[CVE-2026-74482][MODERATE 7.0] mm/huge_memory: unlock i_mmap_rwsem before releasing after-split folios [ Upstream

[CVE-2026-74482][MODERATE 7.0] mm/huge_memory: unlock i_mmap_rwsem before releasing after-split folios [ Upstream

[AL-KERNEL]

CVE: CVE-2026-74482
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: mm/huge_memory: unlock i_mmap_rwsem before releasing after-split folios [ Upstream
Commit: be106f7855f03d3128ed0ce70ba74b484a90b473
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=be106f7855f03d3128ed0ce70ba74b484a90b473
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74482
Analysis date: Sat, 15 Aug 2026 13:54:54 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum**:: Actionable Moderate at minimum. Treat as manual-review UAF in core MM code, not as an ordinary auto-close Moderate.
Manual review required: YES

Summary:
A race in `__folio_split()` can leave `mapping` and `i_mmap_rwsem` accessed after the backing inode is evicted and RCU-freed, causing a local UAF-triggered kernel crash in a difficult shmem THP memory-failure path.

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

The original Linux kernel CVE announcement for CVE-2026-74482 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74482

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-2026-74482	MODERATE	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:H/I:H/A:H';*CWE-416;CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A use-after-free can occur in __folio_split because the code may touch mapping and i_mmap_rwsem after after-split folios have been unlocked and freed. In the reported shmem THP memory_failure path, lock_at may point to a tail page beyond EOF that was already dropped from the page cache, so once the main folio is unlocked there may be no locked in-cache folio pinning the inode. A concurrent final iput can evict and RCU-free the inode before i_mmap_unlock_read(mapping) runs, leading to a slab use-after-free in __up_read. For the CVSS the PR:L is used for the paranoid score because a local user may be able to shape shmem THP and truncation state while the final trigger depends on a difficult memory_failure or hwpoison condition. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to UAF memory corruption, so this should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 4)	YES	READ DANGER UAF MEMORY  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**:: Actionable Moderate at minimum. Treat as manual-review UAF in core MM code, not as an ordinary auto-close Moderate.

## 2. Signal breakdown

Conservative scoring:

* Memory corruption, weak primitive: +1
  Race-dependent UAF is shown by KASAN, but the patch does not show attacker-controlled reclaim, overwrite, callback control, or type confusion.

* Real lifetime corruption: +1
  The mapping/inode can be RCU-freed before `i_mmap_unlock_read(mapping)` touches `i_mmap_rwsem`.

* Reliable kernel crash / strong DoS: +1
  The reported failure is a slab-use-after-free in `__up_read`, likely leading to kernel Oops/panic depending on config.

* Broad/default core MM path: +1
  The affected code is in `mm/huge_memory.c`, involving THP split, shmem, page cache, inode lifetime, and reverse mapping state.

* Important filesystem/storage/MM path: +1
  The bug crosses shmem THP, truncation, page-cache lifetime, and inode eviction semantics.

* Hard race / special timing required: -1
  Requires concurrency between THP split, unlocking after-split folios, final `iput()`, eviction, and delayed `i_mmap_unlock_read()`.

* Special memory_failure / hwpoison condition: -1
  The described trigger involves `memory_failure()` splitting a poisoned shmem THP tail beyond EOF, which is not a normal everyday syscall path.

Paranoid delta:

* Local unprivileged shaping may be relevant: +1
  A local user may be able to shape shmem THP, truncation, EOF state, and inode lifetime, while the final trigger depends on hwpoison or memory-failure conditions.

* Do not subtract the special-trigger penalty in the paranoid score.
  For triage sensitivity, assume the memory-failure condition can occur or be induced in some evaluated environments.

Not awarded:

* Privilege escalation plausible: +0
  LPE is theoretically concerning because this is UAF in core MM code, but the patch does not show reclaim control, write-after-free, object replacement, callback dispatch, or refcount takeover.

* Confidentiality impact: +0 conservative
  No info leak or read-back primitive is shown.

* Integrity impact: +0 conservative
  No attacker-controlled write or object corruption sink is shown.

## 3. Reachability analysis

The realistic trigger is local and difficult. The demonstrated path is `memory_failure()` -> `try_to_split_thp_page()` -> `__folio_split()` on a poisoned shmem THP tail beyond EOF during truncation. Ordinary unprivileged users can potentially influence shmem, THP layout, truncation, and inode lifetime, but reliable triggering appears to require a memory-failure or hwpoison event, which is usually privileged, hardware-driven, or test-environment specific.

Namespaces and containers may lower practical exposure only if container workloads can create suitable shmem THP and influence truncation/lifetime behavior while the host experiences memory-failure handling. There is no network trigger. The affected code is in core MM and shmem/page-cache lifetime handling, so the subsystem importance is high even though the specific trigger is narrow.

Call-site confidence: high for the UAF/crash mechanism because the commit provides the exact stack and lifetime explanation. Medium for attacker reachability because the patch does not provide a normal unprivileged reproducer.

## 4. Severity interpretation

This is not an ordinary low-risk cleanup because it fixes a real UAF in core MM/inode lifetime handling. Realistically, it behaves like a hard-to-trigger local DoS with strong manual-review relevance. Theoretical privilege escalation cannot be dismissed entirely because UAF bugs in core MM code can sometimes become more serious, but the patch does not show the primitives needed to claim likely LPE.

Therefore, conservative handling is borderline manual-review Moderate. Paranoid handling is Actionable Moderate at minimum and should not be auto-closed.

## 5. One-sentence report phrase

A race in `__folio_split()` can leave `mapping` and `i_mmap_rwsem` accessed after the backing inode is evicted and RCU-freed, causing a local UAF-triggered kernel crash in a difficult shmem THP memory-failure path.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is a confirmed UAF in core MM/shmem THP splitting. Even though realistic exploitation appears difficult and mostly DoS-oriented, the affected lifetime boundary is security-sensitive enough to require manual review rather than automatic closure.

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

Patch: mm/huge_memory: unlock i_mmap_rwsem before releasing after-split folios [ Upstream
Commit: be106f7855f03d3128ed0ce70ba74b484a90b473
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=be106f7855f03d3128ed0ce70ba74b484a90b473

Changed files:
  include/linux/fs.h
  mm/huge_memory.c
  mm/memory-failure.c
  mm/shmem.c
  fs/inode.c
  mm/20260710071344.GA106129

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=be106f7855f03d3128ed0ce70ba74b484a90b473

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

The original Linux kernel CVE announcement for CVE-2026-74482 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74482

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:H/UI:N/S:U/C:N/I:N/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: 3
  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: NO
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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).