[CVE-2026-74595][MODERATE 7.0] fscrypt: use the mount idmap for the owner check in fscrypt_ioctl_set_policy()

[CVE-2026-74595][MODERATE 7.0] fscrypt: use the mount idmap for the owner check in fscrypt_ioctl_set_policy()

[AL-KERNEL]

CVE: CVE-2026-74595
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: fscrypt: use the mount idmap for the owner check in fscrypt_ioctl_set_policy()
Commit: 33b7e810ce09955aa02f3b632455cf5e7ac990a9
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=33b7e810ce09955aa02f3b632455cf5e7ac990a9
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74595
Analysis date: Sat, 22 Aug 2026 13:45:28 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
`fscrypt_ioctl_set_policy() used the wrong mount idmap for the owner check, allowing an unrelated local caller on an idmapped mount to be incorrectly authorized to set an encryption policy while the real mapped owner may be denied.`

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

The original Linux kernel CVE announcement for CVE-2026-74595 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74595

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: NO

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-2026-74595	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:L';CWE-863;CWE-284;CWE-269;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '5.3';DESCR 'fscrypt_ioctl_set_policy() used nop_mnt_idmap for the owner check instead of the idmap of the mount where the ioctl was issued. On idmapped mounts this can compare the caller fsuid with the unmapped on disk inode owner, so the real mapped owner can be denied and an unrelated local caller can be incorrectly allowed to set an fscrypt encryption policy. For the CVSS the PR:L is used because a local user or process must access the idmapped mount and issue the ioctl. The issue is not network reachable and does not indicate memory corruption. Impact is mainly an authorization bypass affecting encryption policy control, with possible integrity and availability impact and limited confidentiality concern for future data in affected directories.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	DISK SIMPLEFIX LINUS  INCREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN5 DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	YES	NO	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  A local process that can access an already configured idmapped mount can issue the fscrypt policy ioctl on a reachable inode.

* Cross-boundary isolation/policy bypass: +1
  The bug uses `nop_mnt_idmap` instead of `file_mnt_idmap(filp)`, so the owner check is performed in the wrong id mapping context. This can incorrectly allow an unrelated caller or deny the real mapped owner.

* Integrity impact plausible: +1
  An incorrectly authorized caller may set an fscrypt encryption policy where they should not be considered the owner in the mount context.

* Important filesystem/storage path: +1
  The affected path is fscrypt policy control on filesystems such as ext4 and f2fs, which is security-relevant storage policy logic.

Paranoid additional signal:

* Confidentiality or availability impact plausible: +1
  Misapplied fscrypt policy can affect future encrypted data handling or deny legitimate owners policy control. This is not direct secret disclosure, but it is enough to avoid auto-closing without review.

Not counted:

* No generic memory corruption.
  The patch is a one-line authorization-context fix, not UAF, OOB, double-free, or type confusion.

* No privilege escalation signal.
  There is no demonstrated path to root or kernel code execution.

* No remote reachability.
  This is a local filesystem ioctl path.

* No full admin/root penalty.
  Creating idmapped mounts may require privilege, but the vulnerable check matters when an idmapped mount already exists and a less privileged local caller can access it.

## 3. Reachability analysis

The bug is reachable through `fscrypt_ioctl_set_policy()` on filesystems using fscrypt and supporting idmapped mounts, such as ext4 and f2fs. A local caller needs access to the relevant mount and inode and must be able to issue the ioctl. Namespace and container setups can matter because idmapped mounts are often used to make ownership appear different inside a mount context. The issue is not enabled by network traffic and does not affect ordinary non-idmapped mounts in the same way.

## 4. Severity interpretation

This behaves like an actionable authorization bypass rather than memory corruption. The realistic impact is incorrect permission enforcement for fscrypt policy changes on idmapped mounts, with plausible integrity and limited availability or confidentiality consequences for affected directories. It is not an Important memory-corruption candidate, but it should not be auto-closed as a purely cosmetic correctness bug.

## 5. One-sentence report phrase

`fscrypt_ioctl_set_policy() used the wrong mount idmap for the owner check, allowing an unrelated local caller on an idmapped mount to be incorrectly authorized to set an encryption policy while the real mapped owner may be denied.`

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because this is an access-control bug in fscrypt policy enforcement on idmapped mounts, not a simple DoS or cleanup issue. The likely severity depends on how idmapped mounts and fscrypt-protected directories are used in the target product.

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

Patch: fscrypt: use the mount idmap for the owner check in fscrypt_ioctl_set_policy()
Commit: 33b7e810ce09955aa02f3b632455cf5e7ac990a9
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=33b7e810ce09955aa02f3b632455cf5e7ac990a9

Commit description:

fscrypt_ioctl_set_policy() calls inode_owner_or_capable() with
&nop_mnt_idmap before allowing an encryption policy to be set, instead
of the idmap of the mount the ioctl was issued on.

fscrypt is used by filesystems that support idmapped mounts (e.g. ext4,
f2fs), so on such a mount this compares the caller's fsuid against the
unmapped on-disk owner rather than the mapped owner: the actual owner
can be wrongly denied with -EACCES and an unrelated caller wrongly
allowed.  Use file_mnt_idmap(filp) instead.

Fixes: 14f3db5 ("ext4: support idmapped mounts")
Cc: stable@vger.kernel.org
Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
Link: https://patch.msgid.link/20260725080004.929328-1-zhanxusheng1024@gmail.com
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/crypto/policy.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=33b7e810ce09955aa02f3b632455cf5e7ac990a9

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

The original Linux kernel CVE announcement for CVE-2026-74595 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74595

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:L/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:L
  The Best / paranoid CVSS score: 5.3

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: 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: NO
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).