From: AL-KERNEL <[email protected]>
To: [email protected]
Subject: [CVE-2026-98539][MODERATE REGULAR] fs/ntfs3: use d_instantiate_new() in ntfs_create_inode() and murder syzbot's "WARNING in do_new_mount" saga
Date: Sun, 18 Oct 2026 23:09:31 -0400 [thread overview]
Message-ID: <[email protected]> (raw)
CVE: CVE-2026-98539
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: fs/ntfs3: use d_instantiate_new() in ntfs_create_inode() and murder syzbot's "WARNING in do_new_mount" saga
Commit: 6fda7121cb6a8ea6fbd77d0f787f04b8e8b5358e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6fda7121cb6a8ea6fbd77d0f787f04b8e8b5358e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98539
Analysis date: Sun, 18 Oct 2026 23:09:31 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES
Summary:
A race in ntfs3 inode creation can reinitialize an inode lock while another task holds it, causing rwsem count corruption and indefinite filesystem operation stalls for a local user with access to the affected mount.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98539 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98539
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-2026-98539 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-98539 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE RESULT=DIFF (final Legacy and Aegis differ; FINAL: MODERATE, Aegis: MODERATE_KPANIC) 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:N/A:H';CWE-362;CWE-667;*CWE-833;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'A race in ntfs3 inode creation can expose a newly hashed directory inode before its I_NEW state is safely completed. A concurrent mount or ordinary path lookup can acquire inode i_rwsem before unlock_new_inode() reinitializes the lock class and resets the lock state. The later unlock can operate on an rwsem with a cleared owner and count, producing a warning, count underflow, and an indefinite wait in a later inode_lock(). For the CVSS the PR:L value is used because a local user with access to create entries and perform path lookups on an already mounted ntfs3 filesystem may be able to reach the race without full administrative privileges. AC:H is used because successful triggering depends on a concurrent timing window. The issue is not directly network reachable. Impact is denial of service through corrupted lock state and stalled filesystem operations. No supported UAF, OOB access, information leak, or privilege escalation primitive is visible in this path. Automated impact decision path. The original SimpleNet classifier selected IMPORTANT. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 3 with recommendation Borderline / manual review recommended. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage IMPORTANT to MODERATE_KPANIC because the CVSS score, vector or bug class threshold rule was applied. ActionableScore thresholds stage MODERATE_KPANIC to MODERATE because the ActionableScore threshold rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS and DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 and V41_INHERITED_KPANIC_PROMOTION_BLOCKED and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 1 with primitive class CLASS_C and manual review RECOMMENDED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3) MAYBE WRITE OOB LOCK DISK ERRORPATH HARDWARE LINUS SYZBOT LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 3
* Paranoid score: 3
* No floor or cap was applied.
* Final recommended bucket: **Borderline / manual review recommended**::
2. Signal breakdown
* +1 Local unprivileged trigger. A user with write and lookup access to an already mounted ntfs3 filesystem can race directory creation with an ordinary path lookup. The privileged mount race is one reproducer but is not the only documented trigger.
* +1 Weak indirect kernel-state corruption. `unlock_new_inode()` can reinitialize an `i_rwsem` already held by another task, clearing its owner and count. A subsequent `up_write()` underflows the count.
* +1 Strong local DoS. A later `inode_lock()` on the affected directory can wait indefinitely, stalling operations that need that inode.
* 0 timing penalty. A race is required, but the operations can be attempted repeatedly and the evidence does not establish a one-shot or externally uncontrollable window.
* 0 subsystem penalty. ntfs3 must already be mounted, but specialization alone does not justify a deduction.
* No UAF, OOB access, arbitrary write, controlled reclaim, disclosure, authorization bypass, or privilege-escalation primitive is established.
* No extra availability point was awarded because the indefinite lock wait is the same DoS consequence already counted.
* V33 and V32 do not apply. The corrupted state is an inode lock, but no guest, namespace, delegated-interface, packet-metadata, or other security-boundary chain is established.
* NEW52 B5-R does not apply because there is no heap OOB write or retry-accumulated offset.
* NEW53 B3-PENDING does not apply because no shared-backing data write is involved.
* No repeated release or refcount operation exists. There is no first lifetime-ending release, surviving dangling reference, or second lifetime mutation. No UAF floor applies.
3. Reachability analysis
The below-host-admin trigger is supported by the ordinary path-walk variant described in the commit. One local process can create a directory in a writable ntfs3 location while another resolves the new path through `lookup_slow()`. This requires normal filesystem permissions but not UID 0 or a capability.
An ntfs3 filesystem must already be mounted, which normally represents prior administrative setup. The attacker does not need to perform that setup to exercise the vulnerable inode creation and lookup sequence. No namespace capability is needed or relevant to the established trigger. No special hardware, sysctl, or boot mode is required beyond ntfs3 availability.
The race can be retried using repeated create and lookup operations. The supplied evidence does not prove a reliable host-wide hang, but it does establish persistent waiting for later operations on the corrupted inode.
