[CVE-2026-80537][MODERATE 7.0] xfs: fix off-by-one in rtrefcount btree root level validation

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

CVE: CVE-2026-80537
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: xfs: fix off-by-one in rtrefcount btree root level validation
Commit: 8a0ecae2ecda9f9a83a496ed05c42c4b1f5c3f2d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8a0ecae2ecda9f9a83a496ed05c42c4b1f5c3f2d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80537
Analysis date: Wed, 26 Aug 2026 12:19:49 -0400
ActionableScore: 6
ActionableScore lower bound: 3
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
An off-by-one validation bug in the XFS realtime refcount btree root level allows a crafted filesystem image to create an over-deep cursor and trigger a slab out-of-bounds write during mount recovery or fsmap processing, causing a kernel panic and requiring manual review for possible memory-corruption impact beyond DoS.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-80537 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80537

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-80537	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-193;*CWE-787;*CWE-20;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'An off-by-one validation bug in the XFS realtime refcount btree root level can allow a crafted filesystem image to be accepted with bb_level equal to m_rtrefc_maxlevels. When a realtime refcount cursor is later initialized, bc_nlevels becomes one larger than bc_maxlevels and the first btree operation can index past the end of the xfs_rtrefcountbt_cur slab object. The observed failure is a slab out-of-bounds write in xfs_btree_lookup followed by a kernel panic. For the CVSS the PR:L is used in the paranoid score because practical exposure may include delegated mount paths, removable media automount, container root, or a local service that mounts user supplied filesystem images. The issue is not directly network reachable. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality and integrity impact due to kernel memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-80537 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	WRITE KASAN OOB DISK HARDWARE LINUS KPANIC  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN6	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=6
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 6
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative score signals:

* Memory corruption, strong corruption primitive: +2
  The patch and KASAN trace show a real slab out-of-bounds write in `xfs_btree_lookup`, caused by `bc_nlevels` exceeding `bc_maxlevels` by one.

* Reliable kernel crash / strong DoS: +1
  The supplied trace ends in kernel panic after mounting or processing the crafted filesystem image.

* Important filesystem/storage path: +1
  This affects XFS realtime refcount btree validation and can be reached during mount recovery or filesystem mapping queries.

* Requires admin/root/CAP_SYS_ADMIN in typical deployments: -2
  The most direct trigger is mounting a crafted XFS filesystem image, which normally requires administrative mount privileges.

* Rare AND difficult-to-reach subsystem/configuration: -1
  The affected configuration requires XFS rtreflink with metadir, realtime, and reflink support, which is not a broad default filesystem path.

Paranoid score additions and adjustments:

* Local unprivileged trigger: +1
  In some deployments the effective trigger may be lowered through removable media automount, delegated mount services, container root, image scanning services, or other workflows that mount user-supplied filesystem images.

* Weak LPE concern: +1
  The primitive is a kernel slab out-of-bounds write, so privilege escalation cannot be ruled out for triage. However, there is no demonstrated attacker-controlled overwrite target, reclaim primitive, or arbitrary write.

* Availability impact realistic: +1
  The crash is host-wide and directly demonstrated through the mount/recovery path.

Paranoid total: 6

## 3. Reachability analysis

The direct trigger is a crafted XFS filesystem image whose realtime refcount btree root level is accepted due to an off-by-one validation bug. In normal deployments, mounting such an image requires `CAP_SYS_ADMIN`, so the conservative score treats this as a privileged local attack.

The more actionable interpretation is that filesystem image parsing bugs can become reachable from less-privileged actors through containerized mount helpers, removable media automount, VM or container image processing, backup/indexing systems, forensic tooling, or storage services that mount user-controlled images. This does not make the bug network-reachable by itself, but it can lower the practical privilege boundary in real environments.

The path is not default-triggered by ordinary file I/O on a normal filesystem. It requires a crafted XFS image with specific realtime reflink metadata features. Call-site confidence is high because the report includes the mount/recovery call trace and the exact KASAN out-of-bounds write.

## 4. Severity interpretation

This is not an ordinary Moderate validation bug because the downstream primitive is explicitly a slab out-of-bounds write in kernel memory. The realistic demonstrated impact is local denial of service through kernel panic. The theoretical higher impact is plausible because this is kernel memory corruption, but exploitation beyond DoS is not demonstrated by the patch.

Conservatively, this is a borderline issue due to normal mount privilege requirements and a specialized XFS configuration. For triage, it should be treated as Actionable Moderate at minimum and a Strong Important candidate if the evaluated product has any path that mounts user-supplied filesystem images.

