[CVE-2026-74591][MODERATE 7.0] mm/filemap: __filemap_add_folio() restore index before retrying

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

CVE: CVE-2026-74591
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: mm/filemap: __filemap_add_folio() restore index before retrying
Commit: 4917e3ebcab50f0265e8ca01c8567de4c4a47511
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4917e3ebcab50f0265e8ca01c8567de4c4a47511
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74591
Analysis date: Sat, 22 Aug 2026 11:46:06 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in `__filemap_add_folio()` can retry with a stale rounded xarray index after dropping the lock, causing page-cache wrong-offset folio insertion, file-backed data corruption, and possible local DoS or privilege-escalation relevant behavior.

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

The original Linux kernel CVE announcement for CVE-2026-74591 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74591

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-2026-74591	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	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-362;CWE-367;CWE-682;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR '__filemap_add_folio can retry after xas_nomem drops the xarray lock while xas.xa_index still contains an intermediate rounded down index from xas_set_order. If another thread resolves the conflict before the retry, the folio can be inserted at the wrong file offset, causing page cache index corruption and execution of wrong file contents. Observed production symptoms include rare SIGILL/SIGSEGV and executable text found one page away from the expected location. For the CVSS the PR:L is used because a local process that can drive file backed page cache activity can potentially reach the path, while reliable triggering still requires a narrow race and memory allocation retry. The issue is not network reachable. Impact is at least local DoS and file/page cache integrity corruption. In the paranoid score, confidentiality and integrity are raised because wrong-offset page cache mapping may expose or execute unintended file data and should be reviewed as a potential local privilege escalation candidate.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)  SKIP CVE-2026-74591 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	LINUS KPANIC MEMORY  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	-	-	checked

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

ActionableScore=8
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local process can drive file-backed page-cache activity without special privileges.
* Generic memory corruption, strong corruption primitive: +2. The bug can insert a folio into the xarray at the wrong file offset, causing page-cache index corruption.
* Memory layout invariant restoration: +1. The patch restores consistency between the original file index, folio order, and xarray state after `xas_nomem()` drops the lock.
* Important filesystem/storage path: +1. `__filemap_add_folio()` is a core page-cache path.
* Integrity impact plausible: +1. Wrong-offset page-cache insertion can return or execute file contents from the wrong location.
* Availability impact realistic: +1. Production symptoms include rare SIGILL/SIGSEGV, and DEBUG_VM can hit `!folio_contains()` BUG.
* Hard or unreliable race / special timing required: -1. Triggering requires the xarray lock drop, allocation retry, and concurrent conflict resolution.

Paranoid additional signal:

* Privilege escalation plausible: +2. Page-cache corruption involving executable text found one page away from the expected location is a defensible LPE review candidate, even though no direct exploit primitive is demonstrated.

## 3. Reachability analysis

A local unprivileged user is the realistic attacker model if they can create memory pressure and concurrent file-backed page-cache activity. No network reachability is supported by the patch. The affected code is in the common page-cache insertion path, so the subsystem exposure is broad, but the precise trigger requires a narrow race around `xas_nomem()` and conflict splitting. Namespaces or containers may still matter because page-cache behavior can involve shared host-backed files or images, but the patch does not prove a container escape primitive.

Call-site confidence: high, because the affected function and retry behavior are directly described in the commit message and patch.

## 4. Severity interpretation

This is not an ordinary Moderate correctness issue. It is a race in core page-cache/xarray handling that can place a folio at the wrong file offset, with real production symptoms involving misplaced executable text. Realistic evidence supports at least Actionable Moderate due to page-cache integrity corruption and crash behavior. Theoretical escalation is plausible enough for Strong Important candidate review, but not proven as reliable privilege escalation from the patch alone.

## 5. One-sentence report phrase

A race in `__filemap_add_folio()` can retry with a stale rounded xarray index after dropping the lock, causing page-cache wrong-offset folio insertion, file-backed data corruption, and possible local DoS or privilege-escalation relevant behavior.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.

Reason: this is core page-cache corruption with observed executable-text misplacement, not a simple warning, leak, or DoS-only bug.

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

Patch: mm/filemap: __filemap_add_folio() restore index before retrying
Commit: 4917e3ebcab50f0265e8ca01c8567de4c4a47511
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4917e3ebcab50f0265e8ca01c8567de4c4a47511

Commit description:

In __filemap_add_folio()'s split-a-conflict loop, xas_set_order() is
applied repeatedly: each application modifies xas.xa_index, rounding it
down according to the split_order attempted at that stage: and if all goes
as intended, it eventually (or immediately) converges on an
xas_try_split() to the required folio_order, with xas.xa_index now the
same as index: then xas_store() puts the new folio into the xarray there.

But if a new node was needed, and GFP_NOWAIT allocation did not get one,
the lock is dropped, xas_nomem() used to allocate, and sequence retried.
If (that part of) the xarray is unchanged when the lock is reacquired, no
problem.  But what if the conflict was meanwhile resolved by another
thread (perhaps even doing the same thing, inserting a folio at that same
index)?  Isn't there a danger of now putting our folio into the xarray at
an intermediate rounded-down index?  With !folio_contains() bug to follow,
when CONFIG_DEBUG_VM=y is checking for that.

Fix this with an xas_set_order() to restore the original xas.xa_index at
the bottom of the loop, so the retry does a full re-evaluation after
reacquiring the lock, and cannot reach xas_store() with the wrong index.

Production was suffering from rare SIGILLs and SIGSEGVs, executable text
found a page away from where it belonged, !folio_contains() bug hit when
debug enabled: symptoms not seen since this patch went in.

Link: https://lore.kernel.org/562fbfa6-dd6d-0b6a-2461-ed2ff1173bc8@google.com
Fixes: 200a89c ("mm/filemap: use xas_try_split() in __filemap_add_folio()")
Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Cc: Chris J Arges <carges@cloudflare.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Kairui Song <ryncsn@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  mm/filemap.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=4917e3ebcab50f0265e8ca01c8567de4c4a47511

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

The original Linux kernel CVE announcement for CVE-2026-74591 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74591

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: 6
  Paranoid ActionableScore: 8

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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).