[CVE-2026-89891][MODERATE 7.0] media: em28xx: fix use-after-free of dev_next->devlist on disconnect

[CVE-2026-89891][MODERATE 7.0] media: em28xx: fix use-after-free of dev_next->devlist on disconnect

[AL-KERNEL]

CVE: CVE-2026-89891
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: media: em28xx: fix use-after-free of dev_next->devlist on disconnect
Commit: f61720346fdcc1a2deec59a2f6cbb0cc20d1dc18
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f61720346fdcc1a2deec59a2f6cbb0cc20d1dc18
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89891
Analysis date: Wed, 16 Sep 2026 11:08:22 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A malicious USB device can trigger a real use-after-free in the em28xx driver by leaving `dev_next->devlist` linked in the global device list after disconnect, causing a later probe to hit freed list state and crash the kernel.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-89891 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89891

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-89891	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89891 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:P/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-416;*CWE-664;CWE-362;Other CVSS 'AV:P/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.4';DESCR 'A use after free can occur in the em28xx USB media driver because dev_next devlist remains linked in the global em28xx_devlist after device disconnect. After dev_next is freed, the global list can contain a dangling list entry and a later device probe can trigger list corruption or traversal of freed memory. For the CVSS the PR:N value is used for the paranoid score because a malicious USB device can trigger the vulnerable driver path without a user account on the target host. The attack is not network reachable and normally requires physical USB access or an equivalent USB device emulation path. Impact is at least denial of service via kernel crash. In the worst still defensible interpretation, the dangling list entry is a kernel UAF condition that may allow confidentiality or integrity impact if freed memory is reclaimed in an attacker influenced way.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	YES	DANGER USB UAF HARDWARE LINUS SYZBOT KPANIC 	NO	NO	checked

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

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

## 1. ActionableScore

* Raw conservative score: 2
* Conservative score: 4
* Applied conservative floor/cap: Real UAF floor 4 applied because a freed `dev_next` object leaves a dangling `devlist` entry in a live global list and host-admin privilege is not required.
* Raw paranoid score: 3
* Paranoid score: 5
* Applied paranoid floor/cap: Real UAF floor 5 applied because the attacker can plausibly drive the USB probe/disconnect/reprobe lifecycle and the stale object remains in an active global list rather than only causing a local cleanup-time dereference.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

## 2. Signal breakdown

Conservative signals:

* +2 Real UAF / lifetime violation. `dev_next->devlist` remains linked in `em28xx_devlist` after `dev_next` is freed via `kref_put()`.
* +1 Reliable kernel crash / strong DoS. The next probe can trigger a list corruption BUG when `list_add_tail()` detects the freed list node.
* +1 Firmware/device-mediated external influence. The trigger depends on malicious or fuzzed USB descriptors reaching the em28xx probe path.
* -2 Physical-only / device access. Practical attack requires physical USB access or an equivalent USB emulation path, not a network path.

Paranoid additional signal:

* +1 Attacker-influenced repeatable lifetime manipulation. A malicious USB device can plausibly repeat the probe, disconnect, and later probe sequence that creates and consumes the dangling global-list entry.

Important non-awarded signals:

* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Confidentiality and integrity are not awarded as direct score signals because no disclosure path, directed write, callback hijack, or controlled replacement object is established.
* Structured stale-object leverage is not separately awarded because the visible stale consumption is mainly global list corruption and BUG detection, not proven attacker-shaped object reuse.
* Concrete repeated release/refcount signal is not awarded. The bug involves a dangling list entry after `kref_put()`, but the patch does not establish a second lifetime/refcount mutation on the exact same freed reference.
* Extra Availability is not awarded because the same kernel crash is already counted.
* Remote network reachability is not established.

Repeated-release/refcount checklist:

* First release can end lifetime: yes, `kref_put()` can free `dev_next`.
* Exact same reference survives to a second lifetime/refcount operation: no evidence.
* Second operation is a real lifetime/refcount mutation: no.
* +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: yes, but due active global-list UAF plus attacker-driven lifecycle in the paranoid interpretation, not due repeated refcount mutation.

## 3. Reachability analysis

The plausible attacker is a malicious USB device or a local actor able to emulate such a USB device to the host. The attacker does not need a local user account or host administrator credentials if physical USB attachment is possible, so `HostAdminRequired=NO`.

