* [CVE-2026-100082][MODERATE 7.0] xen-blkback: Avoid UAF when reading I/O statistics
@ 2026-11-07 2:03 21% AL-KERNEL
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From: AL-KERNEL @ 2026-11-07 2:03 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-100082
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: xen-blkback: Avoid UAF when reading I/O statistics
Commit: 253cff0526781e28b1b0f9fdacd74ecdc23d3ab7
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=253cff0526781e28b1b0f9fdacd74ecdc23d3ab7
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-100082
Analysis date: Fri, 06 Nov 2026 22:03:59 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate
Manual review required: YES
Summary:
A malicious Xen frontend can race device teardown with backend sysfs statistics traversal, causing a stale read of the freed ring array and potentially crashing or disrupting the storage backend.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-100082 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-100082
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-2026-100082 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-100082 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 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:A/AC:H/PR:L/UI:N/S:C/C:L/I:N/A:H';*CWE-416;CWE-362;**CWE-476;Other CVSS 'AV:A/AC:H/PR:L/UI:N/S:C/C:N/I:N/A:H';BEST CVSS score: '6.5';DESCR 'A race in xen blkback allows sysfs I O statistics code to traverse blkif rings while frontend teardown clears and frees the same array. This creates a confirmed use after free read or NULL dereference in the backend domain and can crash or disrupt the host storage backend. For the CVSS the PR:L value reflects that the attacker must control a Xen frontend domain able to participate in device teardown, while no privileges in the backend domain are required. AC:H reflects the need to overlap teardown with a concurrent host sysfs statistics read. The demonstrated primitive is an invalid read rather than an arbitrary write, so privilege escalation or integrity compromise is not established. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 6.5 and the production ActionableScore returned 4 with recommendation Actionable Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage LOW to MODERATE_KPANIC because the ActionableScore threshold rule was applied. 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 has no review artifact for this run. Exact triggered rule markers were INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4. The independent weaker Aegis shadow evaluated CVSS 2.3 and ActionableScore 4 with primitive class CLASS_B and manual review REQUIRED. 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 (with actual score 4) MAYBE LOCK NULLPTR INIT LEAK UAF HARDWARE LINUS KPANIC INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=REQUIRED
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw base score: 3.
* Conservative score: 4. Real-UAF floor 4 applied because a freed ring array is actually traversed outside teardown synchronization.
* Raw paranoid score: 3.
* Paranoid score: 4. No additional exploit primitive supports a higher score.
* Final recommended bucket: **Actionable Moderate**::
2. Signal breakdown
* +2 — Confirmed UAF: sysfs statistics traversal can retain and read `blkif->rings` while disconnect clears and frees that array.
* +1 — Plausible backend-domain kernel crash or storage-backend disruption from stale or NULL access.
* +1 — Proven Xen frontend-to-backend availability boundary: the supplied XSA analysis grounds malicious frontend participation in teardown.
* -1 — Materially difficult race: frontend teardown must overlap an independent backend sysfs statistics read.
* No repeated release/refcount operation exists; the stale operation only reads ring statistics.
* No OOB write, post-free write, controlled reclaim, replacement contents, callback consumption, disclosure, authorization bypass, or integrity primitive is shown.
* No second availability point is awarded for the same crash.
* NEW52/B5-R and NEW53/B3-PENDING do not apply: there is no write.
* B4-PENDING does not apply because no DENY-to-ALLOW authorization decision exists.
3. Reachability analysis
A malicious Xen frontend can participate in device teardown, while a backend-domain process must concurrently read the world-readable sysfs statistics attribute. This is therefore a compound timing condition; the guest does not itself control the sysfs read. Neither host UID 0 nor host-administrator capability is required for that read, and no Linux namespace-scoped capability is involved. The evidence positively supports frontend-to-backend DoS reachability but not reliable attacker control of the race.
4. Primitive and severity reading
The primitive is a CLASS_B lifetime race involving an active ring array, but the stale use is a shallow statistics read rather than structured post-reuse processing. It is not a second lifetime mutation and does not establish useful corruption past the possible fault. A backend crash is supported; confidentiality, integrity compromise, host code execution, and privilege escalation are unproven. ImportantCandidate is therefore NO, while manual review remains required due to the Xen trust boundary and XSA-confirmed host/backend availability impact. Auto-downgrade remains allowed because the demonstrated primitive is constrained and crash-oriented.
5. One-sentence report phrase
A malicious Xen frontend can race device teardown with backend sysfs statistics traversal, causing a stale read of the freed ring array and potentially crashing or disrupting the storage backend.
6. Manual review advice
MANUAL CHECK REQUIRED
Confirm product-specific backend-domain failure scope and whether operational polling makes the race practically repeatable.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: xen-blkback: Avoid UAF when reading I/O statistics
Commit: 253cff0526781e28b1b0f9fdacd74ecdc23d3ab7
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=253cff0526781e28b1b0f9fdacd74ecdc23d3ab7
Commit description:
Reading I/O statistics via sysfs concurrently with tearing down a
frontend device might result in UAF or a NULL dereference, as the
reading of the statistics data is not synchronized with freeing the
data.
Use a rwlock for synchronizing.
This is XSA-528 / CVE-2026-100082
Reported-by: Yehyeong Lee <[email protected]>
Signed-off-by: Juergen Gross <[email protected]>
Acked-by: Roger Pau Monné <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
drivers/block/xen-blkback/common.h
drivers/block/xen-blkback/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=253cff0526781e28b1b0f9fdacd74ecdc23d3ab7
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-100082 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-100082
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:N/S:C/C:N/I:N/A:H
The Best / paranoid CVSS vector: AV:A/AC:H/PR:L/UI:N/S:C/C:L/I:N/A:H
The Best / paranoid CVSS score: 6.5
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: 4
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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