## 5. One-sentence report phrase

An off-by-one validation bug in the XFS realtime refcount btree root level allows a crafted filesystem image to create an over-deep cursor and trigger a slab out-of-bounds write during mount recovery or fsmap processing, causing a kernel panic and requiring manual review for possible memory-corruption impact beyond DoS.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a crafted filesystem image bug with a demonstrated kernel slab out-of-bounds write, not just a warning or resource leak. Even though the conservative trigger requires mount privileges, practical exposure may be lower in systems that process user-supplied filesystem images.

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

Patch: xfs: fix off-by-one in rtrefcount btree root level validation
Commit: 8a0ecae2ecda9f9a83a496ed05c42c4b1f5c3f2d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8a0ecae2ecda9f9a83a496ed05c42c4b1f5c3f2d

Commit description:

xfs_rtrefcountbt_compute_maxlevels() sets

	mp->m_rtrefc_maxlevels = min(d_maxlevels, r_maxlevels) + 1;

where the trailing "+ 1" already accounts for the inode-root level, so the
deepest valid on-disk root level is m_rtrefc_maxlevels - 1 and a cursor must
satisfy bc_nlevels <= bc_maxlevels (= m_rtrefc_maxlevels).

The two on-disk validation paths, xfs_rtrefcountbt_verify() and
xfs_iformat_rtrefcount(), check the root level with ">" instead of ">=", so a
crafted rtreflink (metadir + realtime + reflink) image whose
/rtgroups/N.refcount inode has bb_level == m_rtrefc_maxlevels is accepted on
mount. xfs_rtrefcountbt_init_cursor() then sets bc_nlevels = bb_level + 1,
exceeding bc_maxlevels by one. Since the xfs_rtrefcountbt_cur slab object is
sized for exactly bc_maxlevels entries, the first btree op on such a cursor
indexes bc_levels[m_rtrefc_maxlevels] past the end of the object. This is
reached by the first rtrefcount cursor built after mount, via log/CoW
recovery (xfs_reflink_recover_cow() during xfs_mountfs()) or an
FS_IOC_GETFSMAP over the realtime device.

Reject a root level equal to m_rtrefc_maxlevels, matching the ">=" form
already used by the sibling data-device refcount/rmap verifiers and the
in-memory rtrmap verifier.

  BUG: KASAN: slab-out-of-bounds in xfs_btree_lookup (fs/xfs/libxfs/xfs_btree.c:2101)
  Write of size 2 at addr ffff888018391658 by task exploit/144
   xfs_btree_lookup (fs/xfs/libxfs/xfs_btree.c:2101)
   xfs_btree_query_range (fs/xfs/libxfs/xfs_btree.c:5308)
   xfs_refcount_recover_cow_leftovers (fs/xfs/libxfs/xfs_refcount.c:2113)
   xfs_reflink_recover_cow (fs/xfs/xfs_reflink.c:1085)
   xlog_recover_finish (fs/xfs/xfs_log_recover.c:3551)
   xfs_mountfs (fs/xfs/xfs_mount.c:1158)
   xfs_fs_fill_super (fs/xfs/xfs_super.c:1940)
   get_tree_bdev_flags (fs/super.c:1634)
   vfs_get_tree (fs/super.c:1694)
   path_mount (fs/namespace.c:4161)
   __x64_sys_mount (fs/namespace.c:4367)
   entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
  The buggy address belongs to the cache xfs_rtrefcountbt_cur of size 216
  The buggy address is located 8 bytes to the right of
   allocated 216-byte region [ffff888018391578, ffff888018391650)
  Kernel panic - not syncing: Fatal exception

Cc: stable@vger.kernel.org # v6.14
Fixes: 9abe03a ("xfs: introduce realtime refcount btree ondisk definitions")
Reported-by: Weiming Shi <bestswngs@gmail.com>
Signed-off-by: Xiang Mei <xmei5@asu.edu>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Carlos Maiolino <cem@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/xfs/libxfs/xfs_btree.c
  fs/xfs/libxfs/xfs_refcount.c
  fs/xfs/xfs_reflink.c
  fs/xfs/xfs_log_recover.c
  fs/xfs/xfs_mount.c
  fs/xfs/xfs_super.c
  fs/super.c
  fs/namespace.c
  arch/x86/entry/entry_64.S
  fs/xfs/libxfs/xfs_rtrefcount_btree.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=8a0ecae2ecda9f9a83a496ed05c42c4b1f5c3f2d

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

The original Linux kernel CVE announcement for CVE-2026-80537 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80537

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 6

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).