[CVE-2026-98741][IMPORTANT] ALSA: ua101: reject mismatched capture/playback packet sizes

[CVE-2026-98741][IMPORTANT] ALSA: ua101: reject mismatched capture/playback packet sizes

[AL-KERNEL]

CVE: CVE-2026-98741
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: ALSA: ua101: reject mismatched capture/playback packet sizes
Commit: fd3a7bf0d988b0878a1957df150edb35f81db87e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fd3a7bf0d988b0878a1957df150edb35f81db87e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98741
Analysis date: Sat, 24 Oct 2026 05:02:45 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A malicious USB audio device can cause a below-host-admin PCM opener to trigger a descriptor-driven heap OOB write before URB length validation.

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

The original Linux kernel CVE announcement for CVE-2026-98741 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98741

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: YES

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

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-98741	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-98741 CHECK WITH IMPACT FROM ORIG NN LOW | 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:P/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-787;*CWE-20;CWE-131;Other CVSS 'AV:P/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '6.6';DESCR 'A mismatched USB audio descriptor can make the ua101 driver derive the playback transfer length from a much larger capture packet geometry while allocating the playback URB buffer from the smaller playback wMaxPacketSize. The demonstrated primitive is a slab out of bounds write through memset in start_usb_playback, and the same size relationship can also drive memcpy beyond the playback buffer in playback_work. Impact is at least a local kernel crash and may include corruption of adjacent heap objects, with a plausible but unproven path to privilege escalation. Triggering normally requires presentation of a crafted USB audio device followed by opening the registered PCM node. For the CVSS the PR:L value reflects that reliable triggering requires a local user able to open the PCM device. AV:P reflects the required attachment or equivalent presentation of a malicious USB device. The base score uses CIA:LLH because memory corruption is confirmed but reliable disclosure or privilege escalation is not. The paranoid score uses CIA:HHH because the concrete kernel heap write can potentially affect security sensitive adjacent objects. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 6.6 and the production ActionableScore returned 5 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to IMPORTANT because the CVSS score, vector or bug class threshold rule was applied. KP and FD specialist checks stage IMPORTANT to IMPORTANT because a specific evidence based protection or manual review veto 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 V42_LEGACY_CLASSA_FD_VETO and MANUAL_REVIEW_REQUIRED and TEMP_KPANIC_MERGE_IMPORTANT_VETO and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 3.2 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 / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	WRITE KASAN OOB USB ERRORPATH HARDWARE LINUS PACKET INCREASED_TO_HIGH_BASED_ON_GUESSCVSS V42_LEGACY_CLASSA_FD_VETO MANUAL_REVIEW_REQUIRED TEMP_KPANIC_MERGE_IMPORTANT_VETO KPANIC_UNSUPPORTED_REMOVED	-	-	checked

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

ActionableScore=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 2.
* Conservative score: 5. The physical-device descriptor heap-overwrite review floor applies: attacker-supplied USB geometry determines an undersized playback allocation and a larger write.
* Raw paranoid score: 2.
* Paranoid score: 5. No additional speculative effects awarded.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 — Confirmed attacker-influenced heap OOB write: a 168-byte playback buffer receives a 2048-byte `memset`, with capture/playback descriptor geometry controlling the mismatch.
* +1 — Plausible kernel crash/strong DoS from the demonstrated slab overwrite.
* +1 — Below-host-admin local trigger is supplied: a user able to open the registered PCM node reaches `start_usb_playback()`.
* -2 — A malicious or equivalently presented physical USB device is required.
* Floor 5 — Physical device descriptors independently determine allocation and write geometry, producing a real heap overwrite. This preserves mandatory Important review despite physical exposure.
* No separate strong-leverage bonus: the demonstrated `memset` payload is fixed, while no adjacent victim, target selection, controlled reclaim, arbitrary write, or LPE is shown.
* The commit also identifies an OOB `memcpy`, but protected-state corruption and C/I impact are not established.
* NEW52/B5-R does not apply: no retry-accumulated index or retry sequence exists.
* No UAF, authorization bypass, or extra availability point is present.

3. Reachability analysis

A malicious USB device supplies mismatched endpoint/channel descriptors; probing registers the PCM node, and a permitted local user triggers the write by opening playback. Host UID 0, host-admin capabilities, namespaces, a race, and prior privileged software configuration are not required by the supplied path. Physical attachment or equivalent device presentation remains mandatory.

