From: AL-KERNEL <[email protected]>
To: [email protected]
Subject: [CVE-2026-68132][LOW] super: fix emergency thaw deadlock on frozen block devices
Date: Mon, 10 Aug 2026 19:58:59 -0400 [thread overview]
Message-ID: <[email protected]> (raw)
CVE: CVE-2026-68132
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: super: fix emergency thaw deadlock on frozen block devices
Commit: 63d78b546eefc38ad9898dc839bfc94811ede547
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=63d78b546eefc38ad9898dc839bfc94811ede547
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68132
Analysis date: Mon, 10 Aug 2026 19:58:59 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO
Summary:
Emergency thaw can deadlock on frozen block devices due to inverted s_umount and bd_fsfreeze_mutex locking, causing a privileged local denial of service for later unmount, freeze, or thaw operations.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-68132 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68132
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: NO
A detailed explanation of the methodology and priority rules is included
at the end of this message.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-68132 LOW CHECK WITH IMPACT FROM ORIG NN MODERATE Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';CWE-667;*CWE-833;**CWE-400;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.4';DESCR 'Emergency thaw on a frozen block device can deadlock because do_thaw_all_callback calls bdev_thaw while holding sb s_umount exclusively. If the last block layer freeze reference is dropped, fs_bdev_thaw can reacquire s_umount through get_bdev_super and bdev_super_lock in the same worker, causing a self deadlock. The same lock ordering can also conflict with concurrent block layer freeze paths. For the CVSS the PR:H is used because reliable triggering normally requires administrative control over filesystem freeze or emergency thaw operations. The issue is not network reachable. Impact is denial of service limited to stuck emergency thaw and later unmount freeze or thaw operations for the affected filesystem and block device.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2) YES DEADLOCK DISK HARDWARE LINUS DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_GUESSCVSS_AND_CIANNH NO NO checked
======================================================================
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 bug causes a real deadlock in the emergency thaw worker and can block later unmount, freeze, or thaw operations for the affected filesystem and block device.
* Important filesystem/storage path: +1
The issue is in core superblock and block-device freeze/thaw handling.
* Availability impact realistic: +1 in paranoid scoring only
The deadlock can persist and affect later filesystem management operations, but it is not a normal remote crash or broad unprivileged host-wide DoS.
Negative signals:
* Requires admin/root/CAP_SYS_ADMIN in typical deployments: -2
Reliable triggering requires control over filesystem or block-device freeze/thaw or emergency thaw behavior, which is normally administrative.
* Rare or difficult-to-reach operational path: -1 in conservative scoring
Emergency thaw on already frozen block devices is not a common default data path.
No memory-corruption signals apply:
* No UAF, OOB write, double free, refcount takeover, stale callback, type confusion, arbitrary write, or page-cache corruption is shown.
* This is a locking-order and self-deadlock bug.
## 3. Reachability analysis
The trigger is local and privileged in realistic deployments. A user normally needs administrative control over block-device or filesystem freeze/thaw operations, or the ability to trigger emergency thaw behavior. Namespaces do not obviously reduce this to unprivileged reachability because filesystem freeze and block-device management are normally gated by elevated capabilities and host-controlled devices. The path is in core filesystem/block infrastructure, but the specific emergency thaw scenario is operationally uncommon and not exposed through packet traffic or a normal unprivileged syscall pattern.
## 4. Severity interpretation
This behaves like an ordinary Moderate or Low-like kernel availability issue, not an Important-class vulnerability. The theoretical impact is persistent local DoS for the affected filesystem and block device management path. The realistic evidence supports deadlock only, with no demonstrated memory corruption, privilege escalation, confidentiality impact, or integrity impact.
## 5. One-sentence report phrase
Emergency thaw can deadlock on frozen block devices due to inverted s_umount and bd_fsfreeze_mutex locking, causing a privileged local denial of service for later unmount, freeze, or thaw operations.
## 6. Manual review recommendation
NO MANUAL CHECK NEEDED
This is a privileged local deadlock with clear locking root cause and no evidence of memory corruption, privilege escalation, network reachability, or sensitive-object access.
