[CVE-2026-93810][MODERATE 7.0] cachefiles: Fix double fput [ Upstream

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

CVE: CVE-2026-93810
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: cachefiles: Fix double fput [ Upstream
Commit: e9c2e68c3890a935abbd5f54dbce653f9a16836c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e9c2e68c3890a935abbd5f54dbce653f9a16836c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93810
Analysis date: Thu, 24 Sep 2026 20:51:27 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
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-2026-93810 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93810

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-2026-93810	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-93810 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:H';CWE-415;*CWE-416;*CWE-672;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.7';DESCR 'A double fput error in cachefiles_create_tmpfile can occur when the cache backend does not provide read_iter or write_iter support. The first fput is executed before the error path, while the common cleanup path releases the same file reference again. This can result in a use-after-free of struct file and at minimum can crash the kernel. For the CVSS the PR:H metric reflects that cachefiles backend setup and reliable triggering normally require administrative control over cache configuration. The issue is not directly network reachable. The realistic impact is local denial of service. The memory lifetime error provides a plausible but unproven path to confidentiality or integrity impact and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	YES	DISK ERRORPATH LINUS  KPANIC INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4	NO	NO	checked

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

ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 5
* Raw paranoid score: 5
* Paranoid score: 5
* Applied conservative floor: 4 -> 5. FLOOR 5 applies because the patch establishes a concrete first fput that can end the file lifetime, followed by a second lifetime-affecting fput on the same surviving reference through the common error cleanup path.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Positive signals:

* +2 Real UAF or equivalent lifetime violation. The first fput(file) can drop the final file reference and end the lifetime of struct file. The same local file reference remains reachable when execution continues to err_unuse, where normal cleanup performs another release of that file reference.
* +1 Concrete repeated release/refcount operation. The commit explicitly fixes a double fput in a deterministic error path. fput is a real file lifetime operation and is not idempotent.
* +1 Reliable kernel crash or strong DoS. A second fput on an object whose lifetime may already have ended can cause refcount corruption, refcount underflow diagnostics, or a stale file-object access leading to kernel fault or panic.
* +1 Paranoid repeated-refcount leverage. The second fput is a concrete write-like lifetime/refcount mutation through storage whose original struct file lifetime may have ended. This is not scored as controlled reclaim or arbitrary write.

Negative signals:

* No host-admin penalty applied. Cachefiles setup is commonly administrator-managed, but the supplied patch does not establish that the complete triggering sequence always requires host-admin-equivalent privilege.
* No race penalty applied. The vulnerable sequence is an ordered error path, not an independently demonstrated narrow timing race.
* No niche-subsystem penalty applied. Cachefiles is specialized, but the supplied evidence does not prove that an attacker cannot reach an already configured cachefiles instance.

Important non-awarded signals:

* Controlled reclaim or controlled replacement of struct file is not demonstrated.
* Arbitrary kernel read or write is not demonstrated.
* Confidentiality and integrity impact are not awarded merely because a double release could theoretically corrupt reused memory.
* Remote reachability is not established.
* Structured stale-object leverage is not separately awarded. The relevant stale operation is the second fput lifetime mutation, not a distinct post-reuse consumption of attacker-controlled file fields.
* Weak LPE is not separately awarded because it would duplicate the repeated-release fact.
* An additional availability point is not awarded because it would double-count the same potential kernel crash.

Repeated-release assessment:

* First release can end lifetime: yes. fput can release the final struct file reference.
* Exact same reference survives: yes. The removed fput does not clear file before control continues to err_unuse.
* Second operation is a real lifetime/refcount mutation: yes. The common cleanup path performs the second fput identified by the commit message.
* Concrete repeated release/refcount signal: awarded, +1.
* Paranoid repeated-refcount leverage: awarded, +1, based on the second write-like mutation through potentially post-lifetime storage.
* FLOOR 5: applied.

3. Reachability analysis

The direct trigger requires cachefiles_create_tmpfile to create a temporary cache file on a backend that lacks read_iter or write_iter support. The vulnerable behavior occurs after the file is obtained and the backend operation check fails.

Cachefiles configuration and cache backend selection are normally performed by a privileged system component. However, the supplied patch does not establish whether an ordinary local user can trigger cache population or temporary-file creation after a privileged administrator has configured the vulnerable backend. Therefore HostAdminRequired is UNKNOWN rather than YES.

No network-delivered input directly reaches this error path according to the supplied evidence. A remote filesystem workload could potentially cause cache activity only if cachefiles is already configured, but this is not sufficient to classify the issue as directly network reachable.

No namespace-specific capability checks or complete namespace-scoped triggering path are shown. NamespaceScopedPrivilege is UNKNOWN.

No special hardware, boot option, narrow race, or device-specific prerequisite is established.

4. Primitive and severity interpretation

The exact primitive is a concrete double lifetime release of struct file through fput. This is not merely an ordinary stale cleanup dereference that immediately reads a poisoned pointer. The first fput can terminate the file lifetime, while the later cleanup path performs a second lifetime/refcount mutation on the same potentially dangling reference.

The affected object is an active kernel struct file object, but no controlled object replacement, attacker-shaped post-free contents, callback dispatch, arbitrary write, information disclosure, or credential manipulation is established. The demonstrated consequence is at least a local kernel fault, refcount corruption diagnostic, or denial of service.

This is CLASS_B rather than CLASS_A. The patch establishes a real use-after-free-equivalent lifetime violation and a second concrete mutation through the stale reference, but does not establish controlled reclaim, refcount takeover, directed corruption, or privilege escalation.

ImportantCandidate is YES because the exact repeated release operation leaves meaningful post-lifetime corruption leverage unresolved. AutoDowngradeAllowed is NO because automatically treating this as an ordinary cleanup crash would discard the concrete second lifetime mutation through a potentially freed object.

Likely operational severity is Actionable Moderate. It should receive manual source-level review of the full cachefiles_create_tmpfile error cleanup path and contemporary fput and file_ref lifetime behavior before being downgraded.

5. One-sentence report phrase

A double fput in the cachefiles temporary-file error path can release the same struct file reference twice, creating a concrete post-lifetime refcount mutation that may cause a local kernel crash and requires manual exploitation review.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The patch establishes a concrete repeated lifetime operation on the same potentially freed struct file reference. Controlled reuse and exploitation are not demonstrated, but automatic downgrade would lose a meaningful unresolved kernel lifetime-corruption primitive.

7. Call-site confidence

Call-site confidence: medium

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

Patch: cachefiles: Fix double fput [ Upstream
Commit: e9c2e68c3890a935abbd5f54dbce653f9a16836c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e9c2e68c3890a935abbd5f54dbce653f9a16836c

Changed files:
  fs/cachefiles/namei.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=e9c2e68c3890a935abbd5f54dbce653f9a16836c

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

The original Linux kernel CVE announcement for CVE-2026-93810 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93810

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:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 6.7

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: 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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).