[CVE-2026-99136][MODERATE REGULAR] dm-crypt: fix a tiny race condition in crypt_dec_pending

[CVE-2026-99136][MODERATE REGULAR] dm-crypt: fix a tiny race condition in crypt_dec_pending

[AL-KERNEL]

CVE: CVE-2026-99136
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: dm-crypt: fix a tiny race condition in crypt_dec_pending
Commit: ac5d887ab93592fc5e3605fe30c1d8d232d609b3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ac5d887ab93592fc5e3605fe30c1d8d232d609b3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99136
Analysis date: Thu, 29 Oct 2026 00:41:32 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
A narrow dm-crypt completion race can report failed, potentially undecrypted or unwritten I/O as successful, causing silent storage-integrity failure without a shown memory-corruption or privilege-escalation primitive.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-99136 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99136

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: YES

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

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-99136	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-99136 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE RESULT=DIFF (final Legacy and Aegis differ; FINAL: MODERATE, Aegis: 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:N/I:H/A:H';CWE-362;CWE-662;CWE-703;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:L';BEST CVSS score: '6.3';DESCR 'A race in crypt_dec_pending can read io error before another completion context records an I O failure and releases its reference. If the stale success status is used when the final reference is dropped, a failed read that was not decrypted or a failed write can be reported as successful, creating silent storage integrity loss and possible availability damage. For the CVSS the PR:L value reflects that practical triggering requires a local user or process able to issue I O through the affected dm crypt mapping. Reliable exploitation also depends on inducing a backing I O failure inside the narrow race window, so AC:H is appropriate. The issue is not directly network reachable. There is no UAF, OOB access, information leak, or arbitrary write primitive supported by the code change, so confidentiality impact and privilege escalation are not assigned. Automated impact decision path. The original SimpleNet classifier selected IMPORTANT. The primary LLM estimated paranoid CVSS 6.3 and the production ActionableScore returned 3 with recommendation Borderline / manual review recommended. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage IMPORTANT to MODERATE_KPANIC because the CVSS score, vector or bug class threshold rule was applied. ActionableScore thresholds stage MODERATE_KPANIC to MODERATE because the ActionableScore 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 LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS and DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 and V41_INHERITED_KPANIC_PROMOTION_BLOCKED and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 1 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 Borderline / manual review recommended (with actual score 3)	MAYBE	DISK RACE LEAK UAF LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=3
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 3
* Conservative score: 3
* Raw paranoid score: 3
* Paranoid score: 3
* No floor or cap applied.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

* +1 Local below-host-admin trigger: supplied evidence grounds a local process issuing I/O through an accessible dm-crypt mapping.
* +1 Integrity plausible: a failed read or write can be completed with a stale success status, hiding an actual storage failure.
* +1 Important storage path: dm-crypt completion status directly governs encrypted block-I/O error reporting.
* +1 Broad exposure: dm-crypt is widely deployed and `crypt_dec_pending()` is on its normal completion path.
* -1 Hard timing requirement: exploitation requires a backing-I/O failure and a narrow ordering between `io->error` update/reference drop and the stale read.

Not awarded: no UAF, OOB access, arbitrary write, kernel crash, disclosure, authorization bypass, protected-victim overwrite, or privilege escalation is shown. Availability damage is possible but not sufficiently concrete for a separate point.

3. Reachability analysis

A local process with permission to issue I/O through an existing dm-crypt-backed resource can reach the completion path without host-administrator powers. Creation of the mapping normally involves prior privileged system configuration, but the later I/O issuer need not create it. Trigger reliability additionally depends on inducing or encountering a backing-device I/O failure during the narrow race window. No namespace capability or special hardware requirement is shown.

4. Primitive and severity reading

The primitive is a stale completion-status race: `io->error` was sampled before the final ordered reference decrement, while another completion context could record an error before the bio was completed. This is semantic active-state consumption, not memory corruption or lifetime misuse, and is classified CLASS_B because it can silently misreport failed storage operations.

The shown impact is integrity loss in I/O completion semantics. Actual corruption of another principal’s protected data, persistent disk corruption, host crash, or LPE remains unproven. The issue therefore does not meet NEW52/B5-R, B3-PENDING, or B4-PENDING. ImportantCandidate is NO, and auto-downgrade remains allowed, although human review is advisable because silent successful completion can conceal real storage failures.

5. One-sentence report phrase

A narrow dm-crypt completion race can report failed, potentially undecrypted or unwritten I/O as successful, causing silent storage-integrity failure without a shown memory-corruption or privilege-escalation primitive.

6. Manual review advice

MANUAL CHECK RECOMMENDED

Confirm whether realistic workloads let a below-host-admin I/O issuer induce repeatable backing failures and whether stale success can affect shared or security-sensitive storage consumers.

7. Call-site confidence

Call-site confidence: high

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

Patch: dm-crypt: fix a tiny race condition in crypt_dec_pending
Commit: ac5d887ab93592fc5e3605fe30c1d8d232d609b3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ac5d887ab93592fc5e3605fe30c1d8d232d609b3

Commit description:

crypt_dec_pending reads io->error before calling atomic_dec_and_test.
Another context, for example crypt_endio called from an interrupt, may
set io->error and drop its reference between the read and the
decrement. crypt_dec_pending then drops the last reference and completes
the bio with the stale status - so a read that failed and was never
decrypted, or a write that failed, is reported as successful.

The read was placed before the decrement by commit b35f8ca ("dm
crypt: wait for endio to complete before destruction"), because that
commit freed dm_crypt_io before calling bio_endio. This is no longer the
case, dm_crypt_io lives in the per-bio data now.

Read io->error after atomic_dec_and_test instead. atomic_dec_and_test is
fully ordered, so no additional barrier is needed.

Fixes: b35f8ca ("dm crypt: wait for endio to complete before destruction")
Cc: stable@vger.kernel.org
Reviewed-by: Jose Fernandez (Anthropic) <jose.fernandez@linux.dev>
Signed-off-by: Ben Cressey <ben@cressey.dev>
Assisted-by: Claude:unspecified
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/md/dm-crypt.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=ac5d887ab93592fc5e3605fe30c1d8d232d609b3

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

The original Linux kernel CVE announcement for CVE-2026-99136 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99136

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:H/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:H
  The Best / paranoid CVSS score: 6.3

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: 3

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).