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: super: fix emergency thaw deadlock on frozen block devices
Commit: 63d78b546eefc38ad9898dc839bfc94811ede547
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=63d78b546eefc38ad9898dc839bfc94811ede547
Commit description:
do_thaw_all_callback() calls bdev_thaw() while holding sb->s_umount
exclusively. If the block device was frozen via bdev_freeze() dropping
the last block layer freeze reference calls fs_bdev_thaw() which
reacquires s_umount:
do_thaw_all_callback(sb)
super_lock_excl(sb) # holds sb->s_umount
bdev_thaw(sb->s_bdev)
mutex_lock(&bdev->bd_fsfreeze_mutex)
# bd_fsfreeze_count drops 1 -> 0
bd_holder_ops->thaw == fs_bdev_thaw
get_bdev_super(bdev)
bdev_super_lock(bdev, true)
super_lock(sb, true)
down_write(&sb->s_umount) # same task: deadlock
The emergency thaw worker deadlocks against itself holding both
s_umount and bd_fsfreeze_mutex. That fscks any subsequent unmount,
freeze, or thaw of that filesystem and block device.
[ 81.878470] sysrq: Show Blocked State
[ 81.880140] task:kworker/0:1 state:D stack:0 pid:11 tgid:11 ppid:2 task_flags:0x4208060 flags:0x00080000
[ 81.884876] Workqueue: events do_thaw_all
[ 81.886656] Call Trace:
[ 81.887759] <TASK>
[ 81.888763] __schedule+0x579/0x1420
[ 81.890372] schedule+0x3a/0x100
[ 81.891794] schedule_preempt_disabled+0x15/0x30
[ 81.893848] rwsem_down_write_slowpath+0x1ea/0x900
[ 81.895191] ? __pfx_do_thaw_all_callback+0x10/0x10
[ 81.896528] down_write+0xbd/0xc0
[ 81.897505] super_lock+0x91/0x180
[ 81.898457] ? __mutex_lock+0xa99/0x1140
[ 81.900748] ? __mutex_unlock_slowpath+0x1f/0x400
[ 81.902069] bdev_super_lock+0x5b/0x150
[ 81.903132] get_bdev_super+0x10/0x60
[ 81.904042] fs_bdev_thaw+0x23/0xf0
[ 81.904755] bdev_thaw+0x82/0x100
[ 81.905484] do_thaw_all_callback+0x2c/0x50
[ 81.906298] __iterate_supers+0x5d/0x130
[ 81.907067] do_thaw_all+0x20/0x40
[ 81.907739] process_one_work+0x206/0x5e0
[ 81.908545] worker_thread+0x1e2/0x3c0
[ 81.909339] ? __pfx_worker_thread+0x10/0x10
[ 81.910171] kthread+0xf4/0x130
[ 81.910799] ? __pfx_kthread+0x10/0x10
[ 81.911528] ret_from_fork+0x2e2/0x3b0
[ 81.912259] ? __pfx_kthread+0x10/0x10
[ 81.913010] ret_from_fork_asm+0x1a/0x30
[ 81.913806] </TASK>
bdev_super_lock() even documents the violated requirement with
lockdep_assert_not_held(&sb->s_umount).
Acquiring bd_fsfreeze_mutex under s_umount also inverts the
bd_fsfreeze_mutex vs. s_umount ordering established by
bdev_{freeze,thaw}() and can thus ABBA against a concurrent block-layer
freeze even when the recursive path isn't hit.
Fix this by not holding s_umount around the bdev_thaw() loop at all. Pin
the superblock with an active reference instead as
filesystems_freeze_callback() does. The active reference keeps the
superblock from being shut down and so ->s_bdev stays valid without
holding s_umount. The block-layer-held freeze is dropped by
fs_bdev_thaw() with FREEZE_MAY_NEST | FREEZE_HOLDER_USERSPACE exactly as
a regular unfreeze would and thaw_super_locked() handles
filesystem-level freezes as before.
The emergency thaw path has deadlocked like this in one form or
another for a long long time but the current exclusively-held
shape dates back to commit [1] where thaw_bdev() already ended in
thaw_super() with s_umount held by do_thaw_all_callback().
Fixes: 08fdc8a ("buffer.c: call thaw_super during emergency thaw") [1]
Cc: [email protected]
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Christian Brauner (Amutable) <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
fs/super.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=63d78b546eefc38ad9898dc839bfc94811ede547
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-68132 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68132
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:L
The Best / paranoid CVSS vector: AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H
The Best / paranoid CVSS score: 4.4
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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