[CVE-2026-89747][MODERATE 7.0] tracing: Fix use-after-free in trace_pipe read on sub-buffer order change

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-89747
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: tracing: Fix use-after-free in trace_pipe read on sub-buffer order change
Commit: 05ebe1e1d7d38c2612bacfefe043a33572ae34cb
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=05ebe1e1d7d38c2612bacfefe043a33572ae34cb
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89747
Analysis date: Fri, 11 Sep 2026 22:00:59 -0400
ActionableScore: 5
ActionableScore lower bound: 4
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-2026-89747 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89747

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-2026-89747	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89747 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:H';*CWE-416;CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:N/A:H';BEST CVSS score: '6.3';DESCR 'A use after free can occur in the tracing trace_pipe read path when a reader keeps a pointer to a ring buffer event while another local actor changes buffer_subbuf_size_kb and replaces the ring buffer sub buffers. The reader can later dereference freed memory through ring_buffer_event_length() or ring_buffer_event_data(), which can cause a kernel crash and may expose stale kernel memory through trace output in a worst case interpretation. For the CVSS the PR:L is used for the paranoid score because tracefs access may be delegated to a non root local user or service account in some deployments. The issue is not network reachable and requires local interaction with tracing control files. Impact is at least local denial of service via kernel crash, with possible confidentiality impact from reading freed or reused kernel memory.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)  SKIP CVE-2026-89747 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	READ TRACE RACE UAF HARDWARE LINUS SYZBOT KPANIC 	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 4
* Raw paranoid score: 5
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

Conservative floor applied: 3 -> 4 because a real kernel UAF is established, the stale reference is actually dereferenced, and host-admin-equivalent privilege is not positively proven from the supplied evidence.

No paranoid floor or cap was applied.

2. Signal breakdown

Conservative positive signals:

* +2 Real UAF / active lifetime violation. `ring_buffer_subbuf_order_set()` frees ring-buffer sub-buffer pages, including the reader page, while a `trace_pipe` reader can still hold a stale event pointer returned by `ring_buffer_peek()`.
* +1 Reliable kernel crash / strong DoS. KASAN reports a use-after-free read, and the stale dereference can realistically cause Oops or kernel crash.

Conservative negative signals:

* No -2 host-admin penalty. Writing `buffer_subbuf_size_kb` is normally tracefs control-plane access, often restricted, but the supplied evidence does not positively establish that the complete trigger always requires host-admin-equivalent privilege.
* No -1 race penalty. The issue is a race, but both sides can plausibly be driven repeatedly by a local actor with tracefs access. The evidence does not establish a one-shot or externally uncontrollable timing window.

Paranoid additional positive signals:

* +1 Structured stale-object read-path leverage. The stale pointer is not merely passed to a destructor. It is consumed by `ring_buffer_event_length()` and `ring_buffer_event_data()` in the trace output path, so stale structured event metadata can affect later reads.
* +1 Confidentiality plausible. Because the stale object is consumed in a userspace-facing `trace_pipe` read path, a worst-case but still defensible interpretation is that freed or reused kernel memory could be exposed through trace output before or instead of a crash.

Important non-awarded signals:

* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Privilege escalation not demonstrated.
* Concrete repeated release/refcount mutation not established.
* No second lifetime/refcount operation on the exact same dangling reference.
* Extra availability was not awarded because the same crash is already counted once.
* Remote reachability not established.
* Namespace-scoped privilege not established.
* Strong Class A exploitation primitive not established.

Repeated release/refcount checklist:

* First release can end lifetime: yes for the freed ring-buffer page storage.
* Exact same reference survives: yes, the reader can retain an event pointer into the freed page.
* Second operation is a real lifetime/refcount mutation: no, the stale use is a dereference/read, not a second `put`, `free`, or refcount mutation.
* +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: no, because the dedicated repeated-release/refcount condition is not present.

3. Reachability analysis

The trigger requires two local activities against tracing: a `trace_pipe` reader holding an event pointer during its peek-and-print loop, and a writer changing `buffer_subbuf_size_kb`, which causes the ring buffer sub-buffer order change and frees/replaces sub-buffer pages.

This is not network reachable. It is a local tracing control-plane issue.

The exact privilege boundary is deployment-dependent. Tracefs control files are commonly restricted to root or tracing administrators, but tracefs access can be delegated to a group or service account. Therefore `HostAdminRequired=UNKNOWN` is more accurate than forcing `YES` or `NO`.

No supplied evidence shows that the complete trigger is namespace-scoped. Tracefs is not established here as an attacker-created user or network namespace interface, so `NamespaceScopedPrivilege=NO`.

No special hardware requirement is present. The race is repeatable in principle because the reader and writer operations can be attempted in loops by an actor with the required local tracing access.

4. Primitive and severity interpretation

The exact primitive is a race-based use-after-free read in the tracing ring-buffer path. A reader obtains an event pointer under `cpu_buffer->reader_lock`, but the pointer remains in use after that lock is dropped. A concurrent sub-buffer order change can free the page backing that event pointer, and the reader later dereferences freed memory.

This is closer to an active structured stale read than to a simple fixed cleanup pointer fault, because the stale event is processed by `ring_buffer_event_length()` and `ring_buffer_event_data()` in a userspace-facing trace read path. However, controlled replacement, callback control, arbitrary read/write, and LPE are not demonstrated.

Primitive class is `CLASS_B`: a real kernel lifetime/UAF primitive exists and needs exploitation-leverage review, but no Class A primitive is established.

Demonstrated consequence is local DoS through KASAN UAF/Oops/kernel crash. Speculative but technically defensible paranoid consequence is possible information exposure through stale or reused memory being interpreted as trace event data.

`ImportantCandidate=YES` because this is not a mere warning or passive state bug: it is a real UAF in a kernel read path with possible user-visible stale data exposure. `AutoDowngradeAllowed=NO` because automatically closing it as ordinary Moderate could lose a meaningful unresolved lifetime/disclosure primitive.

Likely operational severity: actionable Moderate. It is not a strong Important by itself without evidence of controlled reclaim, arbitrary kernel read/write, or practical privilege escalation.

5. One-sentence report phrase

A race between `trace_pipe` readers and `buffer_subbuf_size_kb` updates can leave readers dereferencing freed ring-buffer event memory, causing a local kernel crash and possibly exposing stale kernel data through trace output.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a confirmed kernel UAF in a userspace-facing tracing read path, with DoS demonstrated and possible confidentiality impact requiring human review.

7. Call-site confidence

Call-site confidence: high

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

Patch: tracing: Fix use-after-free in trace_pipe read on sub-buffer order change
Commit: 05ebe1e1d7d38c2612bacfefe043a33572ae34cb
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=05ebe1e1d7d38c2612bacfefe043a33572ae34cb

Commit description:

Writing to buffer_subbuf_size_kb calls ring_buffer_subbuf_order_set(),
which frees every sub-buffer of the ring buffer, including the reader
page, and replaces them with newly allocated ones.

Readers of trace_pipe hold pointers into those pages. ring_buffer_peek()
looks up an event under cpu_buffer->reader_lock but returns the event
pointer after dropping the lock, and peek_next_entry() then calls
ring_buffer_event_length() and ring_buffer_event_data() on it. If the
sub-buffer order is changed in that window, the reader dereferences
freed memory:

  BUG: KASAN: use-after-free in ring_buffer_peek+0x3e0/0x430
  Read of size 1 at addr ffff88802a4cf010 by task syz-executor989/6002

  Freed by:
   free_buffer_page kernel/trace/ring_buffer.c:398 [inline]
   ring_buffer_subbuf_order_set+0x1325/0x18e0 kernel/trace/ring_buffer.c:7444
   buffer_subbuf_size_write+0x182/0x280 kernel/trace/trace.c:8221

Take trace_access_lock(RING_BUFFER_ALL_CPUS) around the order change.
This is the lock trace_pipe readers already hold across their entire
peek-and-print loop, so the swap can no longer race with a reader that
is dereferencing a peeked event.

Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260817140655.5694-1-kartikey406@gmail.com
Fixes: f9b94da ("ring-buffer: Set new size of the ring buffer sub page")
Reported-by: syzbot+685955db58555575fdd2@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=685955db58555575fdd2
Tested-by: syzbot+685955db58555575fdd2@syzkaller.appspotmail.com
Reviewed-by: Bradley Morgan <include@grrlz.net>
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  kernel/trace/ring_buffer.c
  kernel/trace/trace.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=05ebe1e1d7d38c2612bacfefe043a33572ae34cb

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

The original Linux kernel CVE announcement for CVE-2026-89747 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89747

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:H/UI:N/S:U/C:L/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:H
  The Best / paranoid CVSS score: 6.3

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: 4
  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).