6
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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