The complete trigger is not namespace-scoped. It is a hardware or USB-emulation path, not a user or network namespace capability path, so `NamespaceScopedPrivilege=NO`.

No direct network trigger is supported. The vulnerable path is reached through USB device probing, endpoint parsing, disconnect, and later probing. Prior host setup is limited to having the em28xx driver available and bindable to the presented device. The path was exposed by USB endpoint normalization that can make fuzzed descriptors enter the `is_audio_only + has_dual_ts` path.

The race/timing requirement is not the main issue. The relevant sequence is ordered lifecycle behavior: probe adds both list entries, disconnect removes only one, `dev_next` is freed, and a later probe consumes the stale global-list entry.

## 4. Primitive and severity interpretation

The exact primitive is a real kernel use-after-free / dangling list entry. `dev_next` is freed while `dev_next->devlist` remains linked in the global `em28xx_devlist`. A later probe can operate on that global list and encounter freed memory.

This is an active structured-object lifetime issue rather than fixed stale storage. The stale object is not merely a dead buffer touched once by hardware. It is a list node retained in a live global kernel list used by later driver operations.

The demonstrated consequence is kernel crash / DoS via list corruption BUG. The speculative consequence is stronger post-free list corruption if the freed memory is reclaimed in a useful way, but controlled reclaim or attacker-shaped replacement contents are not shown.

This is `CLASS_B`: a real kernel lifetime/corruption primitive needing exploitation-leverage review, but not `CLASS_A` because arbitrary read/write, controlled reclaim, callback control, refcount takeover, or demonstrated LPE are not established.

`ImportantCandidate=YES` because an active global-list UAF with attacker-driven device lifecycle should not be automatically reduced to an ordinary crash-only bug without manual review. `AutoDowngradeAllowed=NO` because automatic closure would discard unresolved post-lifetime global-list corruption leverage.

Likely operational severity is Actionable Moderate. Strong Important remains possible only if further analysis shows controlled reclaim, useful list manipulation, or another non-crash exploitation primitive.

## 5. One-sentence report phrase

A malicious USB device can trigger a real use-after-free in the em28xx driver by leaving `dev_next->devlist` linked in the global device list after disconnect, causing a later probe to hit freed list state and crash the kernel.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a real kernel UAF involving a dangling entry in a live global list, and the visible crash-only symptom does not fully resolve whether post-free list corruption has stronger exploitation leverage.

## 7. Call-site confidence

Call-site confidence: high

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

Patch: media: em28xx: fix use-after-free of dev_next->devlist on disconnect
Commit: f61720346fdcc1a2deec59a2f6cbb0cc20d1dc18
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f61720346fdcc1a2deec59a2f6cbb0cc20d1dc18

Commit description:

When a device with has_dual_ts=1 is probed and the is_audio_only path
is taken, both dev and dev->dev_next are added to the global
em28xx_devlist via em28xx_init_extension(). However, during disconnect,
em28xx_close_extension(dev) only calls list_del(&dev->devlist), leaving
dev->dev_next->devlist still linked in the global list. When dev_next is
subsequently freed via kref_put(), its devlist entry becomes a dangling
pointer in em28xx_devlist. The next device probe that calls
em28xx_init_extension() triggers a list corruption BUG when list_add_tail
detects the freed node.

This bug was exposed by commit a368ecd ("USB: core: Fix duplicate
endpoint bug by clearing reserved bits in the descriptor") which clears
reserved bits in bEndpointAddress during endpoint parsing. This causes
fuzzed endpoint addresses like 0xf3 to be normalized to 0x83, which
em28xx interprets as a vendor audio endpoint, enabling the
is_audio_only + has_dual_ts code path that was previously unreachable
with such descriptors.

Fix this by removing dev->dev_next->devlist from the global list in
em28xx_close_extension() before the device is freed.

Fixes: f410b40 ("media: em28xx: split up em28xx_dvb_init to reduce stack size")
Cc: stable@vger.kernel.org
Reported-by: syzbot+99d6c66dbbc484f50e1c@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=99d6c66dbbc484f50e1c
Signed-off-by: Jiangong.Han <jiangong.han@windriver.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/media/usb/em28xx/em28xx-core.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=f61720346fdcc1a2deec59a2f6cbb0cc20d1dc18

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

The original Linux kernel CVE announcement for CVE-2026-89891 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89891

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:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:P/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 6.4

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).