[CVE-2026-72135][MODERATE 7.0] tpm: Make the TPM character devices non-seekable

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

CVE: CVE-2026-72135
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: tpm: Make the TPM character devices non-seekable
Commit: ed0ffc2c016629e40ba041ed0424a772d8b02e2c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ed0ffc2c016629e40ba041ed0424a772d8b02e2c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72135
Analysis date: Sun, 16 Aug 2026 07:47:14 -0400
ActionableScore: 8
ActionableScore lower bound: 7
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A local OOB heap read and constrained OOB zero-write in the TPM character-device read path can be triggered via positional I/O on a sequential command-response interface, exposing kernel heap data and creating a plausible local privilege-escalation or kernel-crash risk.

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

The original Linux kernel CVE announcement for CVE-2026-72135 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72135

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-2026-72135	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-125;*CWE-787;CWE-823;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:H';BEST CVSS score: '7.8';DESCR 'A local out of bounds heap access exists in the TPM character device read path because positional I/O was allowed on a sequential command response interface. After a TPM command leaves a response pending, pread with a large offset can make tpm_common_read compute data_buffer plus the attacker controlled offset without validating that the offset is inside the response buffer. This can expose kernel heap data through copy_to_user and can also perform an out of bounds zero write through the following memset if the copy succeeds. For the CVSS the PR:L is used because exploitation requires local code execution and access to the TPM character device, which may be granted to non root users through device permissions or a TPM related group. The issue is not network reachable. Impact includes kernel information disclosure, kernel memory corruption, denial of service, and a plausible local privilege escalation path that should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)	YES	WRITE KASAN OOB HARDWARE LINUS KPANIC  DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=7

## 1. ActionableScore

* Conservative score: 7
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local process can trigger the issue through TPM character device access if permissions allow access to `/dev/tpm*` or `/dev/tpmrm*`.
* Constrained kernel buffer overwrite: +1. The write primitive is an out-of-bounds zero-write through `memset()`, not an arbitrary attacker-controlled payload.
* Weak LPE concern: +1. The combination of kernel heap disclosure and constrained heap corruption makes local privilege escalation plausible enough for manual review, but no reliable exploit primitive is demonstrated.
* Reliable kernel crash / strong DoS: +1. Out-of-bounds heap access in a kernel character-device path can realistically crash the kernel.
* Confidentiality impact plausible: +1. The unchecked offset can cause an out-of-bounds heap read through `copy_to_user()`.
* Integrity impact plausible: +1. The following `memset()` can perform an out-of-bounds zero-write.
* Broad/common subsystem exposure: +1. TPM character devices are common on modern systems, though device permissions vary.

Paranoid adjustment:

* Generic memory corruption, stronger interpretation: +1 additional. In the paranoid view, the paired OOB heap read plus OOB zero-write is treated as a stronger memory-corruption candidate because heap disclosure may help shape later exploitation.

## 3. Reachability analysis

The bug is local, not network reachable. Triggering requires local code execution and access to the TPM character device. This is normally not full root-equivalent access, because some systems expose TPM access to users or services through device permissions, a TPM-related group, or service-specific delegation.

Namespaces and containers may affect practical reachability only if the TPM device node is passed through or exposed into the container. Without device access, the bug is not reachable from an ordinary unprivileged process.

The affected path is not an obscure debug-only interface. TPM devices are common, but access policy is deployment-dependent.

Call-site confidence: medium-high. The patch only changes the open handlers, but the commit message gives the concrete downstream path through `tpm_common_read()`, `copy_to_user()`, and `memset()`.

## 4. Severity interpretation

This behaves more like an Important-class local kernel vulnerability than an ordinary Moderate. The realistic impact includes local kernel heap disclosure and a constrained OOB zero-write, with likely DoS and plausible LPE concern.

Theoretical exploitation is stronger than a simple crash because both read and write primitives are present. Realistic exploitation is still constrained because the write payload is zeroes and the target is offset-based rather than an obvious arbitrary write.

## 5. One-sentence report phrase

A local OOB heap read and constrained OOB zero-write in the TPM character-device read path can be triggered via positional I/O on a sequential command-response interface, exposing kernel heap data and creating a plausible local privilege-escalation or kernel-crash risk.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.

YES REQUIRES MANUAL CHECK because this is not DoS-only: the patch context explicitly supports both an OOB heap read to userspace and an OOB zero-write in kernel heap memory.

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

Patch: tpm: Make the TPM character devices non-seekable
Commit: ed0ffc2c016629e40ba041ed0424a772d8b02e2c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ed0ffc2c016629e40ba041ed0424a772d8b02e2c

Commit description:

The TPM character devices expose a sequential command/response
interface, but their open handlers leave FMODE_PREAD and FMODE_PWRITE
enabled.

After a command leaves a response pending, pread(fd, buf, 16, 0x1400)
passes 0x1400 as *off to tpm_common_read(). The transfer length is
bounded by response_length, but the offset is used unchecked when
forming data_buffer + *off. A sufficiently large offset therefore causes
an out-of-bounds heap read through copy_to_user() and, if the copy
succeeds, an out-of-bounds zero-write through the following memset().

Positional I/O does not provide coherent semantics for this interface.
An arbitrary pread offset cannot represent how much of a response has
been consumed sequentially. The write callback always stores a command
at the start of data_buffer, while pwrite() does not update file->f_pos
and can leave the sequential read cursor stale.

Call nonseekable_open() from both open handlers. This removes
FMODE_PREAD and FMODE_PWRITE, causing positional reads and writes to
fail with -ESPIPE before reaching the TPM callbacks, and explicitly
marks the files non-seekable. Normal read() and write() continue to use
the existing sequential f_pos cursor, leaving the response state machine
unchanged.

Tested on Linux 6.12 with KASAN and a swtpm TPM2 device:

 - sequential partial reads returned the complete response
 - pread() and preadv() with offset 0x1400 returned -ESPIPE
 - pwrite() and pwritev() with offset zero returned -ESPIPE
 - the pending response remained intact after the rejected operations
 - a subsequent normal command/response cycle completed normally
 - no KASAN report was produced.

Fixes: 9488585 ("tpm: add support for partial reads")
Link: https://lore.kernel.org/all/20260710090217.191289-1-yong010301@gmail.com/
Cc: stable@vger.kernel.org
Signed-off-by: Jaewon Yang <yong010301@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/char/tpm/tpm-dev.c
  drivers/char/tpm/tpmrm-dev.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=ed0ffc2c016629e40ba041ed0424a772d8b02e2c

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

The original Linux kernel CVE announcement for CVE-2026-72135 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72135

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:L/PR:L/UI:N/S:U/C:H/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 7
  Paranoid ActionableScore: 8

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).