* [CVE-2026-99180][IMPORTANT] unicode: free the map after a grace period
@ 2026-10-29 4:10 21% AL-KERNEL
0 siblings, 0 replies; 1+ results
From: AL-KERNEL @ 2026-10-29 4:10 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-99180
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: unicode: free the map after a grace period
Commit: 2b54d3c971a1e5df3c9b0bcc10311ae6c29e79d8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2b54d3c971a1e5df3c9b0bcc10311ae6c29e79d8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99180
Analysis date: Thu, 29 Oct 2026 00:10:18 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate
Manual review required: NO
Summary:
A difficult local race can free a filesystem Unicode map before an RCU casefold pathwalk finishes using it, causing a demonstrated slab UAF and possible kernel crash.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99180 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99180
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: IMPORTANT
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-2026-99180 IMPORTANT https://www.kernelcve.org/list/?q=CVE-2026-99180 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=IMPORTANT RESULT=DIFF INDEPENDENT_LEGACY=IMPORTANT INDEPENDENT_RESULT=DIFF TEMP_KPANIC_MERGE=IMPORTANT_HARD_VETO PRE_MERGE_LEGACY=IMPORTANT (final Legacy and Aegis differ; FINAL: IMPORTANT, 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:L/I:L/A:H';*CWE-416;CWE-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'This is a race driven use after free in the Unicode casefold lookup path. generic_ci_d_hash and generic_ci_d_compare can dereference sb s_encoding under rcu_read_lock while filesystem teardown frees the same unicode map through utf8_unload. A local attacker must race an RCU pathwalk against release of the final reference to a casefolded ext4 f2fs or tmpfs mount. The reachable case described by the commit involves lazy unmount followed by close or chdir releasing the last mount reference. For the CVSS the PR:L value reflects the need for a local user or process able to perform path lookups and influence the relevant mount lifetime. AC:H reflects the timing dependent race window. The demonstrated primitive is a stale read from freed slab memory and the confirmed result is a kernel use after free with possible crash. No arbitrary write or reliable information disclosure primitive is established. The paranoid score retains the same use after free class and accounts for possible limited confidentiality or integrity impact if the freed object is reallocated before the stale traversal completes. Automated impact decision path. The original SimpleNet classifier selected MODERATE. The primary LLM estimated paranoid CVSS 5.8 and the production ActionableScore returned 5 with recommendation Actionable Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE to IMPORTANT because the CVSS score, vector or bug class threshold rule was applied. final reconciliation stage IMPORTANT to IMPORTANT because a specific evidence based protection or manual review veto was applied and KP specialist did not establish a qualifying panic. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist produced a review artifact and its decision was processed by the applicable downgrade and preservation rules. Exact triggered rule markers were INCREASED_TO_HIGH_BASED_ON_GUESSCVSS and TEMP_KPANIC_MERGE_IMPORTANT_VETO and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 1.9 and ActionableScore 4 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is IMPORTANT.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate (with actual score 5) MAYBE READ DISK INIT RACE LEAK UAF LINUS INCREASED_TO_HIGH_BASED_ON_GUESSCVSS TEMP_KPANIC_MERGE_IMPORTANT_VETO KPANIC_UNSUPPORTED_REMOVED - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw base score: 3.
* Conservative score: 5. Applied active-structured RCU lifetime calibration: 3 -> 5.
* Raw paranoid score: 3.
* Paranoid score: 5. No additional exploit primitive supports a higher score.
* Final recommended bucket: **Actionable Moderate**::
2. Signal breakdown
* +2 — Real UAF: `utf8_unload()` can free `unicode_map` while an RCU pathwalk still dereferences it through `sb->s_encoding`.
* +1 — Concrete RCU referenced-object lifetime mismatch: the superblock/map association remains observable by pathwalk readers, while the referenced map was released before their grace period. Moving the free and module-reference drop into `call_rcu()` directly establishes this invariant.
* +1 — Reliable kernel crash/strong DoS: KASAN demonstrates a slab-use-after-free in `utf8nlookup()`.
* -1 — Materially difficult race: the commit says ordinary unmount waits for a grace period; triggering instead requires final mount-reference release through a narrower lazy-unmount/close-or-`chdir` sequence while a pathwalk is held in casefold processing.
* No extra Availability point: it would duplicate the demonstrated crash.
* No structured-stale leverage point: active pointer/table fields are read, but attacker-controlled reclaim, replacement contents, or another useful post-free operation is not shown.
* No arbitrary write, disclosure path, controlled reclaim, authorization bypass, or C/I impact is established.
