[CVE-2026-52961][MODERATE REGULAR] ceph: fix BUG_ON in __ceph_build_xattrs_blob() due to stale blob size

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

CVE: CVE-2026-52961
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ceph: fix BUG_ON in __ceph_build_xattrs_blob() due to stale blob size
Commit: 7eb72425c4e3234926502eb262f9d6193ccd572c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7eb72425c4e3234926502eb262f9d6193ccd572c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52961
Analysis date: Wed, 24 Jun 2026 14:13:43 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline Moderate, manual review recommended
Manual review required: YES

Summary:
A race in CephFS xattr rebuilding can leave `required_blob_size` stale after an MDS cap grant updates the xattr blob, allowing a local CephFS user to trigger a `BUG_ON()` in `__ceph_build_xattrs_blob()` and crash the kernel.

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

The original Linux kernel CVE announcement for CVE-2026-52961 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52961

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 REGULAR
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-2026-52961	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-362;CWE-131;CWE-617;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'A race in the CephFS xattr path can leave required_blob_size stale after __build_xattrs releases and reacquires i_ceph_lock. During that window an MDS cap grant can replace the xattr blob and increase the rebuilt xattr tree size, so the later prealloc_blob size check may use an obsolete smaller value. This can reach BUG_ON in __ceph_build_xattrs_blob during cap flush and crash the kernel. For the CVSS the PR:L is used because a local user or local process with access to the mounted CephFS file and xattr operation is the realistic trigger, while full administrator privileges are not inherently required for user xattrs on accessible files. The issue is not directly reachable from the public network, but the race depends on CephFS client and MDS interaction inside a Ceph storage environment. Impact is at least local denial of service via kernel crash and in worst case may allow limited confidentiality or integrity impact due to stale size handling around a preallocated blob, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline Moderate, manual review recommended (with actual score 3)	YES	DANGER DISK RACE HARDWARE LINUS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=3
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: **2**
* Paranoid score: **3**
* Final recommended bucket: **Borderline Moderate, manual review recommended**

## 2. Signal breakdown

Conservative signals:

* **Local unprivileged trigger: +1**
  A local user or process with access to a mounted CephFS filesystem and permission to set xattrs can plausibly reach `__ceph_setxattr()` without full administrator privileges.

* **Reliable kernel crash / strong DoS: +1**
  The commit provides a concrete `BUG_ON()` crash in `__ceph_build_xattrs_blob()` and notes that generic/642 reproduces it.

* **Important filesystem/storage path: +1**
  The bug is in CephFS xattr handling and cap flush interaction, which is a filesystem metadata path.

* **Hard or timing-dependent race: -1**
  The issue depends on `__build_xattrs()` dropping and reacquiring `i_ceph_lock` while an MDS cap grant updates the xattr blob.

Paranoid-only signal:

* **Weak/indirect corruption candidate: +1**
  The stale size check concerns a preallocated xattr blob and rebuilt metadata tree. However, the demonstrated outcome is a `BUG_ON()` before an overflow, so this is not scored as strong memory corruption.

Not awarded:

* **No remote reachable +2**
  The affected client path depends on CephFS and MDS interaction, but this is not a public network protocol parser reachable by arbitrary remote hosts.

* **No strong LPE +2**
  There is no demonstrated UAF reclaim, arbitrary write, type confusion, callback hijack, or attacker-controlled overwrite.

* **No generic memory corruption +2**
  The patch and trace show stale accounting leading to `BUG_ON()`, not a proven OOB write or UAF.

## 3. Reachability analysis

The realistic trigger is a local user or process operating on xattrs of files in a mounted CephFS filesystem while MDS cap updates occur concurrently. Mounting CephFS itself is usually privileged, but once mounted, ordinary users may be able to operate on files and user xattrs depending on permissions.

Namespaces or containers can matter if a container or service is given access to an existing CephFS mount. In that case, the practical privilege requirement may be lower than host root, but the bug still requires local filesystem access rather than unauthenticated network access.

