[CVE-2026-99155][MODERATE REGULAR] scsi: xen/scsifront: Check for a NULL shadow entry on a backend response

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

CVE: CVE-2026-99155
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: scsi: xen/scsifront: Check for a NULL shadow entry on a backend response
Commit: 1d91ec793d6c80eca963d426e0da832d40a062df
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1d91ec793d6c80eca963d426e0da832d40a062df
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99155
Analysis date: Fri, 30 Oct 2026 10:48:48 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
A malicious Xen SCSI backend can return an in-range non-outstanding request ID and NULL-dereference the guest frontend’s shadow entry, crashing the guest kernel.

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

The original Linux kernel CVE announcement for CVE-2026-99155 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99155

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 REGULAR
Manual review required: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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

CVE-2026-99155	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-99155 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=2 LEGACY=MODERATE RESULT=DIFF TEMP_DISTANCE_MERGE_SKIP=AEGIS_NO_POSITIVE_SEVERITY_EVIDENCE (final Legacy and Aegis differ; FINAL: MODERATE, 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-476;*CWE-20;CWE-754;Other CVSS 'AV:A/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.7';DESCR 'A NULL pointer dereference occurs because scsifront_do_response checks that the backend supplied rqid is in range but dereferences info shadow rqid lacking verification that the slot contains an outstanding request. A malicious or buggy Xen SCSI backend can send a spurious response before any command or duplicate a completed response and crash the guest kernel. For the CVSS the PR:H base value reflects that reliable triggering normally requires control of a privileged backend or driver domain. For the CVSS the PR:L paranoid value reflects deployments where backend access is delegated to a less trusted service or isolated driver domain. The demonstrated impact is guest denial of service only. There is no evidence of UAF, OOB access, information disclosure, or an arbitrary write primitive. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 5.7 and the production ActionableScore returned 1 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were final reconciliation stage LOW to MODERATE because 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 V30_AEGIS_DISTANCE_REVIEW_MODERATE and V41_INHERITED_KPANIC_PROMOTION_BLOCKED and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 5.7 and ActionableScore 2 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 MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	MAYBE	SIMPLEFIX LEAK HARDWARE LINUS TEST  V30_AEGIS_DISTANCE_REVIEW_MODERATE V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED	-	-	checked

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

ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 1.
* Conservative score: 1; no floor or cap applies.
* Raw paranoid score: 1.
* Paranoid score: 1; delegated backend control changes exposure but establishes no additional scored primitive.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Reliable kernel crash / strong DoS: an in-range backend-controlled `rqid` can select a NULL `info->shadow[]` slot, which is dereferenced while testing `->inflight`, crashing the frontend guest kernel.
* No privilege penalty: controlling the Xen SCSI backend is privileged in typical deployments, but the evidence also permits a delegated driver/backend domain; effective host-administrator privilege is therefore unresolved rather than proven.
* No OOB-write or B5-R signal: the index is range-checked, and the defect is a NULL dereference, not retry-accumulated heap corruption.
* No UAF, stale-object mutation, arbitrary write, disclosure, authorization bypass, or protected-state corruption is shown.
* No extra availability point: guest crash and guest denial of service are the same demonstrated effect.
* No Important-preserving cross-boundary corruption: the backend-to-guest boundary is real, but only guest crash is demonstrated.

3. Reachability analysis

A malicious or buggy Xen SCSI backend can submit a spurious or duplicate response with any in-range, non-outstanding `rqid`. The normal backend may be host-privileged, while some deployments delegate it to a less-trusted driver domain. The supplied evidence does not establish ordinary guest-user, namespace-capability, or remote-network reachability. No race or special hardware timing is required once backend response control exists.

4. Primitive and severity reading

The exact primitive is a deterministic NULL pointer dereference of an unpopulated shadow slot. It is CLASS_C crash/DoS behavior, not active structured-state corruption or a lifetime violation. The shown impact is denial of service to the frontend guest; host compromise, host crash, memory corruption, and confidentiality or integrity impact are unsupported. Consequently this is not an Important candidate and may be downgraded, while backend trust-boundary exposure warrants review.

5. One-sentence report phrase

A malicious Xen SCSI backend can return an in-range non-outstanding request ID and NULL-dereference the guest frontend’s shadow entry, crashing the guest kernel.

6. Manual review advice

MANUAL CHECK RECOMMENDED

Confirm whether the product delegates backend control to a less-trusted driver domain and whether guest availability has elevated operational importance.

7. Call-site confidence

Call-site confidence: high

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

Patch: scsi: xen/scsifront: Check for a NULL shadow entry on a backend response
Commit: 1d91ec793d6c80eca963d426e0da832d40a062df
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1d91ec793d6c80eca963d426e0da832d40a062df

Commit description:

scsifront_do_response() validates the rqid in a backend response against
VSCSIIF_MAX_REQS before using it to index info->shadow[], but then
dereferences the entry to test ->inflight without checking that the slot
is populated.  Shadow slots are NULL before a command is submitted (the
host private area is zeroed at allocation) and are reset to NULL in
_scsifront_put_rqid() once a request completes, so an in-range rqid that
does not correspond to an outstanding request makes the frontend
dereference a NULL pointer.

A malicious or buggy backend can thus crash the guest by returning a
response whose rqid is in range but not in flight -- for example a
spurious response before any command has been issued, or a duplicate of
one already completed.  The ring has only VSCSIIF_MAX_REQS (16) slots,
so before the first command every in-range rqid selects a NULL slot.

Reject a response whose shadow slot is not populated.

Fixes: 6d1c2f4 ("xen/scsifront: harden driver against malicious backend")
Cc: stable@vger.kernel.org
Signed-off-by: Yehyeong Lee <yhlee@isslab.korea.ac.kr>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://patch.msgid.link/20261005123555.253995-1-yhlee@isslab.korea.ac.kr
Signed-off-by: Martin K. Petersen (Oracle) <mkp@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/scsi/xen-scsifront.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=1d91ec793d6c80eca963d426e0da832d40a062df

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

The original Linux kernel CVE announcement for CVE-2026-99155 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99155

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:H/UI:N/S:U/C:N/I:N/A:H
  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: 1

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).