[CVE-2026-89542][MODERATE 7.0] SUNRPC: harden gss_krb5_unwrap_v2 against short tokens

[CVE-2026-89542][MODERATE 7.0] SUNRPC: harden gss_krb5_unwrap_v2 against short tokens

[AL-KERNEL]

CVE: CVE-2026-89542
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: SUNRPC: harden gss_krb5_unwrap_v2 against short tokens
Commit: dddcb0f4b7e27fac16a78ea9a1c8ec2e8a241087
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=dddcb0f4b7e27fac16a78ea9a1c8ec2e8a241087
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89542
Analysis date: Fri, 11 Sep 2026 20:36:00 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89542 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89542

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-2026-89542	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89542 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-125;CWE-191;CWE-369;*CWE-787;Other CVSS 'AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '8.8';DESCR 'A memory safety flaw in SUNRPC gss_krb5_unwrap_v2 can occur when a short or malformed Kerberos wrap token reaches the unwrap path. The code may read token header fields before confirming that the token is at least GSS_KRB5_TOK_HDR_LEN bytes long, and later arithmetic can underflow movelen so memmove() may operate with a very large length. This can cause at least a remote denial of service through kernel crash and may allow confidentiality or integrity impact in a worst case memory corruption scenario. For the CVSS the PR:L is used because reliable triggering normally requires access to an RPCSEC_GSS protected SUNRPC path or a valid context rather than only raw packet reachability. The issue is network reachable when an affected NFS or SUNRPC service using Kerberos integrity or privacy is reachable by the attacker. Such services are usually deployed on internal enterprise or storage networks rather than exposed to the public Internet.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 7)	YES	REMOTE OOB UAF NETWORK KPANIC  DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	NO	NO	checked

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

ActionableScore=7
ActionableScoreLower=6
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 6
* Conservative score: 6
* Raw paranoid score: 7
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate**::

No floor or cap changed the conservative score. No floor or cap changed the paranoid score.

2. Signal breakdown

Positive conservative signals:

* +2 Strong kernel corruption primitive. The patch-supported primitive is not only an OOB read or divide-by-zero. The dead BUG_ON and unsigned underflow of movelen can drive a very large memmove() in kernel memory.
* +2 Remote reachable network attack surface. Malformed RPCSEC_GSS Kerberos wrap tokens can reach the SUNRPC unwrap path when an affected SUNRPC or NFS service using Kerberos integrity or privacy is reachable.
* +1 Reliable kernel crash / strong DoS. The supplied context supports OOB access, divide-by-zero, and oversized memmove() failure modes that can crash the kernel.
* +1 Memory layout invariant restoration. The patch restores consistency between token length, offset, xdr_buf length, rotate base, and head cleanup length before pointer arithmetic and memmove().

Additional paranoid signal:

* +1 Weak LPE concern / unresolved exploitation leverage. The oversized memmove() is a write-like memory corruption sink and may corrupt adjacent kernel memory before faulting. Controlled target selection, arbitrary write, or controlled reclaim is not demonstrated, so this is not scored as an additional strong primitive.

Non-awarded signals:

* RealUAF not awarded. No use-after-free or stale lifetime object use is established.
* Controlled reclaim not demonstrated.
* Arbitrary or directed kernel write not demonstrated.
* Confidentiality bonus not awarded solely for OOB read because no read-back or disclosure path is shown.
* Integrity bonus not separately awarded because the kernel overwrite is already counted as the corruption primitive.
* Extra availability point not awarded because the same crash is already counted as reliable DoS.
* Race/timing penalty not applied because this is input validation and bounds handling, not a narrow race.
* Host-admin penalty not applied because host-admin-equivalent privilege is not required to send protocol input to a reachable service.
* Namespace-scoped privilege not relevant.

3. Reachability analysis

The likely attacker is a remote client that can send malformed or short RPCSEC_GSS Kerberos wrap tokens to an affected SUNRPC/NFS service. The CVSS-like PR:L interpretation is reasonable because reliable triggering normally requires access to an RPCSEC_GSS protected context or valid authentication state, not just arbitrary unauthenticated packets.

Host administrator privilege is not required on the victim. Namespace-scoped privilege is not part of the trigger model. Prior host setup exists only in the operational sense that the victim must be running an affected SUNRPC/NFS service with Kerberos GSS enabled and reachable. This is a common enterprise/storage deployment pattern but is usually internal rather than public Internet exposed.

No special hardware is required. No race repeatability issue is involved.