The path is not default-enabled on all Linux systems because it requires CephFS usage. In Ceph deployments it is a normal filesystem path, and the crash is reproducible by a filesystem test case, making the DoS realistic under the right environment.

## 4. Severity interpretation

This behaves more like a **borderline Moderate requiring manual review**, not an Important-class vulnerability based on the provided patch alone.

The demonstrated impact is a local kernel crash through `BUG_ON()`. The stale size computation is concerning because it involves preallocation and filesystem metadata rebuilding, but the patch context does not show an actual write past the allocated buffer, UAF, object reuse, or privilege escalation primitive. Therefore, the conservative score remains low, while the paranoid score is raised only to manual-review territory.

## 5. One-sentence report phrase

A race in CephFS xattr rebuilding can leave `required_blob_size` stale after an MDS cap grant updates the xattr blob, allowing a local CephFS user to trigger a `BUG_ON()` in `__ceph_build_xattrs_blob()` and crash the kernel.

## 6. Manual review recommendation

**MANUAL CHECK RECOMMENDED**

Manual review is recommended because this is a race in a filesystem metadata path involving stale size accounting and a preallocated buffer, but the available evidence supports DoS rather than confirmed memory corruption or privilege escalation.

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

Patch: ceph: fix BUG_ON in __ceph_build_xattrs_blob() due to stale blob size
Commit: 7eb72425c4e3234926502eb262f9d6193ccd572c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7eb72425c4e3234926502eb262f9d6193ccd572c

Commit description:

The generic/642 test-case can reproduce the kernel crash:

