[CVE-2026-72151][MODERATE 7.0] tpm: tpm2-sessions: wait for async KPP completion in tpm_buf_append_salt

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-72151
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: tpm: tpm2-sessions: wait for async KPP completion in tpm_buf_append_salt
Commit: 111e520efbe82b324bc42b1999b723c0619eea6d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=111e520efbe82b324bc42b1999b723c0619eea6d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72151
Analysis date: Sat, 15 Aug 2026 21:25:51 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A TPM2 session salt generation bug can free a KPP request before asynchronous ECDH completion finishes, allowing a deferred crypto worker to dereference freed kernel memory and potentially crash the system, with worst-case UAF impact requiring manual review.

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

The original Linux kernel CVE announcement for CVE-2026-72151 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72151

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-2026-72151	MODERATE	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-252;CWE-362;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A use-after-free can occur in the TPM2 session salt generation path because tpm_buf_append_salt starts KPP public key generation and shared secret computation without waiting for asynchronous completion. When an asynchronous ecdh-nist-p256 backend returns EINPROGRESS, the kpp_request can be freed before the deferred crypto worker finishes and later dereferenced by that worker. For the CVSS the PR:L value is used for the paranoid score because reliable attacker driven triggering would normally require local code or a local process context, even though the hwrng kernel thread may reach the path automatically during entropy polling. The issue is not network reachable and depends on TPM2 session use with an asynchronous KPP backend. Impact is at least a local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to kernel use-after-free memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)  SKIP CVE-2026-72151 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 1085b8276bb4239daa7008f0dcd5c973e4bd690f YES  NO NO unknown 	MAYBE	INIT NOMEMCHK UAF LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=6
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: **5**
* Paranoid score: **6**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* **Memory corruption, strong corruption primitive: +2**
  The bug is a real use-after-free: `kpp_request_free()` can run while an asynchronous KPP backend still owns and later dereferences the request.

* **Real lifetime corruption: +1**
  This is an async completion lifetime mismatch: the request lifetime ends before deferred crypto completion.

* **Reliable kernel crash / strong DoS: +1**
  A deferred worker dereferencing a freed `kpp_request` is a credible kernel crash path.

* **Weak LPE concern: +1**
  UAF in kernel heap memory can theoretically become more than DoS, but the patch does not show attacker-controlled reclaim, object replacement, arbitrary write, or callback control.

* **Rare AND difficult-to-reach subsystem/configuration: -1**
  The issue requires an asynchronous `ecdh-nist-p256` KPP backend such as `atmel-ecc`, `HPRE`, or `keembay-ocs`, not the common synchronous software backend.

Paranoid additional signal:

* **Local unprivileged trigger: +1**
  The path may fire automatically from `hwrng_fillfn` via TPM random polling, and local activity may plausibly increase pressure on RNG paths without needing administrative control. This is still not a remote trigger.

## 3. Reachability analysis

The affected path is local and kernel-internal, not network reachable. It is reached through TPM2 session salt generation via `tpm_get_random -> tpm2_get_random -> tpm2_start_auth_session -> tpm_buf_append_salt`.

The strongest trigger condition is not ordinary user input but system behavior using TPM RNG and an asynchronous KPP backend. Namespaces and containers do not obviously expose a direct trigger, although local unprivileged activity may indirectly increase entropy or random polling in some deployments.

The bug is configuration dependent. Systems using synchronous ECDH backends are not expected to hit the UAF. Systems with supported asynchronous crypto hardware backends have a more actionable risk.

## 4. Severity interpretation

This is not an ordinary Moderate cleanup issue because the patch fixes a concrete kernel use-after-free. Realistic impact is most clearly local DoS through kernel crash. Privilege escalation is plausible only as a paranoid concern because the primitive is UAF, but exploitability is not demonstrated and attacker control over reclaim or replacement is unclear.

Overall this fits **Actionable Moderate at minimum**, with **Strong Important candidate** handling justified for triage because kernel lifetime bugs in async completion paths are historically under-scored.

## 5. One-sentence report phrase

A TPM2 session salt generation bug can free a KPP request before asynchronous ECDH completion finishes, allowing a deferred crypto worker to dereference freed kernel memory and potentially crash the system, with worst-case UAF impact requiring manual review.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: this is a real async use-after-free in kernel crypto and TPM code. Even though reliable LPE is not demonstrated, the memory lifetime primitive is strong enough that it should not be auto-closed.

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

Patch: tpm: tpm2-sessions: wait for async KPP completion in tpm_buf_append_salt
Commit: 111e520efbe82b324bc42b1999b723c0619eea6d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=111e520efbe82b324bc42b1999b723c0619eea6d

Commit description:

tpm_buf_append_salt() in drivers/char/tpm/tpm2-sessions.c calls
crypto_kpp_generate_public_key() and crypto_kpp_compute_shared_secret()
without installing a completion callback, discards both return values,
and immediately frees the kpp_request via kpp_request_free(). When the
resolved ecdh-nist-p256 KPP backend is asynchronous (atmel-ecc, HPRE,
keembay-ocs), either operation returns -EINPROGRESS and the deferred
completion worker dereferences the freed request.

The path fires automatically from the hwrng_fillfn kernel thread via
tpm_get_random -> tpm2_get_random -> tpm2_start_auth_session ->
tpm_buf_append_salt on every entropy poll, without any userland action.

Install crypto_req_done as the completion callback, wrap both KPP
operations in crypto_wait_req(), and propagate errors to the caller.
The wait is a no-op for synchronous backends.

Fixes: 1085b82 ("tpm: Add the rest of the session HMAC API")
Cc: stable@vger.kernel.org # v6.10+
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Assisted-by: Claude:claude-opus-4-7
Reviewed-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:
  drivers/char/tpm/tpm2-sessions.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=111e520efbe82b324bc42b1999b723c0619eea6d

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

The original Linux kernel CVE announcement for CVE-2026-72151 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72151

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: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: 5
  Paranoid ActionableScore: 6

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).