* [CVE-2026-99203][MODERATE 7.0] pipe: preserve existing fasync readers on writer setup failure [ Upstream
@ 2026-10-29 4:04 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-29 4:04 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-99203
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: pipe: preserve existing fasync readers on writer setup failure [ Upstream
Commit: 4c42ce499ce4b1c787e61082e3b13df4f30c4eaa
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4c42ce499ce4b1c787e61082e3b13df4f30c4eaa
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99203
Analysis date: Thu, 29 Oct 2026 00:04:12 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
An unprivileged fasync setup race can leave a pipe notification entry referencing a freed file, causing a later `send_sigio()` UAF and kernel crash with unresolved active-object reuse leverage.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99203 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99203
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-2026-99203 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-99203 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:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;CWE-703;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in pipe fasync setup can leave a writer notification entry referencing a freed struct file. Two concurrent F_SETFL or FIOASYNC enables on one O_RDWR FIFO must overlap and the duplicate writer registration must fail allocation after the reader entry was updated. A later pipe notification can reach send_sigio through the stale entry and dereference freed file or f_owner memory. For the CVSS the PR:L value is selected because reliable triggering requires a local unprivileged process able to create or access the FIFO and issue the relevant file control operations. AC:H reflects the required concurrency window and memory allocation failure. The demonstrated impact is a kernel crash, while allocator reuse of the freed kernel object creates a plausible but unproven path to confidentiality or integrity impact and local privilege escalation. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 5 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. 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 2.5 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 / Strong Important candidate requiring manual review (with actual score 5) MAYBE DISK LEAK ERRORPATH UAF LINUS KPANIC INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw base score: 4.
* Conservative score: 5. Real-UAF floor 5 applies because an unprivileged attacker drives the lifetime sequence and later active `send_sigio()` processing dereferences the stale `file`/`f_owner`.
* Raw paranoid score: 4.
* Paranoid score: 5. No additional speculative reuse or LPE point is awarded.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
* +2 — Real UAF: the writer fasync entry can retain a pointer after `__fput()` frees the associated `struct file`; later notification dereferences it.
* +1 — Reliable kernel crash/strong DoS: the unprivileged reproducer reaches a KASAN slab-use-after-free in `send_sigio()`.
* +1 — Local unprivileged trigger: FIFO access, `F_SETFL`/`FIOASYNC`, memory pressure, close, and notification are described as exercised without host-admin privilege.
* +1 — Attacker-influenced lifetime sequence: the attacker drives concurrent enables, allocation pressure, file release, and later pipe notification.
* -1 — Difficult timing/state preconditions: two enables must overlap and the writer-entry allocation must fail at the relevant point.
* Floor 5 — The stale object reaches nontrivial signal-notification processing rather than only a cleanup fault.
Not awarded: controlled reclaim, attacker-shaped replacement contents, arbitrary write, confidentiality/integrity impact, LPE, remote reachability, or a second repeated release/refcount mutation. Extra availability is not counted separately from the same crash.
3. Reachability analysis
A normal local process able to create or access the O_RDWR FIFO can invoke the complete sequence. No capability, namespace privilege, prior privileged setup, special hardware, or unusual boot configuration is shown. The concurrency and allocation-failure requirements raise complexity, but the supplied unprivileged memory-pressure reproducer establishes practical reachability.
4. Primitive and severity reading
This is an active structured-object UAF: a surviving fasync registration points to a freed `struct file`, and `send_sigio()` later consumes the stale file or `f_owner`. It is broader than a cleanup-only stale dereference, but no controlled reclaim, callback replacement, arbitrary read/write, or proven privilege escalation is shown.
There is no repeated put/free of the exact dangling reference. The primitive is therefore CLASS_B, not CLASS_A. ImportantCandidate is YES because unprivileged control reaches active post-lifetime signal processing and exploit leverage beyond the demonstrated crash remains unresolved. Auto-downgrade is unsafe pending reuse and `send_sigio()` field-consumption analysis.
No authorization decision changes from DENY to ALLOW, so B4-PENDING does not apply.
5. One-sentence report phrase
An unprivileged fasync setup race can leave a pipe notification entry referencing a freed file, causing a later `send_sigio()` UAF and kernel crash with unresolved active-object reuse leverage.
6. Manual review advice
MANUAL CHECK REQUIRED
Review allocator reuse and the exact stale `file`/`f_owner` accesses in the signal-delivery path.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: pipe: preserve existing fasync readers on writer setup failure [ Upstream
Commit: 4c42ce499ce4b1c787e61082e3b13df4f30c4eaa
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4c42ce499ce4b1c787e61082e3b13df4f30c4eaa
Changed files:
fs/pipe.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=4c42ce499ce4b1c787e61082e3b13df4f30c4eaa
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99203 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99203
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:L/I:L/A:H
The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
The Best / paranoid CVSS score: 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: 5
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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