[40243.605254] ------------[ cut here ]------------
[40243.605956] kernel BUG at fs/ceph/xattr.c:918!
[40243.607142] Oops: invalid opcode: 0000 [#1] SMP PTI
[40243.608067] CPU: 7 UID: 0 PID: 498762 Comm: kworker/7:1 Not tainted 7.0.0-rc7+ #3 PREEMPT(full)
[40243.609700] Hardware name: QEMU Ubuntu 25.10 PC v2 (i440FX + PIIX, + 10.1 machine, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[40243.611820] Workqueue: ceph-msgr ceph_con_workfn
[40243.612715] RIP: 0010:__ceph_build_xattrs_blob+0x1b8/0x1e0
[40243.613731] Code: 0f 84 82 fe ff ff e9 cf 8e 56 ff 48 8d 65 e8 31 c0 5b 41 5c 41 5d 5d 31 d2 31 c9 31 f6 31 ff 45 31 c0 45 31 c9 c3 cc cc cc cc <0f> 0b 4c 8b 62 08 41 8b 85 24 07 00 00 49 83 c4 04 41 89 44 24 fc
[40243.616888] RSP: 0018:ffffcc80c4d4b688 EFLAGS: 00010287
[40243.617773] RAX: 0000000000010026 RBX: 0000000000000001 RCX: 0000000000000000
[40243.618928] RDX: ffff8a773798dee0 RSI: 0000000000000000 RDI: 0000000000000000
[40243.620158] RBP: ffffcc80c4d4b6a0 R08: 0000000000000000 R09: 0000000000000000
[40243.621573] R10: 0000000000000000 R11: 0000000000000000 R12: ffff8a75f3b58000
[40243.622907] R13: ffff8a75f3b58000 R14: 0000000000000080 R15: 000000000000bffd
[40243.624054] FS:  0000000000000000(0000) GS:ffff8a787d1b4000(0000) knlGS:0000000000000000
[40243.625331] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[40243.626269] CR2: 000072f390b623c0 CR3: 000000011c02a003 CR4: 0000000000372ef0
[40243.627408] Call Trace:
[40243.627839]  <TASK>
[40243.628188]  __prep_cap+0x3fd/0x4a0
[40243.628789]  ? do_raw_spin_unlock+0x4e/0xe0
[40243.629474]  ceph_check_caps+0x46a/0xc80
[40243.630094]  ? __lock_acquire+0x4a2/0x2650
[40243.630773]  ? find_held_lock+0x31/0x90
[40243.631347]  ? handle_cap_grant+0x79f/0x1060
[40243.632068]  ? lock_release+0xd9/0x300
[40243.632696]  ? __mutex_unlock_slowpath+0x3e/0x340
[40243.633429]  ? lock_release+0xd9/0x300
[40243.634052]  handle_cap_grant+0xcf6/0x1060
[40243.634745]  ceph_handle_caps+0x122b/0x2110
[40243.635415]  mds_dispatch+0x5bd/0x2160
[40243.636034]  ? ceph_con_process_message+0x65/0x190
[40243.636828]  ? lock_release+0xd9/0x300
[40243.637431]  ceph_con_process_message+0x7a/0x190
[40243.638184]  ? kfree+0x311/0x4f0
[40243.638749]  ? kfree+0x311/0x4f0
[40243.639268]  process_message+0x16/0x1a0
[40243.639915]  ? sg_free_table+0x39/0x90
[40243.640572]  ceph_con_v2_try_read+0xf58/0x2120
[40243.641255]  ? lock_acquire+0xc8/0x300
[40243.641863]  ceph_con_workfn+0x151/0x820
[40243.642493]  process_one_work+0x22f/0x630
[40243.643093]  ? process_one_work+0x254/0x630
[40243.643770]  worker_thread+0x1e2/0x400
[40243.644332]  ? __pfx_worker_thread+0x10/0x10
[40243.645020]  kthread+0x109/0x140
[40243.645560]  ? __pfx_kthread+0x10/0x10
[40243.646125]  ret_from_fork+0x3f8/0x480
[40243.646752]  ? __pfx_kthread+0x10/0x10
[40243.647316]  ? __pfx_kthread+0x10/0x10
[40243.647919]  ret_from_fork_asm+0x1a/0x30
[40243.648556]  </TASK>
[40243.648902] Modules linked in: overlay hctr2 libpolyval chacha libchacha adiantum libnh libpoly1305 essiv intel_rapl_msr intel_rapl_common intel_uncore_frequency_common skx_edac_common nfit kvm_intel kvm irqbypass joydev ghash_clmulni_intel aesni_intel rapl input_leds mac_hid psmouse vga16fb serio_raw vgastate floppy i2c_piix4 pata_acpi bochs qemu_fw_cfg i2c_smbus sch_fq_codel rbd dm_crypt msr parport_pc ppdev lp parport efi_pstore
[40243.654766] ---[ end trace 0000000000000000 ]---

Commit d93231a ("ceph: prevent a client from exceeding the MDS
maximum xattr size") moved the required_blob_size computation to before
the __build_xattrs() call, introducing a race.

__build_xattrs() releases and reacquires i_ceph_lock during execution.
In that window, handle_cap_grant() may update i_xattrs.blob with a
newer MDS-provided blob and bump i_xattrs.version.  When
__build_xattrs() detects that index_version < version, it destroys and
rebuilds the entire xattr rb-tree from the new blob, potentially
increasing count, names_size, and vals_size.

The prealloc_blob size check that follows still uses the stale
required_blob_size computed before the rebuild, so it passes even when
prealloc_blob is too small for the now-larger tree. After __set_xattr()
adds one more xattr on top, __ceph_build_xattrs_blob() is called from
the cap flush path and hits:

    BUG_ON(need > ci->i_xattrs.prealloc_blob->alloc_len);

Fix this by recomputing required_blob_size after __build_xattrs()
returns, using the current tree state. Also re-validate against
m_max_xattr_size to fall back to the sync path if the rebuilt tree now
exceeds the MDS limit.

[Commit description truncated; see the upstream URL below]

Changed files:
  fs/ceph/xattr.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=7eb72425c4e3234926502eb262f9d6193ccd572c

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

The original Linux kernel CVE announcement for CVE-2026-52961 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52961

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:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 2
  Paranoid ActionableScore: 3

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 REGULAR

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).