[CVE-2026-68148][MODERATE 7.0] fscrypt: Add missing superblock check in find_or_insert_direct_key()

[CVE-2026-68148][MODERATE 7.0] fscrypt: Add missing superblock check in find_or_insert_direct_key()

[AL-KERNEL]

CVE: CVE-2026-68148
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: fscrypt: Add missing superblock check in find_or_insert_direct_key()
Commit: 330249609b70778094a7a36f5b6bcfa6362121d4
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=330249609b70778094a7a36f5b6bcfa6362121d4
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68148
Analysis date: Mon, 10 Aug 2026 10:10:27 -0400
ActionableScore: 6
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A missing superblock check in fscrypt direct-key lookup can cause direct keys to be shared across different superblocks, leaving a stale superblock pointer that may be used after free during inode eviction and causing at least a local kernel crash with possible higher impact.

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

The original Linux kernel CVE announcement for CVE-2026-68148 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68148

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-2026-68148	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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-697;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'fscrypt direct key lookup for v1 direct key policies reused a cached fscrypt_direct_key across different superblocks because find_or_insert_direct_key() compared the key descriptor, mode, and raw key but did not compare the superblock. The cached object stores dk_sb and later uses it during inode eviction and inline crypto key destruction, so sharing the object across filesystems can keep a pointer to a superblock after that superblock has been released. For the CVSS the PR:L is used for the paranoid score because a local user or service with access to fscrypt policy and key operations may be able to create encrypted inodes and trigger the lifetime mismatch, while full administrative control is not always required in delegated or containerized environments. The issue is not network reachable. Impact is at least local denial of service via a kernel crash and in the worst case may allow privilege escalation due to use after free of a kernel superblock pointer.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)	YES	DISK UAF LINUS KPANIC  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=6
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 6
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Generic memory corruption: +2. The commit explicitly describes a use-after-free caused by reusing one `fscrypt_direct_key` across different `super_block` objects.
* Real lifetime corruption: +1. The stale `dk_sb` pointer may outlive the referenced superblock and be used later during inode eviction.
* Reliable kernel crash / strong DoS: +1. A stale superblock pointer used during cleanup is a credible kernel crash path.
* Important filesystem/storage path: +1. The bug is in fscrypt key lifetime handling.
* Hard or special triggering conditions: -1. It requires legacy v1 fscrypt direct-key policy, matching key parameters across different superblocks, and a later eviction/destruction path.
* Legacy / non-default execution mode: -1. The affected path is the legacy v1 direct-key policy mechanism, not the v2 per-superblock key path.

Paranoid additional signal:

* Weak LPE concern: +1. The bug is a real kernel UAF involving a high-value lifetime object pointer, but the patch does not show controlled reclaim, arbitrary write, callback control, or type confusion. This is enough for manual-review sensitivity, not enough to claim strong LPE.

## 3. Reachability analysis

The issue is local, not network reachable. A realistic trigger likely requires access to fscrypt policy and key setup operations on filesystems using legacy v1 direct-key policies. Full host root is not necessarily required in all deployments because users or services may be allowed to create encrypted directories or use fscrypt keys on already mounted filesystems, but creating the exact cross-superblock conditions may depend on administrator-provided mounts or delegated environments. Containers and reduced-capability service accounts can matter if they have access to encrypted filesystem trees and key operations, so treating this as purely PR:H would likely understate practical risk.

Call-site confidence: medium. The patch provides the affected helper and explains the lifetime consequence, but not the full eviction call path.

## 4. Severity interpretation

This is more than an ordinary Moderate cleanup because the patch fixes an explicit use-after-free in kernel filesystem encryption key lifetime handling. The conservative interpretation is local DoS with a difficult trigger. The paranoid interpretation is Actionable Moderate because kernel UAF bugs involving stale filesystem lifetime pointers can sometimes become security-relevant if object reuse is controllable, even though this patch does not demonstrate a strong exploitation primitive. It should not be auto-closed without manual review.

## 5. One-sentence report phrase

A missing superblock check in fscrypt direct-key lookup can cause direct keys to be shared across different superblocks, leaving a stale superblock pointer that may be used after free during inode eviction and causing at least a local kernel crash with possible higher impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: explicit kernel UAF in filesystem key lifetime management, local trigger conditions may be configuration-dependent, and the worst-case impact depends on whether stale superblock memory reuse can be influenced.

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

Patch: fscrypt: Add missing superblock check in find_or_insert_direct_key()
Commit: 330249609b70778094a7a36f5b6bcfa6362121d4
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=330249609b70778094a7a36f5b6bcfa6362121d4

Commit description:

The legacy 'fscrypt_direct_keys' table caches master keys that are used
by v1 encryption policies that have FSCRYPT_POLICY_FLAG_DIRECT_KEY.
It's just a global table for all filesystems (since the keys can be
provided by the legacy process-subscribed keyrings mechanism, which
makes it difficult to reuse super_block::s_master_keys).

The entries in it ('struct fscrypt_direct_key') do contain a super_block
pointer, though, for passing to fscrypt_destroy_inline_crypt_key() when
the last inode that references the key is evicted.

However, when finding the fscrypt_direct_key for an inode, we weren't
actually comparing the super_block pointer.  As a result, inodes with
different super_blocks could point to the same fscrypt_direct_key.  That
could extend the lifetime of a fscrypt_direct_key beyond the
super_block it points to, causing a use-after-free later.

Fix this by creating distinct fscrypt_direct_key structs for distinct
super_block structs.

Note that this problem doesn't exist in the v2 policy equivalent
("per-mode keys"), since the data structures there are per super_block.

Fixes: 22e9947 ("fscrypt: stop holding extra request_queue references")
Cc: stable@vger.kernel.org
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260717044303.425265-1-ebiggers%40kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20260719033120.122120-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/crypto/keysetup_v1.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=330249609b70778094a7a36f5b6bcfa6362121d4

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

The original Linux kernel CVE announcement for CVE-2026-68148 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68148

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: 4
  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).