4. Primitive and severity reading

The primitive is an attacker-influenced heap OOB write into and beyond a fixed playback URB buffer, not a UAF or cleanup-only stale access. The write is confirmed before `usb_submit_urb()` rejects the oversized transfer. This is CLASS_A because descriptor-controlled allocation/write geometry produces a substantial slab overwrite. Crash and adjacent-object corruption are credible; arbitrary kernel write, disclosure, privilege escalation, and a named corrupted consumer remain unproven. ImportantCandidate is YES and auto-downgrade is disallowed because the concrete overwrite would otherwise be lost, notwithstanding physical exposure.

5. One-sentence report phrase

A malicious USB audio device can cause a below-host-admin PCM opener to trigger a descriptor-driven heap OOB write before URB length validation.

6. Manual review advice

MANUAL CHECK REQUIRED

The confirmed heap overwrite requires exploit-leverage and adjacent-object analysis.

7. Call-site confidence

Call-site confidence: high

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

Patch: ALSA: ua101: reject mismatched capture/playback packet sizes
Commit: fd3a7bf0d988b0878a1957df150edb35f81db87e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fd3a7bf0d988b0878a1957df150edb35f81db87e

Commit description:

detect_usb_format() cross-checks bSubframeSize, bBitResolution and
tSamFreq between the capture and playback interfaces, but never relates
the two endpoints' wMaxPacketSize and bNrChannels.  Each playback URB
gets a buffer of ua->playback.max_packet_bytes, while the number of
bytes written into it is derived from the capture stream:
capture_urb_complete() computes frames from the received capture packet
and capture.frame_bytes, and start_usb_playback() and playback_work()
multiply that by playback.frame_bytes.

A device declaring a large capture wMaxPacketSize with few capture
channels and a small playback wMaxPacketSize with many playback channels
therefore memset()s and memcpy()s past the end of the playback buffer,
in open() of the PCM node the driver registers during probe.
usb_submit_urb() rejects the over-long iso_frame_desc[0].length with
-EMSGSIZE, but only after the write.

Reject such descriptors at probe time.  Genuine UA-101/UA-1000 hardware
declares proportional packet sizes and is unaffected.

  BUG: KASAN: slab-out-of-bounds in start_usb_playback (sound/usb/misc/ua101.c:586)
  Write of size 2048 at addr ffff8881098f3c00 by task exploit/5021
  Call Trace:
   __asan_memset (mm/kasan/shadow.c:84)
   start_usb_playback (sound/usb/misc/ua101.c:586)
   playback_pcm_open (sound/usb/misc/ua101.c:679)
   snd_pcm_open_substream (sound/core/pcm_native.c:2829)
   snd_pcm_open (sound/core/pcm_native.c:2865 sound/core/pcm_native.c:2932)
   snd_pcm_playback_open (sound/core/pcm_native.c:2891)
   snd_open (sound/core/sound.c:166)
   chrdev_open (fs/char_dev.c:411)
   do_dentry_open (fs/open.c:996)
   vfs_open (fs/open.c:1101)
   path_openat (fs/namei.c:4837 fs/namei.c:5000)
   do_file_open (fs/namei.c:5029)
   do_sys_openat2 (fs/open.c:1417)
   __x64_sys_openat (fs/open.c:1423 fs/open.c:1439 fs/open.c:1434)
   do_syscall_64 (arch/x86/entry/syscall_64.c:61 arch/x86/entry/syscall_64.c:84)
   entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)

  The buggy address belongs to the object at ffff8881098f3c00
   which belongs to the cache kmalloc-192 of size 192
  The buggy address is located 0 bytes inside of
   allocated 168-byte region [ffff8881098f3c00, ffff8881098f3ca8)

Cc: stable@vger.kernel.org
Fixes: 63978ab ("sound: add Edirol UA-101 support")
Reported-by: <co+24304d323d28f156@bugs.sh>
Assisted-by: LLM
Signed-off-by: Xiang Mei <xmei5@asu.edu>
Link: https://patch.msgid.link/20260918225753.1278505-1-xmei5@asu.edu
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  sound/usb/misc/ua101.c
  mm/kasan/shadow.c
  sound/core/pcm_native.c
  sound/core/sound.c
  fs/char_dev.c
  fs/open.c
  fs/namei.c
  arch/x86/entry/syscall_64.c
  arch/x86/entry/entry_64.S

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=fd3a7bf0d988b0878a1957df150edb35f81db87e

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

The original Linux kernel CVE announcement for CVE-2026-98741 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98741

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:P/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:P/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 6.6

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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).