[CVE-2026-89746][MODERATE 7.0] tracing: Fix use-after-free with same-name named triggers

[CVE-2026-89746][MODERATE 7.0] tracing: Fix use-after-free with same-name named triggers

[AL-KERNEL]

CVE: CVE-2026-89746
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: tracing: Fix use-after-free with same-name named triggers
Commit: 477d762be4b0ffaf01429ccd88859e93b0ec28cc
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=477d762be4b0ffaf01429ccd88859e93b0ec28cc
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89746
Analysis date: Sat, 12 Sep 2026 00:48:17 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89746 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89746

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: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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

CVE-2026-89746	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89746 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:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;*CWE-672;*CWE-664;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.8';DESCR 'A use-after-free in the tracing histogram trigger path can occur when two hist triggers on different events use the same name. The second trigger may be freed in the named_data path while its hist_data entry remains linked in tr hist_vars, leaving a dangling pointer that is later dereferenced by find_var_file when another trigger references a variable. This is a real kernel heap UAF and the demonstrated outcome is a kernel panic. For the CVSS the PR:L is used for the paranoid score because some deployments may delegate tracefs write access or tracing control to non root local users or service accounts, although the typical default configuration requires administrative tracing privileges. The issue is not network reachable and requires local access to the tracing control plane. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to memory corruption and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	YES	READ OOB TRACE LEAK UAF HARDWARE LINUS  KPANIC INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4	YES	NO	checked

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

ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 5
* Applied conservative floor/cap: REAL UAF FLOOR 5 applied because the bug is a real active-object UAF, the attacker-controlled trigger sequence can create the dangling hist_vars entry, and the stale object is later consumed by non-cleanup parsing logic in find_var_file.
* Raw paranoid score: 5
* Paranoid score: 5
* Applied paranoid floor/cap: none
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Positive signals:

* +2 Real UAF / active lifetime violation. The second same-name hist trigger frees its hist_data via destroy_hist_data while the same hist_data remains linked from tr->hist_vars.
* +1 Reliable kernel crash / strong DoS. The supplied trace shows slab-use-after-free in find_var_file followed by Oops and kernel panic.
* +1 Attacker-influenced active-object lifetime manipulation. The trigger writer controls the sequence of hist trigger creation and the later variable-reference trigger that walks the stale tr->hist_vars entry.
* +1 Paranoid structured stale-object leverage. The stale object is not merely a fixed buffer or shallow cleanup pointer. It is a tracing histogram object later traversed by active variable-resolution logic. This gives unresolved exploitation surface, but not enough for CLASS_A.

Negative or non-awarded signals:

* 0 controlled reclaim not demonstrated. No evidence shows attacker-controlled replacement of the freed hist_data allocation.
* 0 arbitrary write not demonstrated. The demonstrated stale use is a read/dereference during variable lookup.
* 0 confidentiality and integrity not independently awarded. They are possible worst-case concerns for manual review, but no disclosure path or directed corruption primitive is established.
* 0 extra Availability not awarded. The panic is already counted as the reliable crash signal.
* 0 concrete repeated release/refcount signal not awarded. The patch establishes remove_hist_vars before destroy_hist_data, not a second put/free/refcount mutation on the same dangling reference.
* 0 Weak LPE not separately awarded. Generic UAF-to-LPE speculation is not scored without controlled reuse, callback influence, arbitrary R/W, or another concrete primitive.
* 0 remote reachability not established. The trigger path is tracefs control-plane writes, not packet or network input.
* No privilege penalty applied. Host-admin-equivalent privilege is plausible in default tracefs deployments, but the complete privilege boundary is not positively established from the supplied evidence.

3. Reachability analysis

The demonstrated reproducer is local and uses writes to tracefs event trigger files under /sys/kernel/tracing. Default systems commonly restrict this interface to root or tracing administrators, but the supplied evidence does not prove that host-admin-equivalent privilege is always required for the complete operation. Therefore HostAdminRequired is UNKNOWN rather than YES.

No namespace-scoped trigger is established. Tracefs tracing controls are not shown to be fully usable from attacker-created namespaces, and the supplied reproducer does not establish a namespace-contained capability path. NamespaceScopedPrivilege is NO.

Prior host setup may be needed in practice if tracefs is not mounted or if permissions are not delegated. That affects deployment exposure, but it does not remove the concrete UAF primitive. No special hardware, network exposure, or boot-only feature is required beyond tracing histogram support.

This is not a timing race. The supplied sequence is an ordered control-plane sequence using same-name hist triggers and a later variable reference. No hard-race penalty is applied.

