[CVE-2026-89763][MODERATE REGULAR] KEYS: trusted: Fix TPM teardown ordering

[CVE-2026-89763][MODERATE REGULAR] KEYS: trusted: Fix TPM teardown ordering

[AL-KERNEL]

CVE: CVE-2026-89763
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: KEYS: trusted: Fix TPM teardown ordering
Commit: 753c978f2400f9783eb524842a975d3ac950d511
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=753c978f2400f9783eb524842a975d3ac950d511
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89763
Analysis date: Sat, 12 Sep 2026 02:54:13 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89763 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89763

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 REGULAR
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-2026-89763	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89763 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:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A use-after-free can occur in the trusted TPM key backend because trusted_tpm_exit can free the digest array and drop the TPM chip reference before unregistering the trusted key type. A concurrent trusted key operation that already passed key_type_lookup can continue into trusted_tpm_seal and pcrlock, then call tpm_pcr_extend with freed backend state. KASAN confirms a slab-use-after-free read in tpm_pcr_extend through the add_key path. For the CVSS the PR:L is used for the paranoid scenario because the racing key operation can be initiated by a local user through add_key when trusted keys are available, while teardown may be reachable in systems that delegate module or service lifecycle control to non full administrators. The issue is not network reachable and requires local interaction with the kernel key management path. Impact is at least local denial of service via kernel crash and in the worst case may allow confidentiality or integrity impact due to kernel UAF conditions, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	YES	READ DANGER INIT UAF HARDWARE LINUS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	NO	NO	checked

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

ActionableScore=1
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=YES
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 1
* Paranoid score: 1
* Final recommended bucket: **Ordinary Moderate**::

No floor or cap changed the score. REAL UAF FLOOR 4 does not apply because host-admin-equivalent module teardown is positively established by the delete_module path.

2. Signal breakdown

Positive signals:

* +2 Real UAF / lifetime violation. trusted_tpm_exit() can free digests and drop the TPM chip reference before unregister_key_type() waits for in-flight trusted key operations.
* +1 Reliable kernel crash / strong DoS. KASAN reports slab-use-after-free in tpm_pcr_extend() through trusted_tpm_seal() and add_key().

Negative signals:

* -2 Host-admin-equivalent trigger requirement. The freeing side is trusted_tpm_exit() reached from __do_sys_delete_module(), which normally requires host CAP_SYS_MODULE or equivalent host-admin control.

Important non-awarded signals:

* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Confidentiality and integrity impact not awarded because the established stale use is a freed digest-array read, not a demonstrated disclosure or write primitive.
* Local unprivileged trigger not awarded because add_key() can start the key operation, but the complete vulnerable interleaving also requires trusted TPM module teardown.
* Remote reachability not established.
* Structured stale-object exploitation leverage not awarded because no attacker-shaped replacement object or callback/ops consumption is shown.
* Concrete repeated release/refcount signal not awarded. The visible confirmed issue is kfree(digests) followed by stale dereference, not a second lifetime/refcount mutation on the same dangling reference.
* Weak LPE not awarded solely from UAF class.
* Extra Availability not awarded because the same crash is already counted.
* Race penalty not applied separately. The bug is a teardown race, but the supplied KASAN trace and explicit interleaving establish the primitive without proving a uniquely hard or externally uncontrollable timing condition.

Repeated release/refcount checklist:

* First release can end lifetime: yes for kfree(digests), and put_device() may drop the chip reference.
* Exact same reference survives: yes for digests as backend global state used by in-flight operation.
* Second operation is a real lifetime/refcount mutation: no, the demonstrated second operation is dereference/read in tpm_pcr_extend().
* +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: no.

3. Reachability analysis

The confirmed freeing path is module teardown via trusted_tpm_exit(), with the trace showing __do_sys_delete_module(). Reliable self-triggering therefore requires host-admin-equivalent module lifecycle control, normally CAP_SYS_MODULE.

The concurrent use path is a trusted key operation through add_key(), trusted_instantiate(), trusted_tpm_seal(), pcrlock(), and tpm_pcr_extend(). A local user may be able to initiate the key operation when the trusted key type is available, but that alone does not complete the trigger because the backend teardown must occur concurrently.

