* [CVE-2026-97902][LOW] fs: don't return -EINVAL for successful nested thaw
@ 2026-09-25 17:17 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-25 17:17 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-97902
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: fs: don't return -EINVAL for successful nested thaw
Commit: 111339816138f042a59778322f941edc26903484
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=111339816138f042a59778322f941edc26903484
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97902
Analysis date: Fri, 25 Sep 2026 13:17:40 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-97902 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97902
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-2026-97902 LOW https://www.kernelcve.org/list/?q=CVE-2026-97902 CHECK WITH IMPACT FROM ORIG NN LOW 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-911;CWE-703;**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 'A logic error in nested filesystem thaw handling causes thaw_super_locked to drop a valid freeze reference but return EINVAL while another freezer remains. In the device mapper path this incorrect result can prevent bdev_thaw from decrementing bd_fsfreeze_count, leaving the block device permanently marked as frozen after the other holder releases its freeze. The affected filesystem can then be unmounted but cannot be mounted again until the block device is destroyed or the system is rebooted. For the CVSS the PR:H is selected because reliable triggering normally requires CAP_SYS_ADMIN and permission to freeze a mounted filesystem and suspend or resume a device mapper device. The issue is not network reachable and does not expose a memory corruption primitive. Impact is denial of service against the affected block device, with no supported confidentiality or integrity impact.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1) SKIP CVE-2026-97902 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES NO NO unknown MAYBE DISK LEAK HARDWARE LINUS BOOT - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=YES
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
1. ActionableScore
* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 1
* Paranoid score: 1
* No floor or cap was applied.
* Final recommended bucket: **Ordinary Moderate**::
2. Signal breakdown
* +1 Concrete persistent kernel resource/reference leak. `freeze_dec()` releases the filesystem holder, but the incorrect `-EINVAL` return prevents `bdev_thaw()` from decrementing `bd_fsfreeze_count`. Normal cleanup cannot recover this count.
* +1 Reliable strong device-level DoS. The affected block device remains marked frozen and cannot be mounted again until it is destroyed or the system is rebooted.
* +1 Important filesystem/storage path. The defect affects filesystem freezing and device-mapper or LVM block-device lifecycle operations.
* -2 Proven host-admin-equivalent privilege. The demonstrated sequence requires filesystem freeze and device-mapper management operations normally gated by host `CAP_SYS_ADMIN`.
* No additional Availability point. Persistent block-device unavailability is already represented by the strong DoS signal.
* No UAF, OOB access, stale post-lifetime object use, arbitrary write, controlled corruption, disclosure, or authorization bypass is established.
* No remote or ordinary unprivileged trigger is established.
* No race penalty applies because the reproducer is an ordered and deterministic sequence, not a timing race.
3. Reachability analysis
The demonstrated attacker must freeze a mounted filesystem, suspend and resume a device-mapper target, unfreeze and unmount the filesystem, and then attempt another mount. These operations require host-level storage administration and normally `CAP_SYS_ADMIN` in the initial user namespace.
The complete sequence is not available through an attacker-created user or mount namespace because device-mapper target management operates on host block devices. Prior creation and mounting of a device-mapper or LVM volume is also required. No network trigger, special hardware, boot option, or timing window is involved. Once the required administrative access exists, the sequence is deterministic and repeatable.
4. Primitive and severity interpretation
The exact primitive is a return-value and freeze-accounting mismatch. A valid nested thaw decrements the per-holder freeze count, but the erroneous failure return causes the caller to retain a separate block-device freeze count.
This is not a memory-lifetime violation. `freeze_dec()` legitimately modifies a live counter and does not leave a dangling object or cause a second operation on freed storage. Therefore, the repeated-release/refcount UAF rules and UAF floors do not apply.
The demonstrated consequence is persistent unavailability of one affected block device until destructive administrative recovery or reboot. No host crash, memory corruption, confidentiality loss, integrity violation, or privilege escalation is supported. The issue is Class C because it creates an invalid persistent state and device-level DoS without an independently useful corruption primitive.
`ImportantCandidate=NO` because the trigger already requires host storage-administration privileges and no security boundary is crossed. `AutoDowngradeAllowed=YES` because no unresolved memory-corruption, ownership, authorization, or post-lifetime primitive remains.
5. One-sentence report phrase
A nested filesystem thaw can incorrectly return EINVAL after successfully dropping its hold, causing device-mapper to leak the block-device freeze count and leave the affected volume unavailable until device destruction or reboot.
6. Manual review recommendation
NO MANUAL CHECK NEEDED
The patch establishes a deterministic host-admin-only accounting bug with persistent device-level DoS but no memory corruption, authorization bypass, or unresolved exploitation primitive.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: fs: don't return -EINVAL for successful nested thaw
Commit: 111339816138f042a59778322f941edc26903484
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=111339816138f042a59778322f941edc26903484
Commit description:
Commit 7366f8b ("fs: handle freezing from multiple devices")
replaced the freeze_holders bitmask with per-holder counters to allow
nested freezes. In the bitmask version, a thaw that released a shared
hold while another holder remained returned 0. Since the rework,
thaw_super_locked() drops the freeze reference via freeze_dec() but
then returns -EINVAL when other freezers remain, misinforming the
caller: the thaw did succeed, the superblock just stays frozen for the
remaining holders.
This breaks bdev-initiated freezing. When a filesystem is frozen with
FIFREEZE and additionally frozen via bdev_freeze() -- which nests by
design, see fs_bdev_freeze() -- the subsequent bdev_thaw() receives
-EINVAL from the holder op although its freeze reference was dropped,
and therefore keeps bd_fsfreeze_count elevated. Then device-mapper's
unlock_fs() ignores bdev_thaw()'s return value, so nothing rebalances
the count. After the user's FITHAW and umount, the block device can
never be mounted again:
dm-1: Can't mount, blockdev is frozen
There is no way for userspace to drop the leaked count; only
destroying the block device (or a reboot) recovers the device.
Reproducer (any kernel since v6.8):
dmsetup create dut --table "0 $(blockdev --getsz "$DEV") linear $DEV 0"
mkfs.ext4 /dev/mapper/dut
mount /dev/mapper/dut /mnt
fsfreeze --freeze /mnt # freeze_ucount == 1
dmsetup suspend dut # bd_fsfreeze_count == 1, ucount == 2
dmsetup resume dut # ucount 2 -> 1, but thaw_super()
# returns -EINVAL, so bdev_thaw()
# keeps bd_fsfreeze_count at 1
fsfreeze --unfreeze /mnt # filesystem thaws fine
umount /mnt
mount /dev/mapper/dut /mnt # EBUSY, forever
The same happens with fsfreeze held across an LVM snapshot of the
origin volume.
fs_bdev_thaw()'s documentation already describes the intended
semantics: "If this function returns zero it doesn't mean that the
filesystem is unfrozen as it may have been frozen multiple times".
Restore them by returning 0 when a nested thaw drops its hold while
other freezers remain. Thawing without holding a freeze still fails
with -EINVAL as may_unfreeze() rejects that case before the reference
count is touched.
Fixes: 7366f8b ("fs: handle freezing from multiple devices")
Cc: [email protected] # needs adjustments for < 6.17 (no may_unfreeze())
Signed-off-by: Moritz Tanner <[email protected]>
Link: https://patch.msgid.link/[email protected]
Tested-by: Lars Ellenberg <[email protected]>
Reviewed-by: Lars Ellenberg <[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=111339816138f042a59778322f941edc26903484
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-97902 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97902
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: unknown
Paranoid ActionableScore: 1
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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