4. Primitive and severity interpretation

The exact primitive is a dangling pointer to freed hist_data left in tr->hist_vars. The stale object is an active structured tracing object, not fixed stale storage. The stale use occurs when find_var_file walks tr->hist_vars during later hist trigger parsing and dereferences the freed hist_data.

This is broader than an ordinary cleanup-only stale dereference because the stale entry remains reachable after the freeing operation and is consumed by later active parsing and variable-resolution logic. However, it is not CLASS_A because controlled reclaim, attacker-shaped replacement-object consumption, arbitrary kernel read/write, callback control, or demonstrated LPE is not established.

PrimitiveClass is CLASS_B. The demonstrated consequence is local kernel panic / DoS. The speculative consequence is possible exploitation beyond crash due to real kernel heap UAF in active tracing state, but that requires manual analysis and is not scored as proven LPE.

ImportantCandidate is YES because this is a real active-object UAF with attacker-influenced trigger state and unresolved post-free exploitation leverage. AutoDowngradeAllowed is NO because automatically closing it as ordinary crash-only Moderate would discard a concrete kernel lifetime primitive requiring review.

5. One-sentence report phrase

A same-name tracing histogram trigger sequence can leave freed hist_data linked in tr hist_vars, allowing a later tracefs trigger write to dereference the dangling pointer in find_var_file and crash the kernel, with unresolved UAF exploitation leverage requiring manual review.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: real kernel heap UAF in active tracing histogram state, not just NULL dereference or resource leak. Demonstrated impact is DoS, but exploitation leverage beyond crash remains unresolved.

7. Call-site confidence

Call-site confidence: high

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

Patch: tracing: Fix use-after-free with same-name named triggers
Commit: 477d762be4b0ffaf01429ccd88859e93b0ec28cc
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=477d762be4b0ffaf01429ccd88859e93b0ec28cc

Commit description:

When two hist triggers on different events are registered with the same
name=, the second one reuses the first as named_data.  Both are added to
tr->hist_vars by save_hist_vars() during event_hist_trigger_parse(),
because save_hist_vars() is called before event_trigger_register() while
the named reuse is only detected later, in hist_register_trigger().

In the named-data branch hist_register_trigger() then frees the second
histogram's hist_data via destroy_hist_data(), but never removes its
tr->hist_vars list entry, leaving a dangling pointer and leaking the
trace_array reference it holds.

A later hist trigger that references a variable makes find_var_file()
walk tr->hist_vars and dereference the freed hist_data.  The bug is
reproducible from userspace by writing three hist triggers to tracefs:

  cd /sys/kernel/tracing
  echo 'hist:keys=common_pid:x=common_pid:name=mh' > events/sched/sched_switch/trigger
  echo 'hist:keys=common_pid:x=common_pid:name=mh' > events/sched/sched_process_fork/trigger
  echo 'hist:keys=common_pid:vals=$x' > events/sched/sched_process_exit/trigger

The third write panics the kernel:

  BUG: KASAN: slab-use-after-free in find_var_file.part.0+0x272/0x290
  Read of size 8 at addr ffff888001f8a0e0 by task sh/1
  CPU: 1 UID: 0 PID: 1 Comm: sh Tainted: G      D          N
  Call Trace:
    find_var_file.part.0
    find_event_var
    parse_atom
    parse_expr
    __create_val_field
    event_hist_trigger_parse
    trigger_process_regex
    event_trigger_write
    vfs_write
    ksys_write
    do_syscall_64
    entry_SYSCALL_64_after_hwframe
  Allocated by task 1:
    event_hist_trigger_parse
  Freed by task 1:
    hist_register_trigger+0x618/0xa30
    event_hist_trigger_parse
  The buggy address belongs to freed 2048-byte region
  Oops: general protection fault ... RIP: find_var_file.part.0
  Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b

Fix by removing the hist_data from tr->hist_vars and releasing the
trace_array reference in the named-data branch of hist_register_trigger()
before freeing the hist_data.

Cc: stable@vger.kernel.org
Fixes: 6f86bde ("tracing: Fix bad hist from corrupting named_triggers list")
Link: https://patch.msgid.link/20260816100427.33642-3-sh_def@163.com
Signed-off-by: Hui Su <sh_def@163.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  kernel/trace/trace_events_hist.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=477d762be4b0ffaf01429ccd88859e93b0ec28cc

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

The original Linux kernel CVE announcement for CVE-2026-89746 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89746

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:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: MODERATE
KPANIC detected for this report: YES
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).