The complete trigger is not namespace-scoped. Module unload is not an attacker-created user namespace capability. No network trigger is shown. A TPM-backed trusted key configuration and unloadable trusted module path are relevant prerequisites. The race is a teardown ordering race between unregistering the key type and freeing backend state.

4. Primitive and severity interpretation

The exact primitive is a real kernel use-after-free of trusted TPM backend state. The freed object confirmed by KASAN is the digest array, later read by tpm_pcr_extend(). The TPM chip reference may also be dropped too early, but the supplied trace concretely establishes the digest-array UAF.

This is CLASS_B because it is a real kernel lifetime violation, but no controlled reclaim, attacker-controlled replacement, directed write, callback control, information disclosure, or demonstrated privilege escalation is established.

This is not an ordinary stale cleanup dereference in the user path: the stale dereference occurs during an active trusted key operation. However, the freeing side is host-admin module teardown, and the demonstrated consequence is crash/DoS.

ImportantCandidate=NO because the established primitive is constrained by host-admin teardown and lacks meaningful unresolved exploitation leverage beyond the UAF read/crash. AutoDowngradeAllowed=YES because no Class A primitive, repeated lifetime mutation, security-boundary bypass, COW violation, or controlled corruption is established.

Likely operational severity is ordinary Moderate or Low-like for security triage, despite being a real UAF, because the complete self-trigger requires host-admin-equivalent teardown control.

5. One-sentence report phrase

A teardown ordering bug in trusted TPM keys can let an in-flight trusted key operation dereference freed TPM backend digest state after module unload begins, causing a local kernel crash, but reliable self-triggering requires host-admin-equivalent module teardown control.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is a confirmed kernel UAF in trusted key backend state, but the currently established complete trigger requires host-admin module teardown and no non-crash exploitation primitive is shown.

7. Call-site confidence

Call-site confidence: high

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

Patch: KEYS: trusted: Fix TPM teardown ordering
Commit: 753c978f2400f9783eb524842a975d3ac950d511
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=753c978f2400f9783eb524842a975d3ac950d511

Commit description:

trusted_tpm_exit() drops the TPM chip reference and frees the digest
array before unregistering the trusted key type. key_type_lookup()
holds key_types_sem for reading until the key operation finishes, while
unregister_key_type() takes it for writing. It therefore provides the
synchronization point that must precede backend teardown.

The current order permits this interleaving:

  CPU 0                              CPU 1
  trusted_tpm_exit()                 key_type_lookup("trusted")
    put_device(&chip->dev)             trusted_tpm_seal()
    kfree(digests)                       pcrlock()
    unregister_key_type()                  tpm_pcr_extend(..., digests)

CPU 1 can consequently dereference the freed digest array. The chip can
also be released before callbacks stop using it.

KASAN reported:

  BUG: KASAN: slab-use-after-free in tpm_pcr_extend+0x1f0/0x200
  Read of size 2 at addr ffff88810872d000 by task poc/89
  Call Trace:
    tpm_pcr_extend+0x1f0/0x200
    pcrlock+0x42/0x70 [trusted]
    trusted_tpm_seal+0x1b6/0x570 [trusted]
    trusted_instantiate+0x293/0x340 [trusted]
    __key_instantiate_and_link+0xb2/0x2b0
    __key_create_or_update+0x61e/0xb50
    __do_sys_add_key+0x1b8/0x310
  Allocated by task 88:
    __kmalloc_noprof+0x1a7/0x490
    do_one_initcall+0xa1/0x390
    do_init_module+0x2df/0x840
  Freed by task 90:
    kfree+0x131/0x3c0
    trusted_tpm_exit+0x59/0xa0 [trusted]
    __do_sys_delete_module+0x346/0x510

Move unregister_key_type() before releasing either resource. This stops
new lookups and waits for in-flight key operations to finish before the
backend state is destroyed.

Fixes: 0b6cf6b ("tpm: pass an array of tpm_extend_digest structures to tpm_pcr_extend()")
Cc: stable@vger.kernel.org
Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
Link: https://lore.kernel.org/r/20260731140925.2973492-1-nicoyip.dev@gmail.com
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Tested-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  security/keys/trusted-keys/trusted_tpm1.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=753c978f2400f9783eb524842a975d3ac950d511

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

The original Linux kernel CVE announcement for CVE-2026-89763 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89763

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:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 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: MODERATE
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).