4. Primitive and severity interpretation
The exact primitive is corruption of active synchronization state. `d_instantiate()` publishes the new dentry while its inode remains `I_NEW`, allowing another task to acquire `i_rwsem`. The later `unlock_new_inode()` reinitializes that held lock, after which the other task performs an unlock against reset state and underflows the rwsem count.
This is broader than a harmless warning but remains CLASS_C because it establishes invalid lock bookkeeping and a hang rather than a lifetime violation or independently useful memory-corruption primitive. It is not a UAF, repeated release, OOB write, arbitrary write, or authorization bypass.
The demonstrated consequence is denial of service affecting operations on the directory and potentially other work dependent on them. A broader filesystem or system stall is plausible but not sufficiently established to add another point. Confidentiality impact, protected-data modification, and privilege escalation are unsupported.
ImportantCandidate is NO because no meaningful Important-grade exploitation leverage or security-boundary primitive remains unresolved. AutoDowngradeAllowed is YES because the established behavior is constrained synchronization corruption and DoS, without UAF, controlled corruption, or cross-boundary integrity impact.
5. One-sentence report phrase
A race in ntfs3 inode creation can reinitialize an inode lock while another task holds it, causing rwsem count corruption and indefinite filesystem operation stalls for a local user with access to the affected mount.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
Confirm the operational scope of the indefinite wait and whether it can block filesystem-wide or host-critical workloads, but the supplied evidence does not justify an Important candidate.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: fs/ntfs3: use d_instantiate_new() in ntfs_create_inode() and murder syzbot's "WARNING in do_new_mount" saga
Commit: 6fda7121cb6a8ea6fbd77d0f787f04b8e8b5358e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6fda7121cb6a8ea6fbd77d0f787f04b8e8b5358e
Commit description:
ntfs_create_inode() creates a new inode via ntfs_new_inode(). It hashes
it with insert_inode_locked() and so it's marked as I_NEW until
unlock_new_inode().
ntfs 3 calls d_instantiate() in between though... Since the dentry was
already hashed by the lookup before the create any path walk finds it
without touching the parent's i_rwsem and so can lock the inode.
If the inode is a directory unlock_new_inode() calls
lockdep_annotate_inode_mutex_key() and marks i_rwsem with the
i_mutex_dir_key class.
That resets the count and the owner of a lock somebody else may already
hold by now...
syzbot has been spamming us with the same godforsaken bug
"WARNING in do_new_mount"
since 2023. I can't take it anymore so I went looking. Afaict, syzbot's
executor chdirs into a freshly mounted ntfs3 image, creates a
directory and then mounts some pseudofs on it. Everytime the mkdir()
takes longer than syzbot waits mount() runs concurrently:
mkdir("./sys") mount(NULL, "./sys", "sysfs")
ntfs_create_inode()
d_instantiate()
user_path_at() finds the dentry
do_lock_mount()
inode_lock(inode)
namespace_lock()
unlock_new_inode()
lockdep_annotate_inode_mutex_key()
init_rwsem(&inode->i_rwsem)
unlock_mount()
inode_unlock(inode)
The mount side then releases a lock that according to the rwsem nobody
holds:
DEBUG_RWSEMS_WARN_ON((rwsem_owner(sem) != current) && ...):
count = 0x0, magic = 0xffff888043a854e8, owner = 0x0,
curr 0xffff888000244880, list empty
WARNING: CPU: 0 PID: 5346 at kernel/locking/rwsem.c:1368 __up_write
Call Trace:
inode_unlock include/linux/fs.h:877 [inline]
unlock_mount fs/namespace.c:2892 [inline]
do_new_mount_fc fs/namespace.c:3828 [inline]
do_new_mount+0x777/0xa40 fs/namespace.c:3887
On PREEMPT_RT the same thing shows up as
DEBUG_LOCKS_WARN_ON(rt_mutex_owner(lock) != current)
WARNING: kernel/locking/rtmutex_common.h:193 at rt_mutex_slowunlock
The up_write() underflows the reset count. A following inode_lock() on
that directory then never returns. A path walk into the new directory
racing with the mkdir() corrupts the lock the same way via
inode_lock_shared() in lookup_slow().
Switch to d_instantiate_new() and drop the trailing unlock_new_inode().
All error paths bail out before that point with I_NEW still set and
keep using discard_new_inode().
May we never see this fscking bug report again.
Link: https://patch.msgid.link/20260909-work-ntfs3-d_instantiate_new-v1-1-2db697162ce8@kernel.org
Fixes: 82cae26 ("fs/ntfs3: Add initialization of super block")
Reviewed-by: Jan Kara <[email protected]>
Cc: [email protected] # v5.15+
Reported-by: [email protected]
Closes: https://lore.kernel.org/[email protected]
Signed-off-by: Christian Brauner (Amutable) <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
kernel/locking/rwsem.c
include/linux/fs.h
fs/namespace.c
kernel/locking/rtmutex_common.h
fs/ntfs3/inode.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=6fda7121cb6a8ea6fbd77d0f787f04b8e8b5358e
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98539 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98539
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:L
The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
The Best / paranoid CVSS score: 4.7
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: 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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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