[CVE-2026-89772][MODERATE 7.0] btrfs: write-protect folios during data writeback

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

CVE: CVE-2026-89772
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: btrfs: write-protect folios during data writeback
Commit: 074c715e0b498891c09fe7f11e1cd9d7a04699bd
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=074c715e0b498891c09fe7f11e1cd9d7a04699bd
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89772
Analysis date: Fri, 11 Sep 2026 19:34:46 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89772 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89772

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: 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
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CVE-2026-89772	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89772 CHECK WITH IMPACT FROM ORIG NN NONE	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:L/A:H';CWE-362;CWE-367;CWE-345;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '5.3';DESCR 'A Btrfs writeback race can occur because folios used for checksumming, compression, inline extent copy, zeroing, or bio submission may remain writable through existing mmap PTEs. A local process can modify an mmaped file while writeback expects the folio contents to be stable, which can cause invalid checksums, corrupted compressed data, lost writes even after fsync, or non zero data past EOF. For the CVSS the PR:L is selected because triggering requires a local user or process with write access to a Btrfs file and the ability to use mmap, but not full administrative privileges. The issue is not network reachable and is triggered through local filesystem writeback behavior. Impact is primarily data integrity loss with possible availability impact from filesystem corruption reports or unreadable data. It is not classified as kernel memory corruption or privilege escalation based on the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate (with actual score 5)  SKIP CVE-2026-89772 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	READ LOCK DEADLOCK DISK WARNONLY SKIP IMPROVEONLY  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4. No floor or cap applied.
* Raw paranoid score: 5
* Paranoid score: 5. No floor or cap applied.
* Final recommended bucket: **Actionable Moderate**::

2. Signal breakdown

Positive conservative signals:

* +1 Local unprivileged trigger. A local user or process with write access to a Btrfs file can mmap the file and race writes against writeback without host-admin privileges.
* +1 Dirty-Pipe-like weaker shared file-backed page corruption. The bug allows writable mmap PTEs to remain active while writeback code expects the folio to be stable.
* +1 Important filesystem/storage path. The affected paths include Btrfs writeback, checksumming, compression, inline extents, EOF zeroing, and bio submission.
* +1 Integrity impact. The commit directly describes invalid checksums, corrupted compressed data, lost writes even after fsync, and nonzero data past EOF.

Additional paranoid signal:

* +1 Availability impact. Persistent corrupted data, checksum failures, or unreadable data can plausibly cause operational availability impact beyond a transient warning.

Non-awarded signals:

* RealUAF not awarded. The patch does not establish use-after-free or stale post-lifetime object use.
* Class A ownership/COW violation not awarded. The evidence shows corruption of data through the same writable mmaped file, not an attacker-authorized write to A modifying independently permissioned B.
* Controlled reclaim not demonstrated.
* Arbitrary kernel read/write not demonstrated.
* Kernel memory corruption not demonstrated.
* Privilege escalation not demonstrated.
* Remote reachability not established.
* Hard race penalty not applied. Although the issue is timing-dependent, the affected mmap writeback interaction can be repeatedly exercised by a local process and is not shown to be a narrow one-shot teardown race.
* Extra crash/strong DoS not awarded. No kernel panic or host-wide fatal crash is established.

3. Reachability analysis

The evaluated attacker is a local user or process that can create or modify a file on a Btrfs filesystem and map it writable with mmap. Host administrator privileges are not required for the core trigger once the filesystem is mounted and the attacker has ordinary file write access.

No namespace-scoped capability is needed for the main path. The vulnerability is not a network-triggered path and does not depend on packet input. Prior privileged setup may exist in the ordinary sense that the system must have a Btrfs filesystem mounted, but that is not a special attacker-controlled prerequisite.

Some concrete manifestations depend on features or states such as large folios, subpage sectorsize, zoned writes, inline extents, compressed extents, or folios spanning EOF. These conditions affect exploitability and reproducibility, but the patch shows multiple normal Btrfs writeback paths where stable-folio assumptions can be violated.

Race repeatability is meaningful because the attacker can repeatedly mmap-write and force or wait for writeback activity. The race is timing-sensitive, but not enough evidence is supplied to apply a hard race penalty under the ActionableScore rules.

4. Primitive and severity interpretation

The exact primitive is a filesystem writeback stable-folio violation. Btrfs writeback code consumes folio contents for checksumming, compression, inline extent copying, EOF zeroing, or bio submission while existing mmap PTEs can still allow concurrent writes without a page fault.

This is not an active structured UAF and not fixed stale storage. It is a shared file-backed page stability and storage-integrity primitive. No concrete repeated release or refcount operation exists. No first lifetime operation frees an object, and no second lifetime/refcount mutation through a dangling reference is present.

PrimitiveClass is CLASS_B because the bug establishes a meaningful filesystem/page-cache consistency primitive with persistent data corruption potential, but not a Class A protected-object overwrite, arbitrary write, controlled reclaim, or authorization bypass.

Demonstrated consequences include invalid checksums, corrupted compressed data, lost writes even after fsync, and persisted nonzero bytes past EOF. Speculative consequences such as privilege escalation or kernel memory corruption are not supported by the patch context.

ImportantCandidate is NO because the evidence supports Actionable Moderate filesystem integrity risk, not an Important-class kernel memory corruption or security-boundary bypass. AutoDowngradeAllowed is NO because automatic closure would discard a concrete filesystem data-corruption primitive in an important storage path.