3. Reachability analysis
A local process can perform pathname lookups, and the commit establishes that the race is reachable through final-reference release after lazy unmount. However, the supplied evidence does not prove that one below-host-admin actor can create/configure the casefolded mount, arrange lazy unmount, control the final reference, and race the lookup. The governing mount/user-namespace privileges and any prior privileged setup therefore remain unresolved. No special hardware is required.
4. Primitive and severity reading
This is a genuine Class B UAF of an active structured `unicode_map`, not fixed stale storage and not cleanup-only use: the stale object and its table pointers are consumed by live casefold hash/compare processing. The shown outcome is an invalid read and possible kernel crash. There is no repeated release/refcount mutation or demonstrated useful reclaimed-object consumption. Important-grade leverage is therefore unproven, so `ImportantCandidate=NO`; constrained downgrade remains allowed, though the active RCU lifetime defect warrants review.
5. One-sentence report phrase
A difficult local race can free a filesystem Unicode map before an RCU casefold pathwalk finishes using it, causing a demonstrated slab UAF and possible kernel crash.
6. Manual review advice
MANUAL CHECK RECOMMENDED
Confirm whether an unprivileged user namespace can complete the entire casefolded-mount and lazy-unmount trigger sequence and whether reclaim can influence the map’s consumed table pointers.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: unicode: free the map after a grace period
Commit: 2b54d3c971a1e5df3c9b0bcc10311ae6c29e79d8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2b54d3c971a1e5df3c9b0bcc10311ae6c29e79d8
Commit description:
generic_ci_d_hash() and generic_ci_d_compare() read sb->s_encoding and
dereference the unicode map while a pathwalk in rcu mode holds nothing
but rcu_read_lock(). The filesystems that use them, ext4, f2fs and
tmpfs, drop the map with utf8_unload() from ->put_super(), which runs
when the last reference to the mount goes away. A lookup that is still
inside such a filesystem at that point reads a freed map.
With a pathwalk held up in utf8_casefold_hash() while the last reference
to a casefolded tmpfs goes away:
BUG: KASAN: slab-use-after-free in utf8nlookup+0x74c/0x950
Read of size 8 at addr ff110001030fe658 by task walker/98
Call Trace:
<TASK>
utf8nlookup+0x74c/0x950
utf8byte+0x160/0x1560
utf8_casefold_hash+0x167/0x230
generic_ci_d_hash+0xdd/0x1c0
link_path_walk+0x11ad/0x1d30
path_lookupat+0x63/0xb00
filename_lookup+0x1f1/0x5b0
vfs_statx+0xd7/0x3b0
[...]
Allocated by task 94:
__kmalloc_cache_noprof+0x249/0x560
utf8_load+0x4e/0x570
shmem_parse_opt_casefold.isra.0+0x92/0x270
[...]
Freed by task 97:
kfree+0x1db/0x5b0
shmem_put_super+0x72/0x170
generic_shutdown_super+0x14c/0x530
kill_anon_super+0x3a/0x60
deactivate_locked_super+0x9c/0xf0
cleanup_mnt+0x9f/0x130
So far that has been hard to hit. umount(2) waits for a grace period
in namespace_unlock() before it drops the mount, so only a mount whose
last reference goes away elsewhere, like the close of a file or a chdir
out of a lazily unmounted directory, can race a walker. So it is
reachable.
Delay the free until a grace period has passed. The map and the data
table it holds a reference to then outlive any pathwalk that found the
superblock before the mount went away. The reference on the data table
module is dropped from the rcu callback as well. utf8-core is built in
whenever CONFIG_UNICODE is set, so there is no module exit that would
have to wait for the callbacks.
Overlayfs points the s_encoding of an overlay at the map of its first
layer without taking a reference of its own. That stays correct. The
layer is held by the overlay, so its map is dropped only after the
overlay is gone.
Fixes: b886ee3 ("ext4: Support case-insensitive file name lookups")
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/unicode/utf8-core.c
include/linux/unicode.h
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=2b54d3c971a1e5df3c9b0bcc10311ae6c29e79d8
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99180 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99180
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:L/I:L/A:H
The Best / paranoid CVSS score: 5.8
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: 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: IMPORTANT
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): IMPORTANT
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).
^ permalink raw reply [relevance 21%]
Results 1-1 of 1 | reverse | sort options + mbox downloads above
-- links below jump to the message on this page --
2026-10-29 4:10 21% [CVE-2026-99180][IMPORTANT] unicode: free the map after a grace period AL-KERNEL
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox