[CVE-2026-99271][MODERATE 7.0] ALSA: line6: Clear prev_fbuf on capture buffer release

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

CVE: CVE-2026-99271
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ALSA: line6: Clear prev_fbuf on capture buffer release
Commit: 862d9268f40a6e10cef24eb226e738fb69ce8aa7
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=862d9268f40a6e10cef24eb226e738fb69ce8aa7
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99271
Analysis date: Sat, 07 Nov 2026 22:16:29 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
ALSA Line6 retains a freed capture-buffer pointer that playback later dereferences for a two-byte stale read, potentially causing a fault or limited output-dependent disclosure.

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

The original Linux kernel CVE announcement for CVE-2026-99271 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99271

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-2026-99271	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-99271 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=MODERATE_KPANIC RESULT=SAME V39_AEGIS_ONE_STEP_UP=MODERATE->MODERATE_KPANIC (final reconciliation agrees; DECISION: both MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:H';*CWE-416;*CWE-672;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:L';BEST CVSS score: '6.1';DESCR 'ALSA Line6 keeps prev_fbuf pointing into the capture buffer after line6_buffer_release frees that buffer. Starting playback later can make submit_audio_out_urb read two bytes through the stale pointer while generating an impulse test signal. For the CVSS the PR:L value reflects that an attacker needs local access to the affected ALSA Line6 device and its PCM or mixer controls. The demonstrated primitive is a slab use after free read rather than a write. It can cause a KASAN failure or possible denial of service and may expose limited stale data through output dependent processing, but privilege escalation and host wide failure are not established. Automated impact decision path. The original SimpleNet classifier selected MODERATE. The primary LLM estimated paranoid CVSS 6.1 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 final reconciliation stage MODERATE to MODERATE_KPANIC because the cross model evidence reconciliation 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 V39_AEGIS_ONE_STEP_UP and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 3.3 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 MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	MAYBE	READ USB LEAK UAF HARDWARE LINUS TEST  KPANIC V39_AEGIS_ONE_STEP_UP MANUAL_REVIEW_REQUIRED	-	-	checked

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

ActionableScore=3
ActionableScoreLower=2
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=REQUIRED
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 2.
* Conservative score: 2; no floor applies because this is fixed stale storage with a shallow two-byte read.
* Raw paranoid score: 3.
* Paranoid score: 3; no floor or cap changes it.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

* +2 Real UAF: `prev_fbuf` remains pointed into the capture buffer after `kfree()`, then `submit_audio_out_urb()` reads through it.
* +1 Local below-host-admin trigger: supplied evidence grounds local access to the affected PCM/mixer controls.
* +1 Plausible crash/DoS: KASAN detects the stale slab read; a production host-wide panic is not established.
* -2 Rare physical hardware dependency: the vulnerable path requires an affected Line6 USB audio device.
* Paranoid +1 Confidentiality concern: two stale bytes influence generated playback output, providing a technically plausible but unproven, very limited observation channel.

Not awarded: no write, controlled reclaim, attacker-shaped replacement, arbitrary access, LPE, authorization bypass, or separate availability point. The confidentiality effect is not proven strongly enough for the conservative score.

3. Reachability analysis

A local user with access to the affected ALSA Line6 PCM or mixer controls can release the capture buffer and later start the playback-side impulse path. Host UID 0 or host-administrator capability is not shown as necessary. No namespace capability is involved. Physical presence of supported Line6 hardware and device-node/control access are prerequisites; no race is required by the described sequence.

4. Primitive and severity reading

The primitive is a real slab UAF read from fixed stale capture-buffer storage. It is broader than bookkeeping but remains a shallow two-byte read, with no shown post-free write or structured replacement consumption. The demonstrated effect is KASAN detection and possible denial of service; limited audio-dependent stale-data exposure is plausible but unconfirmed. This is constrained CLASS_B, not CLASS_A. ImportantCandidate is NO because material leverage beyond fault or limited stale reading is absent. Auto-downgrade is allowed, although manual review remains required by the supplied triage disposition.

5. One-sentence report phrase

ALSA Line6 retains a freed capture-buffer pointer that playback later dereferences for a two-byte stale read, potentially causing a fault or limited output-dependent disclosure.

6. Manual review advice

MANUAL CHECK REQUIRED

Confirm production crash behavior, device permissions, and whether stale sample data can be meaningfully recovered from playback output.

7. Call-site confidence

Call-site confidence: high

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

Patch: ALSA: line6: Clear prev_fbuf on capture buffer release
Commit: 862d9268f40a6e10cef24eb226e738fb69ce8aa7
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=862d9268f40a6e10cef24eb226e738fb69ce8aa7

Commit description:

A KASAN slab-use-after-free read was reported in submit_audio_out_urb()
when generating an impulse test signal:

  BUG: KASAN: slab-use-after-free in create_impulse_test_signal sound/usb/line6/playback.c:76 [inline]
  BUG: KASAN: slab-use-after-free in submit_audio_out_urb+0x1848/0x1b20 sound/usb/line6/playback.c:249
  Read of size 2 at addr ffff888008d74870 by task syz.2.3214/7785

  Allocated by task 11:
   kmem_cache_alloc_noprof+0x2f0/0x390 mm/slub.c:4144
   line6_buffer_acquire+0x1a1/0x220 sound/usb/line6/pcm.c:158
   line6_pcm_acquire+0xb6/0x230 sound/usb/line6/pcm.c:329
   line6_startup_work+0x3c/0x50 sound/usb/line6/driver.c:728

  Freed by task 7785:
   kfree+0xf8/0x390 mm/slub.c:6566
   line6_buffer_release sound/usb/line6/pcm.c:176 [inline]
   line6_buffer_release+0xa5/0xf0 sound/usb/line6/pcm.c:170
   __line6_pcm_release sound/usb/line6/pcm.c:305 [inline]
   line6_pcm_release+0x5a/0x80 sound/usb/line6/pcm.c:313
   snd_toneport_monitor_put+0x11b/0x190 sound/usb/line6/toneport.c:183

line6pcm->prev_fbuf points into the capture PCM stream buffer
(line6pcm->in.buffer), which is updated during audio_in_callback() to
feed rate synchronization, software monitoring, and impulse test signal
generation on playback side.

When all capture streams are closed, line6_buffer_release() frees
pstr->buffer with kfree().  However, it does not clear line6pcm->prev_fbuf.
If playback is subsequently started (e.g. via impulse test signal
volume control snd_line6_impulse_volume_put()), submit_audio_out_urb()
dereferences the stale pointer line6pcm->prev_fbuf under in.lock,
resulting in a slab-use-after-free read.

Clear prev_fbuf and prev_fsize in line6_buffer_release() when releasing
the capture stream buffer under in.lock, ensuring that no stale buffer
pointer can be accessed by submit_audio_out_urb().

Fixes: 63e20df ("ALSA: line6: Reorganize PCM stream handling")
Cc: stable@vger.kernel.org
Co-developed-by: Bin Yu <byu@xidian.edu.cn>
Signed-off-by: Bin Yu <byu@xidian.edu.cn>
Co-developed-by: Cheng Wen <wencheng@xidian.edu.cn>
Signed-off-by: Cheng Wen <wencheng@xidian.edu.cn>
Signed-off-by: YuanZhe Liu <25031212351@stu.xidian.edu.cn>
Link: https://patch.msgid.link/20261013063554.571-1-25031212351@stu.xidian.edu.cn
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  sound/usb/line6/playback.c
  mm/slub.c
  sound/usb/line6/pcm.c
  sound/usb/line6/driver.c
  sound/usb/line6/toneport.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=862d9268f40a6e10cef24eb226e738fb69ce8aa7

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

The original Linux kernel CVE announcement for CVE-2026-99271 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99271

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:L/UI:N/S:U/C:L/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:H
  The Best / paranoid CVSS score: 6.1

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: 2
  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: 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).