[CVE-2026-64326][MODERATE 7.0] block: skip sync_blockdev() on surprise removal in bdev_mark_dead()

[CVE-2026-64326][MODERATE 7.0] block: skip sync_blockdev() on surprise removal in bdev_mark_dead()

[AL-KERNEL]

CVE: CVE-2026-64326
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: block: skip sync_blockdev() on surprise removal in bdev_mark_dead()
Commit: d6998ddd507c81e3829489a6ead23f17f5acb7fe
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d6998ddd507c81e3829489a6ead23f17f5acb7fe
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64326
Analysis date: Sat, 25 Jul 2026 06:56:07 -0400
ActionableScore: 5
ActionableScore lower bound: 2
Actionable bucket: Ordinary Moderate conservatively / Actionable Moderate for untrusted adjacent storage exposure
Manual review required: NO

Summary:
`bdev_mark_dead() could call sync_blockdev() during surprise block-device removal, causing an indefinite wait on writeback I/O that can no longer complete and wedging the NVMe reset worker or tasks waiting on it.`

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

The original Linux kernel CVE announcement for CVE-2026-64326 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64326

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.

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

CVE-2026-64326	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';**CWE-400;CWE-667;CWE-703;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.3';DESCR 'bdev_mark_dead could call sync_blockdev during surprise removal even though the block device is already gone. This can wait forever on writeback I/O that can no longer complete, for example in the nvme reset path after I/O queues are lost. For the CVSS the PR:N is used in the paranoid interpretation because a malicious or failing storage endpoint may trigger the device loss condition without local privileges on the victim when the storage transport is reachable on an adjacent storage network. The more typical local administration scenario is PR:H because forcing device removal or reset usually requires administrative control. The issue is not a memory corruption primitive and does not indicate privilege escalation or information disclosure. Impact is denial of service through a wedged reset worker and tasks waiting on it.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate conservatively / Actionable Moderate for untrusted adjacent storage exposure (with actual score 4)	YES	SIMPLEFIX HARDWARE  INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	YES	NO	checked

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

ActionableScore=5
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 5
* Final recommended bucket: **Ordinary Moderate conservatively / Actionable Moderate for untrusted adjacent storage exposure**::

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1. `sync_blockdev()` can hang forever in `folio_wait_writeback()` when the device is already gone.
* Availability impact realistic: +1. The reset worker can be wedged and tasks waiting on it can also block.
* Important filesystem/storage path: +1. This is in the block layer removal path and affects writeback/reset behavior.
* Broad/default/common subsystem exposure: +1. Block devices and NVMe reset paths are common, although the exact trigger needs surprise removal.
* Requires admin/root/device-management capability in typical deployments: -2. Reliably forcing local block device removal or reset usually requires administrative control.

Paranoid delta:

* Firmware-mediated external influence: +1. A malicious, faulty, or fuzzed storage endpoint/controller can plausibly cause the “I/O queues lost” / surprise removal condition without local privileges on the victim.
* The admin penalty is not applied in this adjacent-storage interpretation because the trigger comes from the device/storage side rather than a local privileged user.

Not applied:

* Remote reachable / network-triggerable: +0. This is not a normal network packet path. NVMe-oF or storage-fabric exposure would be deployment-specific.
* Generic memory corruption: +0. The patch shows a hang/wait problem, not UAF, OOB, double-free, type confusion, or overwrite.
* Privilege escalation plausible: +0. No credible LPE primitive is shown.
* Confidentiality / integrity: +0. No data disclosure or modification primitive is indicated.

Call-site confidence: high. The commit provides the relevant call chain: `nvme_reset_work()` -> `nvme_mark_namespaces_dead()` -> `blk_mark_disk_dead()` -> `bdev_mark_dead(bdev, true)` -> `sync_blockdev()` -> `folio_wait_writeback()`.

## 3. Reachability analysis

Typical local triggering requires control over block device teardown, reset, or removal, which usually means root or an equivalent administrative/device-management capability. Containers normally do not lower the privilege requirement unless a block device or storage-management interface is delegated into the container.

The paranoid scenario is adjacent-storage or firmware-mediated: a failing or malicious NVMe device, emulated device, or storage endpoint can cause queue loss or surprise removal. This is not Internet-remote by default, but it can matter in virtualized, fuzzed, lab, or storage-fabric environments where the storage endpoint is less trusted than the host.

The path is in a common block/storage subsystem, but exploitation requires specific runtime state: surprise removal plus pending writeback that cannot complete.

## 4. Severity interpretation

This behaves like ordinary Moderate in the conservative local-admin model because it is a DoS-only hang with no memory corruption or privilege escalation evidence. It becomes Actionable Moderate in the paranoid adjacent-storage model because an external storage endpoint may be able to wedge a host reset worker and dependent tasks.

Theoretical memory corruption potential is not supported by the patch. Realistic evidence supports persistent availability impact only.

## 5. One-sentence report phrase

`bdev_mark_dead() could call sync_blockdev() during surprise block-device removal, causing an indefinite wait on writeback I/O that can no longer complete and wedging the NVMe reset worker or tasks waiting on it.`

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

For generic CVE triage this can be auto-handled as DoS-only. Manual review is only recommended for products where untrusted NVMe, NVMe-oF, virtual storage, or firmware-mediated storage endpoints are part of the threat model.

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

Patch: block: skip sync_blockdev() on surprise removal in bdev_mark_dead()
Commit: d6998ddd507c81e3829489a6ead23f17f5acb7fe
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d6998ddd507c81e3829489a6ead23f17f5acb7fe

Commit description:

bdev_mark_dead()'s @surprise == true means the device is already gone.
The filesystem callback fs_bdev_mark_dead() honours this and skips
sync_filesystem(), but the bare block device path (no ->mark_dead op)
lost its !surprise guard when the holder ->mark_dead callback was wired
up (see Fixes), and now calls sync_blockdev() unconditionally, which can
hang forever waiting on writeback that can no longer complete.

syzkaller hit this via nvme_reset_work()'s "I/O queues lost" path:
nvme_mark_namespaces_dead() -> blk_mark_disk_dead() ->
bdev_mark_dead(bdev, true) -> sync_blockdev() blocks in
folio_wait_writeback(), wedging the reset worker and every task waiting
on it.

Skip the sync on surprise removal, matching fs_bdev_mark_dead();
invalidate_bdev() still runs. Orderly removal (surprise == false) is
unchanged.

Found by FuzzNvme.

Fixes: d8530de ("block: call into the file system for bdev_mark_dead")
Acked-by: Sungwoo Kim <iam@sung-woo.kim>
Acked-by: Dave Tian <daveti@purdue.edu>
Acked-by: Weidong Zhu <weizhu@fiu.edu>
Signed-off-by: Chao Shi <coshi036@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20260522220025.1770388-1-coshi036@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
[ adjusted for 6.6.y: bdev_mark_dead() there still unlocks bd_holder_lock
  after the if/else and the else arm is a bare sync_blockdev(), so the
  !surprise guard is added as "else if (!surprise)" instead of inside the
  mainline else block. ]
Signed-off-by: Chao Shi <coshi036@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Changed files:
  block/bdev.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=d6998ddd507c81e3829489a6ead23f17f5acb7fe

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

The original Linux kernel CVE announcement for CVE-2026-64326 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64326

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:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/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: 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: NO
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).