[CVE-2026-99151][LOW] drm/amd/display: Fix NULL sink dereference in USB4 bandwidth functions

[CVE-2026-99151][LOW] drm/amd/display: Fix NULL sink dereference in USB4 bandwidth functions

[AL-KERNEL]

CVE: CVE-2026-99151
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: drm/amd/display: Fix NULL sink dereference in USB4 bandwidth functions
Commit: 4c8b5e5b9776ad08138d79b14238c5742b32911b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4c8b5e5b9776ad08138d79b14238c5742b32911b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99151
Analysis date: Wed, 28 Oct 2026 23:34:13 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
A missing production NULL check in AMD USB4 display bandwidth handling can fault the kernel during a sinkless DPMS transition, with availability impact only.

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

The original Linux kernel CVE announcement for CVE-2026-99151 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99151

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: LOW
Manual review required: NO

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

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

CVE-2026-99151	LOW	https://www.kernelcve.org/list/?q=CVE-2026-99151 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT LOW AEGIS_AS=1 LEGACY=LOW RESULT=SAME (final reconciliation agrees; DECISION: both LOW)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';**CWE-476;CWE-754;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'A NULL sink can reach USB4 bandwidth allocation or deallocation paths after earlier DPMS code already recognized that stream sink may legitimately be absent. Production builds remove ASSERT diagnostics, so dereferencing stream sink link can cause a kernel NULL pointer fault during a USB4 display state transition. For the CVSS the PR:L value reflects that reliable triggering generally requires a local user or process able to control DRM display state. AC:H reflects the required USB4 DP tunnel and missing sink lifecycle condition. The demonstrated primitive is an invalid NULL read with availability impact only. Host wide panic depends on kernel oops policy, and there is no evidence of UAF, OOB access, information disclosure, or write corruption. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 1 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. No severity changing production threshold or specialist override was recorded. 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. The independent weaker Aegis shadow evaluated CVSS 3.1 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was LOW. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	MAYBE	NULLPTR SIMPLEFIX LEAK HARDWARE 	-	-	checked

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

ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 1.
* Conservative score: 1; no floor or cap applied.
* Raw paranoid score: 1.
* Paranoid score: 1; unsupported exploit effects were not added.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Reliable kernel crash/strong DoS: production builds remove `ASSERT()`, allowing `stream->sink->link` to dereference a legitimate NULL sink and cause a kernel fault.
* +0 Weak corrupt concern: this is an invalid NULL read, not a write or useful corruption primitive.
* No extra Availability point: host panic depends on oops policy and is the same fault already counted.
* No local-unprivileged bonus: the evidence does not establish that a normal user can perform the complete USB4 DPMS transition.
* No race penalty: a special sink-lifecycle state is required, but no narrow or uncontrollable race is shown.
* No UAF, OOB access, retry-accumulated write, shared-backing write, authorization bypass, disclosure, or integrity impact is shown.

3. Reachability analysis

The vulnerable operation requires AMD display hardware, USB4 DP tunneling, bandwidth allocation state, and a stream whose sink has become NULL. A DRM master, compositor, or delegated display service may plausibly exercise DPMS, but the supplied evidence does not establish which actor can create the complete state. A normal logged-in user is not proven to have direct access, while host-administrator privilege is also not proven necessary. No namespace-scoped path, prior privileged setup requirement, or timing race is established.

4. Primitive and severity reading

The exact primitive is a NULL pointer dereference through `stream->sink->link` in allocation or deallocation processing. It is neither stale storage nor an active-object UAF, and there is no lifetime mutation or corrupted-state consumption. The patch-supported effect is an Oops and possible host availability loss; stronger memory corruption, confidentiality, integrity, or privilege effects are unsupported. This is CLASS_C, so it is not an Important candidate and may be downgraded automatically.

5. One-sentence report phrase

A missing production NULL check in AMD USB4 display bandwidth handling can fault the kernel during a sinkless DPMS transition, with availability impact only.

6. Manual review advice

NO MANUAL CHECK NEEDED

The patch shows a bounded NULL-dereference crash path without a separate corruption or security-boundary primitive.

7. Call-site confidence

Call-site confidence: high

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

Patch: drm/amd/display: Fix NULL sink dereference in USB4 bandwidth functions
Commit: 4c8b5e5b9776ad08138d79b14238c5742b32911b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4c8b5e5b9776ad08138d79b14238c5742b32911b

Commit description:

allocate_usb4_bandwidth_for_stream() and allocate_usb4_bandwidth() both
use ASSERT(stream->sink) before dereferencing stream->sink->link. ASSERT
is compiled out in production builds, so a NULL stream->sink causes a
NULL pointer dereference.

link_set_dpms_off() already guards two earlier accesses to stream->sink
with explicit NULL checks, confirming that stream->sink can legitimately
be NULL. The subsequent call to deallocate_usb4_bandwidth() had no
equivalent guard.

Replace the ASSERT() calls with proper NULL checks that return false
early, and add a matching stream->sink guard at the call site in
link_set_dpms_off() consistent with the surrounding pattern.

v2: Keep the existing ASSERT() calls for diagnostics (to report
    unexpected NULL sink), and add explicit NULL checks that return false
    before dereferencing stream->sink. Also add a matching stream->sink
    guard at the deallocation call site in link_set_dpms_off(). (Alex)

Fixes: 8fa813b ("drm/amd/display: Remove sink usage from DPMS")
Reported-by: Dan Carpenter <error27@gmail.com>
Cc: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Cc: Dominik Kaszewski <dominik.kaszewski@amd.com>
Cc: George Zhang <george.zhang@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Roman Li <Roman.Li@amd.com>
Cc: Ivan Lipski <ivan.lipski@amd.com>
Cc: Dan Wheeler <daniel.wheeler@amd.com>
Cc: Alex Hung <alex.hung@amd.com>
Cc: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Roman Li <roman.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 98029a0ce61feb52c563f65349dd9b25f672e355)
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/gpu/drm/amd/display/dc/link/link_dpms.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=4c8b5e5b9776ad08138d79b14238c5742b32911b

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

The original Linux kernel CVE announcement for CVE-2026-99151 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99151

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:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 1

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).