* [CVE-2026-90336][MODERATE 7.0] serial: core: clear freed pointers on uart_register_driver() failure [ Upstream
@ 2026-09-18 12:44 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-18 12:44 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-90336
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: serial: core: clear freed pointers on uart_register_driver() failure [ Upstream
Commit: 06b376432974fe5f25f2843a8e8438c6a1cf3d50
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=06b376432974fe5f25f2843a8e8438c6a1cf3d50
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90336
Analysis date: Fri, 18 Sep 2026 08:44:58 -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-2026-90336 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90336
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-2026-90336 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-90336 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:H/UI:N/S:U/C:H/I:H/A:H';*CWE-416;*CWE-672;CWE-703;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '6.4';DESCR 'A failed uart_register_driver call can leave drv state or drv tty_driver pointing to released memory. A later device probe may treat the non NULL state pointer as evidence that registration already succeeded and can pass the freed state to uart_add_one_port. This creates a use-after-free condition with kernel crash and memory corruption risk. For the CVSS the PR:H is selected because reliable triggering normally requires fault injection or administrative control over serial device unbinding and rebinding. Attack complexity is high because registration must first fail and the affected device must then be probed again while the stale pointer remains. The issue is local and is not network reachable. Impact is at least a local denial of service via kernel crash and in the worst case may include confidentiality and integrity impact or privilege escalation due to reuse of the freed kernel memory.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) SKIP CVE-2026-90336 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES NO NO unknown MAYBE NULLPTR PRINTF-TTY UAF LINUS KPANIC KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS KPANIC LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS_AND_PRH - - 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
* Applied conservative floor: 3 -> 4 under REAL UAF FLOOR 4 because freed structured state is passed to a later active user and host-admin-equivalent privilege is not established as universally required.
* Raw paranoid score: 4
* Paranoid score: 5
* Applied paranoid floor: 4 -> 5 because the demonstrated failure and reprobe sequence can repeatedly drive the lifetime transition, while the stale state enters nontrivial active processing in `uart_add_one_port()` rather than a shallow cleanup dereference.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
Conservative signals:
* +2 Real UAF. `drv->state` is freed but remains non-NULL, allowing a later probe to pass the dangling state to `uart_add_one_port()`.
* +1 Reliable kernel crash or strong DoS. Use of freed UART state during port registration can realistically produce an invalid access or kernel fault.
* No privilege penalty. Failslab and manual device rebinding are privileged in the demonstrated reproducer, but the supplied evidence does not establish that every vulnerable failure and reprobe sequence requires host-admin action.
* Raw conservative total: 2 + 1 = 3.
* FLOOR 4 applied because this is an active structured UAF and not clearly fixed stale storage with immediate crash-only use.
Paranoid-only signal:
* +1 Attacker-influenced repeatable lifetime sequence. A privileged operator can reproduce allocation failure and bind the PL011 port again, repeatedly driving free followed by stale structured use. This is counted only in the paranoid interpretation because below-host-admin control and useful heap placement are not established.
* Raw paranoid total: 2 + 1 + 1 = 4.
* FLOOR 5 applied because the repeated transition feeds freed state into nontrivial port initialization rather than only into cleanup.
Important non-awarded signals:
* Controlled reclaim or attacker-shaped replacement contents are not demonstrated.
* Arbitrary or directed kernel read/write is not demonstrated.
* Structured stale-object leverage is not separately awarded because attacker control over replacement contents or security-sensitive fields is unproven.
* Strong LPE leverage is not awarded merely because this is a UAF.
* Confidentiality and integrity points are not awarded from the paranoid CVSS assumptions alone.
* Remote reachability is not established.
* No race penalty applies because this is an ordered failure and reprobe sequence, not a demonstrated narrow timing race.
* No extra Availability point is awarded because it would double-count the same kernel fault.
* No concrete repeated release/refcount point is awarded. The first operation is `kfree(drv->state)`, but the later established operation is active stale use through `uart_add_one_port()`, not a second `free`, `put`, or refcount mutation on the exact reference.
* Paranoid repeated-refcount leverage is therefore withheld.
* The repeated-release-specific FLOOR 5 rule does not apply. The paranoid floor instead follows repeatable lifetime transition plus nontrivial active processing.
3. Reachability analysis
The demonstrated reproducer uses failslab to force registration failure and then rebinds the PL011 platform device. Those particular operations normally require host-level administrative or kernel-debug control and are not namespace-scoped.
However, the root defect can also arise whenever `tty_alloc_driver()` or `tty_register_driver()` fails and a later probe retries registration while the stale pointer remains. The supplied evidence does not establish whether every practical occurrence requires an attacker to control failslab or device binding. Therefore `HostAdminRequired=UNKNOWN` rather than YES.
The relevant serial platform-driver registration, failslab control, and device rebinding are not operations granted through an attacker-created user or network namespace. Therefore `NamespaceScopedPrivilege=NO`.
No network trigger, remote protocol, special physical serial input, or timing race is established. QEMU raspi1ap and PL011 demonstrate the bug, but the defect is in generic serial core failure handling and is not shown to require rare physical hardware.
4. Primitive and severity interpretation
The exact primitive is a dangling pointer to freed `struct uart_state` storage. The object is active structured state rather than a fixed transfer buffer. A later probe can interpret the non-NULL pointer as successful prior registration and pass the freed state into `uart_add_one_port()`, where normal serial-core semantics involve active port initialization and state linkage.
This is broader than an ordinary cleanup-only stale dereference. It is nevertheless CLASS_B rather than CLASS_A because controlled reclaim, attacker-shaped replacement, callback control, arbitrary kernel access, and privilege escalation are not established.
There is no concrete second release or refcount operation. The sequence is:
A. free the state while retaining its pointer.
B. consume that potentially dangling structured state during a later port-add operation.
The demonstrated practical consequence is kernel crash or memory corruption during reprobe. Privilege escalation remains speculative. The active structured use and unresolved behavior after allocator reuse justify `ImportantCandidate=YES`, required manual review, and `AutoDowngradeAllowed=NO`. The likely operational classification is Actionable Moderate, with Important-grade potential requiring source-level review of all accesses made by `uart_add_one_port()` through the stale state.
5. One-sentence report phrase
A failed UART driver registration can leave freed state referenced by the driver and later pass that dangling structured object into port initialization, creating a local use-after-free with kernel crash and unresolved memory-corruption potential.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Review `uart_add_one_port()` and its downstream operations to determine which fields are read or written through the stale `uart_state`, whether allocator reuse can be influenced, and whether the bug is limited to a fault or provides a useful post-free corruption primitive.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: serial: core: clear freed pointers on uart_register_driver() failure [ Upstream
Commit: 06b376432974fe5f25f2843a8e8438c6a1cf3d50
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=06b376432974fe5f25f2843a8e8438c6a1cf3d50
Changed files:
drivers/tty/serial/serial_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=06b376432974fe5f25f2843a8e8438c6a1cf3d50
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-90336 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90336
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:L/A:H
The Best / paranoid CVSS vector: AV:L/AC:H/PR:H/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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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