Likely operational severity is Actionable Moderate. It should be reviewed by a human, especially for products where untrusted local users can write to Btrfs-backed shared storage, where compression or large folios are enabled, or where data integrity guarantees are security-relevant.

5. One-sentence report phrase

A Btrfs writeback race can leave mmap PTEs writable while folios are used for checksumming, compression, inline copying, EOF zeroing, or bio submission, allowing a local writer to cause persistent file data corruption or lost writes.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

The issue is not kernel memory corruption or privilege escalation, but it is a concrete local filesystem data-integrity bug in Btrfs writeback with persistent corruption and fsync-related consequences.

7. Call-site confidence

Call-site confidence: high

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

Patch: btrfs: write-protect folios during data writeback
Commit: 074c715e0b498891c09fe7f11e1cd9d7a04699bd
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=074c715e0b498891c09fe7f11e1cd9d7a04699bd

Commit description:

commit 095be15 ("btrfs: unify folio dirty flag clearing") replaced
the folio_clear_dirty_for_io() call in extent_write_cache_pages() with a
plain folio_test_dirty() check. Besides clearing the dirty flag,
folio_clear_dirty_for_io() also calls folio_mkclean(), which write-protects
the shared mmap PTEs mapping the folio. Note that we still do call
folio_clear_dirty_for_io() later in submit_one_sector() when we clear
dirty on the last sector of the folio (the only sector for non-subpage
cases). But we lost this early call in extent_write_cache_pages().

Without the extra write-protection, a process with the file mmap-ed can
modify a sector while it is being used by writeback in a way that
expects a stable folio (checksumming, compressing, copying, etc...)
without faulting, which manifests as a handful of concrete bugs.

1. For large folios or subpage sectorsize, it is possible to submit a bio
which does not cover the whole folio. When this happens, we will have a
bio in flight for a folio that we have *not* called
folio_clear_dirty_for_io() on. If a task with an existing mmap-ed PTE
writes (without faulting..) in this window, it can result in
corruptions. If the write arrives while the checksumming or writing itself
is underway, this can result in an invalid checksum and later corruption
reports on read. If the write arrives after checksumming/writing is done
but before the last sector dirty is cleared, then the write is present
in page cache but doesn't affect the dirty tracking and will be lost
when the folio is fully finished being submitted and the dirty bit
is cleared. This results in losing the write even if fsync() is called.

2. For zoned submissions which are done in batch separate from the main
extent_writepage() loop, we also risk csum violations for those
submissions. Zoned writes are clamped to max_zone_append_size and are
not aligned with folios, so a submission can span two folios. The first
folio being processed in extent_write_cache_pages() will call
extent_write_locked_range() which will submit the partial range of the
next folio, while the rest of that folio could still be dirty. So
clearing dirty on the submitted sectors doesn't call
folio_clear_dirty_for_io() and we have the same issue. Since
extent_write_cache_pages() skips these batch submitted folios (they are
already marked for writeback from submission by the preceding folio), we
must add the extra write protection in lock_delalloc_folios().

3. For inline extents this will subtly risk losing writes that happen
after/while we copy the inline extent but before we clear dirty on
the folio.

4. For folios spanning EOF, mmap could tamper with the zeroed bytes past
EOF and cause them to be persisted where future faults would improperly
see them instead of zeros.

5. Finally, for compressed extents, we risk modifying the folios while we
work on compressing them which will result in corrupted compressed data.
Specifically, in run_delalloc_compressed() we queue up work to do
compress_file_range() in BTRFS_COMPRESSION_CHUNK_SIZE (512K) chunks which
will call btrfs_folio_clamp_clear_dirty() on the range. For non-subpage,
this will always clear the whole folio, safely. For subpage, we risk a
partial clear here as well. In particular, imagine a 2M folio broken up
into 512K chunks of work which might start compression work on one chunk
before all the chunks compress_file_range() workers have gotten far
enough to finish clearing all the dirty bitmaps of the folio and getting
to folio_clear_dirty_for_io(). Large folios on the edges of submission
ranges are similarly at risk to be only partly cleared.
This particular gap was introduced by a second patch in the same series:
commit a4ef54d ("btrfs: make extent_range_clear_dirty_for_io() to handle sector size < page size cases")

We cannot simply restore the call to folio_clear_dirty_for_io() because
that also drops the dirty flag off the folio which violates invariants
introduced for large folios by
commit 334509c ("btrfs: use dirty flag to check if an ordered extent needs to be truncated")
and results in failing to invalidate clean folios past i_size, resulting
in deadlocks.

Therefore, to fix it, leave the existing semantics w.r.t. the folio's
dirty flag (to preserve the correct invalidate behavior) but ensure that
the other aspect of folio_clear_dirty_for_io(), folio_mkclean(), is run
on the folio when we lock it for writeback.

Finally, to help prevent similar regressions in the future, add a debug
warning that triggers at the known corruption sites if we have failed to
write protect the folio.

Assisted-by: LLM (debug, reproduce, research fix, review patch)

[Commit description truncated; see the upstream URL below]

Changed files:
  fs/btrfs/extent_io.c
  fs/btrfs/extent_io.h
  fs/btrfs/inode.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=074c715e0b498891c09fe7f11e1cd9d7a04699bd

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

The original Linux kernel CVE announcement for CVE-2026-89772 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89772

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:L/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:H
  The Best / paranoid CVSS score: 5.3

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: 4
  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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).