[CVE-2026-74571][LOW] btrfs: skip global block reserve accounting for rescue mounts [ Upstream

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

CVE: CVE-2026-74571
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: btrfs: skip global block reserve accounting for rescue mounts [ Upstream
Commit: 076349e4c8d11f6b58c4549976a513b2b4dc6df2
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=076349e4c8d11f6b58c4549976a513b2b4dc6df2
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74571
Analysis date: Sat, 15 Aug 2026 16:34:08 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: YES

Summary:
A Btrfs rescue mount with corrupted block group tree or raid stripe tree metadata can dereference a NULL root pointer during global block reserve accounting and crash the kernel, but triggering normally requires local administrative mount capability and there is no evidence of memory corruption beyond NULL dereference.

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

The original Linux kernel CVE announcement for CVE-2026-74571 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74571

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: YES

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

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

CVE-2026-74571	LOW	CHECK WITH IMPACT FROM ORIG NN NONE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';**CWE-476;CWE-754;CWE-703;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A NULL pointer dereference can occur during a Btrfs rescue mount when the block group tree root or raid stripe tree root cannot be read but the corresponding on disk feature bit remains set. btrfs_update_global_block_rsv can then perform global block reserve accounting based on the feature bit and dereference a missing root pointer, causing a kernel crash during mount. For the CVSS the PR:L is used in the paranoid score because some environments may delegate mount capability to a reduced capability root, container root, service account, or storage management process, even though the typical base case requires administrative mount privileges. The issue is not network reachable and requires local ability to mount or influence mounting of a corrupted Btrfs filesystem with rescue options. Impact is denial of service only via kernel crash, with no evidence of UAF, OOB write, information disclosure, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)  SKIP CVE-2026-74571 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 8dbfc14fc736eb701089aff09645c3d4ad3decb1 YES  NO NO unknown 	MAYBE	LOCK DANGER NULLPTR DISK SKIP IMPROVEONLY  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

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

## 2. Signal breakdown

Triggered signals:

* Reliable kernel crash / strong DoS: +1
  The commit shows a reproducible NULL pointer dereference in `btrfs_update_global_block_rsv()` during mount.

* Availability impact realistic: +1
  The failure is a kernel crash during mount, so the availability impact is system-wide if triggered.

* Important filesystem/storage path: +1
  The bug is in Btrfs mount and global block reserve accounting, which is a filesystem metadata and mount-time integrity path.

Subtracted signals:

* Requires admin/root/CAP_SYS_ADMIN in typical deployments: -2
  Triggering requires mounting a corrupted Btrfs filesystem with rescue options, which normally requires administrative mount privileges.

* Non-default rescue mode / special corrupted filesystem condition: -1
  The crash requires `rescue=ibadroots` and a corrupted block group tree root or raid stripe tree root.

Paranoid adjustment:

* The paranoid score uses a reduced penalty for environments where mount operations are delegated to a reduced-capability root, container root, service account, or storage management process. This raises practical concern slightly but does not make the bug unprivileged in normal deployments.

No signals for:

* Generic memory corruption
* Real lifetime corruption
* UAF
* OOB write
* Privilege escalation
* Confidentiality impact
* Integrity impact
* Network reachability

## 3. Reachability analysis

The bug is locally reachable through Btrfs mount processing. In typical deployments, mounting with rescue options requires root or `CAP_SYS_ADMIN`, so the conservative interpretation is privileged local DoS. Namespace or container setups may reduce the practical privilege barrier if a storage management service or container root is allowed to initiate mounts, but this is not the default safe assumption.

The path is not network reachable. It also requires a corrupted Btrfs filesystem with specific missing roots while feature bits remain set, plus a read-only rescue mount mode.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like kernel DoS issue. The demonstrated primitive is a NULL pointer dereference, not UAF, stale pointer reuse, object replacement, OOB write, arbitrary write, or page-cache corruption.

Theoretical impact is limited to crashing the kernel during mount. Realistic exploitation evidence supports DoS only, not privilege escalation or data confidentiality/integrity compromise.

## 5. One-sentence report phrase

A Btrfs rescue mount with corrupted block group tree or raid stripe tree metadata can dereference a NULL root pointer during global block reserve accounting and crash the kernel, but triggering normally requires local administrative mount capability and there is no evidence of memory corruption beyond NULL dereference.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: this is a local privileged rescue-mount NULL pointer DoS with special corrupted-filesystem preconditions and no supported UAF, OOB write, information leak, privilege escalation, or network exposure.
NO MANUAL CHECK REQUIRED COULD BE AUTOCLOSED.

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

Patch: btrfs: skip global block reserve accounting for rescue mounts [ Upstream
Commit: 076349e4c8d11f6b58c4549976a513b2b4dc6df2
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=076349e4c8d11f6b58c4549976a513b2b4dc6df2

Changed files:
  fs/btrfs/block-rsv.c
  fs/btrfs/disk-io.c
  fs/btrfs/fs.h

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=076349e4c8d11f6b58c4549976a513b2b4dc6df2

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

The original Linux kernel CVE announcement for CVE-2026-74571 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74571

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:L/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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).