[CVE-2026-74376][LOW] md/raid10: reset read_slot when reusing r10bio for discard [ Upstream

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

CVE: CVE-2026-74376
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: md/raid10: reset read_slot when reusing r10bio for discard [ Upstream
Commit: 3cb2a606ce4902eceabe68338df0653312f861f8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3cb2a606ce4902eceabe68338df0653312f861f8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74376
Analysis date: Sun, 16 Aug 2026 06:25:04 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
md raid10 discard handling could leak repl_bio references when an r10bio reused after a read keeps a stale non negative read_slot, allowing repeated local discard activity to cause resource exhaustion DoS in affected configurations.

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

The original Linux kernel CVE announcement for CVE-2026-74376 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74376

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.

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

CVE-2026-74376	LOW	CHECK WITH IMPACT FROM ORIG NN MODERATE	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:N/A:H';*CWE-401;*CWE-772;**CWE-400;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'A reference leak in md raid10 discard handling can occur when an r10bio object is reused after a previous read and read_slot still contains an old non negative value. In that state put_all_bios() can release devs bio but skip bio_put() for repl_bio, causing bio references or related memory to accumulate over repeated discard operations. For the CVSS the PR:L is used for the paranoid interpretation because a local user with write access to a filesystem mounted with discard, or a local process able to trigger discard through storage management paths, may be able to exercise the affected block layer path without full administrative control in some deployments. The issue is not network reachable directly and requires a local storage workload on an md raid10 device with discard support. Impact is denial of service only through resource exhaustion, with no concrete evidence of memory corruption, information disclosure, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)	YES	DISK SIMPLEFIX LINUS  DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_GUESSCVSS_AND_CIANNH	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

Conservative signals:

* Important filesystem/storage path: +1. The bug is in md/raid10 discard handling in the block layer.
* Availability impact realistic: +1. A skipped bio_put() on repl_bio can leak bio references or related memory and may cause resource exhaustion after repeated discard activity.
* Rare AND difficult-to-reach subsystem/configuration: -1. Triggering requires md/raid10 with discard support and reuse of an r10bio that previously carried a read_slot state.

Paranoid additional signal:

* Local unprivileged trigger: +1. If a filesystem on md/raid10 is mounted with discard or otherwise issues discard from ordinary file deletion or truncation, a local user with write access may indirectly exercise the affected path.

Not awarded:

* No generic memory corruption. This is a reference leak, not UAF, double free, OOB write, or type confusion.
* No privilege escalation plausible. No reclaim, overwrite, stale callback, or attacker-controlled reuse primitive is shown.
* No confidentiality or integrity impact. The patch only fixes missing release of repl_bio.
* No remote/network trigger.

## 3. Reachability analysis

The realistic trigger is local storage activity that causes discard BIOs on an md/raid10 device. In many deployments discard is driven by privileged tools such as fstrim, but in configurations with mounted discard, ordinary local file operations may generate discard indirectly. Namespaces or containers do not normally provide direct access to host md/raid10 block discard paths unless storage is delegated. The affected path is not network reachable and depends on a specific RAID10 plus discard configuration.

Call-site confidence: medium. The commit message clearly describes put_all_bios() cleanup behavior, but the full cleanup call graph is not included in the provided patch.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like resource leak. The theoretical worst case is local DoS through sustained resource exhaustion, but there is no evidence of memory corruption, information disclosure, or privilege escalation. The paranoid score only adds local unprivileged reachability for deployments where discard can be triggered through normal filesystem writes.

## 5. One-sentence report phrase

md raid10 discard handling could leak repl_bio references when an r10bio reused after a read keeps a stale non negative read_slot, allowing repeated local discard activity to cause resource exhaustion DoS in affected configurations.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This can be handled as non-urgent unless the affected product heavily exposes md/raid10 discard to untrusted local workloads.

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

Patch: md/raid10: reset read_slot when reusing r10bio for discard [ Upstream
Commit: 3cb2a606ce4902eceabe68338df0653312f861f8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3cb2a606ce4902eceabe68338df0653312f861f8

Changed files:
  drivers/md/raid10.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=3cb2a606ce4902eceabe68338df0653312f861f8

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

The original Linux kernel CVE announcement for CVE-2026-74376 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74376

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:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 1
  Paranoid ActionableScore: 2

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).