[CVE-2026-99062][LOW] vsock/virtio: account only unread bytes in read_skb()

[CVE-2026-99062][LOW] vsock/virtio: account only unread bytes in read_skb()

[AL-KERNEL]

CVE: CVE-2026-99062
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: vsock/virtio: account only unread bytes in read_skb()
Commit: e4a0028b0d54b2c45e1dc8c0dd95fe3e9236f55b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e4a0028b0d54b2c45e1dc8c0dd95fe3e9236f55b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99062
Analysis date: Mon, 26 Oct 2026 16:35:29 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
A partial VSOCK stream read can underflow receive accounting and cause incorrect socket readiness or temporary loss of access to queued data without demonstrated memory corruption or privilege bypass.

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

The original Linux kernel CVE announcement for CVE-2026-99062 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99062

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.

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

CVE-2026-99062	LOW	https://www.kernelcve.org/list/?q=CVE-2026-99062 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT LOW AEGIS_AS=2 LEGACY=LOW RESULT=SAME (final reconciliation agrees; DECISION: both LOW)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';CWE-191;CWE-682;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '3.3';DESCR 'A partial VSOCK stream receive can leave rx_bytes tracking only the unread suffix while virtio_transport_read_skb subtracts the full packet length. This causes an integer underflow or wrap in receive accounting and can produce an invalid SIOCINQ result, ELOOP from an empty receive, incorrect poll readiness, or temporary hiding of queued bytes. For the CVSS the PR:L value reflects that the demonstrated trigger requires a local process able to communicate through an affected CID_LOCAL stream and reach the specific SK_SKB verdict path. The demonstrated impact is limited availability loss or incorrect behavior for the affected socket. There is no supported UAF, OOB access, information leak, arbitrary write, kernel panic, or privilege escalation primitive. Automated impact decision path. The original SimpleNet classifier selected MODERATE. The primary LLM estimated paranoid CVSS 3.3 and the production ActionableScore returned 1 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE to LOW because the CVSS score, vector or bug class threshold 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_LOW_FROM_MODERATE_BASED_ON_GUESSCVSS. The independent weaker Aegis shadow evaluated CVSS 3.3 and ActionableScore 2 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was LOW. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	MAYBE	READ OOB VIRT SIMPLEFIX SKB LINUS PACKET  DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_GUESSCVSS	-	-	checked

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

ActionableScore=1
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: 1.
* Conservative score: 1; no floor or cap applied.
* Raw paranoid score: 1.
* Paranoid score: 1; no stronger patch-supported primitive exists.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Availability impact: subtracting the full packet length from an `rx_bytes` value that represents only the unread suffix causes underflow/wrap, incorrect poll/SIOCINQ behavior, `ELOOP`, and temporary hiding of queued socket data.
* No crash point: no kernel Oops, panic, host-wide DoS, or persistent outage is shown.
* No corruption point: the patch fixes scalar receive accounting, not an OOB write, UAF, pointer corruption, or shared-backing write.
* No local-unprivileged bonus: local CID_LOCAL communication is demonstrated, but the complete privilege/setup requirements for the SK_SKB verdict path are not established.
* No C/I impact, authorization bypass, retry-accumulated OOB (B5-R), shared-backing write (B3-PENDING), or active-state cross-boundary corruption is shown.

3. Reachability analysis

A local process communicating over a connected CID_LOCAL VSOCK stream can exercise the receive behavior once the relevant SK_SKB verdict path exists. The evidence does not establish who installs or attaches that verdict program, whether all setup is available below host administrator, or whether namespace-scoped capabilities suffice. Consequently, host-admin and namespace privilege requirements remain unknown. No remote trigger, special hardware requirement, race, or retry-sensitive corruption sequence is shown.

4. Primitive and severity reading

The exact defect is unsigned receive-accounting underflow caused by subtracting `pkt_len` rather than the unread length `skb->len - offset`. The malformed counter is subsequently consumed by socket query, receive, and poll logic, but its demonstrated effects remain socket-local incorrect readiness and temporary availability loss. This is CLASS_C invalid-state behavior, not active structured memory corruption. ImportantCandidate is NO because there is no memory-safety, ownership, authorization, or credible cross-boundary primitive. AutoDowngradeAllowed is YES because the shown consequence is constrained and recoverable.

5. One-sentence report phrase

A partial VSOCK stream read can underflow receive accounting and cause incorrect socket readiness or temporary loss of access to queued data without demonstrated memory corruption or privilege bypass.

6. Manual review advice

NO MANUAL CHECK NEEDED

The patch and reproducer support only constrained socket-accounting availability effects.

7. Call-site confidence

Call-site confidence: high

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

Patch: vsock/virtio: account only unread bytes in read_skb()
Commit: e4a0028b0d54b2c45e1dc8c0dd95fe3e9236f55b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e4a0028b0d54b2c45e1dc8c0dd95fe3e9236f55b

Commit description:

After a partial stream receive, rx_bytes tracks the unread suffix while
buf_used and peer credit still cover the whole packet.
virtio_transport_read_skb() dequeues that packet but passes pkt_len for
both counters, underflowing rx_bytes.

On a connected CID_LOCAL stream with an SK_SKB verdict, a 40-byte read
from a 100-byte packet made SIOCINQ report -40, an empty recv return
ELOOP, and poll report the empty socket readable.  After verdict detach,
receiving 40 bytes wrapped the counter to zero and hid those queued bytes
until one more byte arrived.  The fixed run kept the counter balanced
and exposed all queued data immediately.

This was found during an LLM-assisted manual source audit of VSOCK
receive accounting while re-evaluating virtio_transport_read_skb() after
CVE-2024-50169 .

Subtract only skb->len minus the VSOCK offset from rx_bytes.  Retain the
full packet length for buf_used and peer credit.

Fixes: 45ca7e9 ("vsock/virtio: fix `rx_bytes` accounting for stream sockets")
Cc: stable@vger.kernel.org
Reviewed-by: Luigi Leonardi <leonardi@redhat.com>
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
Link: https://patch.msgid.link/20261001151310.4101006-1-4ncienth@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/vmw_vsock/virtio_transport_common.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=e4a0028b0d54b2c45e1dc8c0dd95fe3e9236f55b

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

The original Linux kernel CVE announcement for CVE-2026-99062 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99062

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:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS score: 3.3

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: 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).