[CVE-2026-53397][MODERATE 7.0] nfsd: fix posix_acl leak on SETACL decode failure

[CVE-2026-53397][MODERATE 7.0] nfsd: fix posix_acl leak on SETACL decode failure

[AL-KERNEL]

CVE: CVE-2026-53397
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: nfsd: fix posix_acl leak on SETACL decode failure
Commit: b2eb1ffd511d1b3c3e21122f97cbbccea411e277
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b2eb1ffd511d1b3c3e21122f97cbbccea411e277
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53397
Analysis date: Sun, 19 Jul 2026 09:07:07 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A remote NFS client can trigger a server-lifetime posix_acl memory leak by causing SETACL decode failure after the first ACL allocation succeeds, potentially leading to cumulative memory exhaustion and denial of service on exposed NFSv2 or NFSv3 ACL services.

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

The original Linux kernel CVE announcement for CVE-2026-53397 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53397

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-2026-53397	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';*CWE-401;*CWE-772;**CWE-400;Other CVSS 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.9';DESCR 'A memory leak in nfsd SETACL decoding can occur when the first ACL decode succeeds and the second ACL decode fails. In that path pc_decode returns false, svc_process skips the SETACL handler, and the old pc_release callback only releases the file handle while the allocated posix_acl object is lost for the lifetime of the server. A remote client can potentially trigger this by sending malformed NFS ACL SETACL RPC requests to an exposed NFSv2 or NFSv3 ACL service. For the CVSS the PR:N value is used because the vulnerable decode path can be reached by a network client without local code execution on the target, assuming the NFS service is reachable. Impact is denial of service through cumulative memory exhaustion rather than direct memory corruption, information disclosure, or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	DISK LEAK NETWORK KPANIC 	NO	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

Triggered signals:

* Remote reachable / network-triggerable: +2
  The vulnerable path is in nfsd ACL SETACL RPC decoding. A remote NFS client can potentially send malformed SETACL requests to an exposed NFSv2/NFSv3 ACL service.

* Availability impact realistic: +1
  A successful malformed request can leak a posix_acl object for the lifetime of the server. Repeated requests can accumulate memory pressure and cause resource exhaustion.

* Broad/default/common subsystem or broadly deployed exposure: +1
  NFS server deployments are common in enterprise and storage environments, even though nfsd is not enabled by default on all systems.

Paranoid-only signal:

* Reliable kernel crash / strong DoS: +1
  Not from a single request, but sustained remote abuse can plausibly exhaust kernel memory because leaked ACL objects are not released until nfsd restart or system reboot.

Not applied:

* No generic memory corruption. This is a leak of an allocated posix_acl reference, not UAF, OOB write, double free, or type confusion.
* No privilege escalation signal. The patch does not show attacker-controlled reuse, overwrite, stale callback, refcount takeover, or another LPE primitive.
* No confidentiality or integrity impact. The observable impact is resource exhaustion.

CWE: CWE-401, CWE-772, CWE-400

Call-site confidence: high. The commit message explicitly describes svc_process_common, pc_decode failure, skipped pc_func, and pc_release behavior.

## 3. Reachability analysis

The bug can be triggered by a remote client that can reach the NFS ACL SETACL RPC procedure. Local code execution on the target is not required, so PR is effectively none for network-exposed vulnerable services.

Realistic exposure depends on whether nfsd with NFSv2 or NFSv3 ACL support is enabled and reachable. NFS servers are usually deployed inside trusted storage or data-center networks rather than directly exposed to the public Internet. Namespaces and containers do not materially reduce the privilege requirement for a remote attacker, but containerized nfsd deployments are typically privileged and configuration-specific.

## 4. Severity interpretation

This behaves like an actionable Moderate issue, not an Important memory-corruption vulnerability. The vulnerability is remotely reachable and can cause cumulative server-lifetime memory leakage, which makes it more actionable than a purely local or low-rate leak.

The theoretical impact remains DoS. There is no evidence in the patch of UAF, arbitrary write, object confusion, information disclosure, or privilege escalation. The paranoid score is raised because repeated malformed network requests can plausibly exhaust resources, but it remains capped below Important-class memory-corruption handling.

## 5. One-sentence report phrase

A remote NFS client can trigger a server-lifetime posix_acl memory leak by causing SETACL decode failure after the first ACL allocation succeeds, potentially leading to cumulative memory exhaustion and denial of service on exposed NFSv2 or NFSv3 ACL services.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue is network reachable and affects nfsd server resource lifetime, so exposure and rate-limit behavior should be checked manually. It should not be treated as LPE or memory corruption based on this patch.

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

Patch: nfsd: fix posix_acl leak on SETACL decode failure
Commit: b2eb1ffd511d1b3c3e21122f97cbbccea411e277
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b2eb1ffd511d1b3c3e21122f97cbbccea411e277

Commit description:

nfsaclsvc_decode_setaclargs() and nfs3svc_decode_setaclargs() each
call nfs_stream_decode_acl() twice, first for NFS_ACL and then for
NFS_DFACL.  Each successful call transfers ownership of a freshly
allocated posix_acl into argp->acl_access or argp->acl_default.  If
the first call succeeds but the second fails, the decoder returns
false and argp->acl_access is left dangling.

ACLPROC2_SETACL.pc_release was wired to nfssvc_release_attrstat and
ACLPROC3_SETACL.pc_release was wired to nfs3svc_release_fhandle.
Both only call fh_put() and have no knowledge of the ACL fields on
argp.  The posix_acl_release() pairs sat at the out: labels inside
nfsacld_proc_setacl() and nfsd3_proc_setacl(), but svc_process()
skips pc_func when pc_decode returns false, so that cleanup is
unreachable on decode failure:

    svc_process_common()
      pc_decode()                  /* decode_setaclargs: false */
      /* pc_func skipped */
      pc_release()                 /* fh_put only -- ACLs leaked */

The orphaned posix_acl is leaked for the lifetime of the server.

Fix by adding nfsaclsvc_release_setacl() and nfs3svc_release_setacl(),
which release both argp->acl_access and argp->acl_default in addition
to fh_put(), and wiring them as pc_release for their respective SETACL
procedures.  pc_release runs on every path svc_process() takes after
decode, including decode failure, so the posix_acl_release() pairs are
removed from the proc functions' out: labels to keep ownership in one
place.  This matches the existing release_getacl() pattern used by
the sibling GETACL procedures.

Fixes: a257cdd ("[PATCH] NFSD: Add server support for NFSv3 ACLs.")
Cc: stable@vger.kernel.org
Assisted-by: kres:claude-opus-4-7
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/nfsd/nfs2acl.c
  fs/nfsd/nfs3acl.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=b2eb1ffd511d1b3c3e21122f97cbbccea411e277

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

The original Linux kernel CVE announcement for CVE-2026-53397 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53397

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:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.9

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: 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).