[CVE-2026-46066][MODERATE REGULAR] ceph: fix num_ops off-by-one when crypto allocation fails

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

CVE: CVE-2026-46066
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ceph: fix num_ops off-by-one when crypto allocation fails
Commit: 6200f41d6fcf2ac7e24866431e381cbc914560e4
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6200f41d6fcf2ac7e24866431e381cbc914560e4
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46066
Analysis date: Wed, 27 May 2026 11:16:53 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A local user writing sparse data to fscrypt-enabled CephFS files under memory pressure can trigger a writeback `num_ops` accounting mismatch and kernel panic, with no demonstrated memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-46066 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46066

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-2026-46066	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:N/I:L/A:H';CWE-193;CWE-617;CWE-755;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.3';DESCR 'A local denial of service can occur in CephFS encrypted writeback when bounce buffer allocation fails after ceph_wbc num_ops has already been incremented for a new discontiguous extent. The failed folio is redirtied and not added to the batch, but the stale num_ops value can later make ceph_submit_write() hit BUG_ON and panic the kernel. For the CVSS the PR:L is used because a local user with write access to an fscrypt enabled CephFS file can trigger the writeback pattern, although reliable reproduction also requires memory pressure and a sparse 4KiB write pattern. The issue is not directly network reachable and does not involve UAF, out of bounds write, arbitrary write, information disclosure, or privilege escalation. Impact is primarily denial of service via kernel panic, with only weak integrity concern from writeback accounting mismatch.';NO MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 3)	YES	WRITE OOB DANGER DISK KERNEL_PANIC_PLUS_UAF LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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

ActionableScore=3
ActionableScoreLower=2

## 1. ActionableScore

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

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  A local user with write access to an fscrypt-enabled CephFS file can create the sparse write pattern involved in the bug.

* Reliable kernel crash / strong DoS: +1
  The described failure reaches `BUG_ON(ceph_wbc->op_idx + 1 != req->r_num_ops)`, causing a kernel panic.

* Availability impact realistic: +1
  Successful triggering can crash the kernel.

* Important filesystem/storage path: +1
  This is in CephFS encrypted writeback and write extent accounting.

Subtracting signals:

* Hard/special condition: -1
  Reliable reproduction requires encrypted CephFS, a 4KiB written / 4KiB skipped pattern, and memory pressure sufficient to make bounce buffer allocation fail.

* Configuration-dependent exposure: -1
  Requires CephFS with fscrypt-enabled files and affected kernel versions where multi-folio encrypted writeback is active.

Paranoid additional signal:

* Weak integrity concern: +1
  The root cause is writeback extent accounting mismatch. The demonstrated result is panic, not data corruption, but filesystem writeback accounting bugs deserve limited manual scrutiny.

No signals apply for UAF, OOB write, arbitrary write, page-cache corruption, information disclosure, remote reachability, or privilege escalation.

## 3. Reachability analysis

The trigger is local writeback activity against fscrypt-enabled CephFS files. A user who can write to such files can shape the sparse write pattern, but reliable triggering also needs memory pressure to make the encryption bounce buffer allocation fail.

Namespaces and containers matter only if they can write to the affected CephFS mount and induce memory pressure. The issue is not directly network reachable, although CephFS itself is a network filesystem.

## 4. Severity interpretation

This behaves primarily as a local filesystem DoS. The patch fixes an off-by-one accounting error in `ceph_wbc->num_ops` after a failed encrypted writeback folio move.

Theoretical integrity concern is limited because the bug is in writeback accounting, but the commit specifically shows a `BUG_ON` panic before successful submission with mismatched ops. There is no evidence of memory corruption, data disclosure, or privilege escalation.

## 5. One-sentence report phrase

A local user writing sparse data to fscrypt-enabled CephFS files under memory pressure can trigger a writeback `num_ops` accounting mismatch and kernel panic, with no demonstrated memory corruption or privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is DoS-only based on the patch, but it affects encrypted CephFS writeback accounting and can be triggered by local file writes under specific conditions, so limited manual validation of deployment exposure is useful.

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

Patch: ceph: fix num_ops off-by-one when crypto allocation fails
Commit: 6200f41d6fcf2ac7e24866431e381cbc914560e4
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6200f41d6fcf2ac7e24866431e381cbc914560e4

Commit description:

move_dirty_folio_in_page_array() may fail if the file is encrypted, the
dirty folio is not the first in the batch, and it fails to allocate a
bounce buffer to hold the ciphertext. When that happens,
ceph_process_folio_batch() simply redirties the folio and flushes the
current batch -- it can retry that folio in a future batch.

However, if this failed folio is not contiguous with the last folio that
did make it into the batch, then ceph_process_folio_batch() has already
incremented `ceph_wbc->num_ops`; because it doesn't follow through and
add the discontiguous folio to the array, ceph_submit_write() -- which
expects that `ceph_wbc->num_ops` accurately reflects the number of
contiguous ranges (and therefore the required number of "write extent"
ops) in the writeback -- will panic the kernel:

    BUG_ON(ceph_wbc->op_idx + 1 != req->r_num_ops);

This issue can be reproduced on affected kernels by writing to
fscrypt-enabled CephFS file(s) with a 4KiB-written/4KiB-skipped/repeat
pattern (total filesize should not matter) and gradually increasing the
system's memory pressure until a bounce buffer allocation fails.

Fix this crash by decrementing `ceph_wbc->num_ops` back to the correct
value when move_dirty_folio_in_page_array() fails, but the folio already
started counting a new (i.e. still-empty) extent.

The defect corrected by this patch has existed since 2022 (see first
`Fixes:`), but another bug blocked multi-folio encrypted writeback until
recently (see second `Fixes:`). The second commit made it into 6.18.16,
6.19.6, and 7.0-rc1, unmasking the panic in those versions. This patch
therefore fixes a regression (panic) introduced by cac190c .

Cc: stable@vger.kernel.org
Fixes: d552077 ("ceph: add encryption support to writepage and writepages")
Fixes: cac190c ("ceph: fix write storm on fscrypted files")
Signed-off-by: Sam Edwards <CFSworks@gmail.com>
Reviewed-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Changed files:
  fs/ceph/addr.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=6200f41d6fcf2ac7e24866431e381cbc914560e4

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

The original Linux kernel CVE announcement for CVE-2026-46066 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46066

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: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: 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).