4. Primitive and severity interpretation

The exact primitive is malformed-token driven memory safety failure in gss_krb5_unwrap_v2(). The failures include short-token OOB reads at ptr+4 and ptr+6, rotate_left() length underflow into xdr_buf_subsegment(), divide-by-zero on zero-length buffers, and the strongest primitive: unsigned underflow of movelen followed by a very large memmove().

This is not fixed stale storage and not a cleanup-only stale dereference. It is an active protocol parsing and unwrap path where attacker-influenced token length and layout can violate kernel buffer bounds.

PrimitiveClass is CLASS_A because the patch establishes a concrete kernel memory corruption primitive through oversized memmove(), not merely a warning or passive invalid read.

Demonstrated consequence is remote kernel crash / DoS. Speculative but technically defensible consequence is broader kernel memory corruption with possible confidentiality or integrity impact, requiring manual exploitation-leverage review. No controlled arbitrary write or privilege escalation chain is proven from the supplied evidence.

ImportantCandidate=YES because a network-reachable kernel memory corruption primitive exists. AutoDowngradeAllowed=NO because automatically closing this as ordinary Moderate would discard a concrete corruption primitive with unresolved exploitation leverage.

Likely operational severity is high for systems exposing Kerberos-protected SUNRPC/NFS services to untrusted or semi-trusted clients.

5. One-sentence report phrase

A malformed RPCSEC_GSS Kerberos wrap token can trigger OOB reads, length underflow, divide-by-zero, and a huge memmove() in SUNRPC gss_krb5_unwrap_v2(), causing remote kernel DoS and requiring manual review for possible memory-corruption impact beyond crash.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch establishes a concrete network-reachable kernel memory corruption sink via underflow-driven memmove(), not just crash-only validation failure.

7. Call-site confidence

Call-site confidence: high

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

Patch: SUNRPC: harden gss_krb5_unwrap_v2 against short tokens
Commit: dddcb0f4b7e27fac16a78ea9a1c8ec2e8a241087
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=dddcb0f4b7e27fac16a78ea9a1c8ec2e8a241087

Commit description:

gss_krb5_unwrap_v2() reads the EC and RRC header fields at ptr+4 and
ptr+6 before validating that the token is at least GSS_KRB5_TOK_HDR_LEN
(16) bytes long, and its rotate_left() helper passes buf->len - base
to xdr_buf_subsegment() without verifying that base <= buf->len. When
a caller hands in a sub-16-byte token, or a token whose declared len
leaves base past the end of the buffer, three distinct failures follow:

    gss_krb5_unwrap_v2(offset, len, buf)
      ptr = buf->head[0].iov_base + offset
      ec  = *(ptr + 4)              /* OOB read on short head */
      rrc = *(ptr + 6)              /* OOB read on short head */
      rotate_left(offset + 16, buf, rrc)
        xdr_buf_subsegment(buf, &subbuf,
                           base, buf->len - base)   /* u32 wrap when base > len */
        _rotate_left(&subbuf, shift)
          shift %= buf->len         /* divide-by-zero when base == len */

After decryption, the cleanup arithmetic has the same shape:

    movelen = min_t(unsigned int, buf->head[0].iov_len, len);
    movelen -= offset + GSS_KRB5_TOK_HDR_LEN + headskip;
    BUG_ON(offset + GSS_KRB5_TOK_HDR_LEN + headskip + movelen >
                                            buf->head[0].iov_len);

The BUG_ON re-adds the value just subtracted, so it reduces to
min(A, B) > A and is permanently false; it cannot catch the unsigned
underflow of movelen, which then drives a ~UINT_MAX-byte memmove().

Add four defense-in-depth guards inside the unwrap core so it is safe
regardless of what its callers validate:

  - reject tokens with len - offset < GSS_KRB5_TOK_HDR_LEN before
    touching ptr+4/ptr+6;
  - bail from rotate_left() when buf->len <= base, covering both the
    underflow and zero-length cases;
  - return early from _rotate_left() when buf->len is zero, so the
    shift %= buf->len modulo cannot fault;
  - replace the dead BUG_ON with a live check that returns
    GSS_S_DEFECTIVE_TOKEN before the movelen subtraction.

Fixes: de9c17e ("gss_krb5: add support for new token formats in rfc4121")
Cc: stable@vger.kernel.org
Assisted-by: kres (claude-opus-4-7)
Signed-off-by: Chris Mason <clm@meta.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260524010213.557424-5-cel@kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/sunrpc/auth_gss/gss_krb5_wrap.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=dddcb0f4b7e27fac16a78ea9a1c8ec2e8a241087

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

The original Linux kernel CVE announcement for CVE-2026-89542 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89542

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:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 8.8

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: 6
  Paranoid ActionableScore: 7

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).