[CVE-2026-89533][MODERATE 7.0] svcrdma: Fix offset arithmetic in read_chunk_range

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

CVE: CVE-2026-89533
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: svcrdma: Fix offset arithmetic in read_chunk_range
Commit: 6ee4dc7476b3abc92ec1d444533a559f33f59561
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6ee4dc7476b3abc92ec1d444533a559f33f59561
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89533
Analysis date: Sat, 12 Sep 2026 06:12:41 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89533 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89533

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-2026-89533	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89533 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';CWE-191;CWE-682;**CWE-400;Other CVSS 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '6.5';DESCR 'An arithmetic bug in svcrdma Read chunk range handling can calculate an incorrect DMA segment length when the requested offset reaches or lies inside a segment. In the boundary case the u32 length can underflow and propagate into a very large nr_bvec value, causing oversized kernel allocations in the RPC over RDMA server path. A remote RDMA client on a reachable NFS/RDMA transport can trigger this by sending crafted Read chunk layout and offset values. For the CVSS the PR:N is used because no local account on the target is required when the attacker can connect as an RDMA client to the exposed service. The issue is best modeled as adjacent network reachable because NFS/RDMA is normally deployed on local storage or data center fabrics rather than the public Internet. Impact is denial of service through memory pressure or kernel allocation failure. No concrete arbitrary write or information leak primitive is shown by the code context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate (with actual score 5)	YES	REMOTE OOB NETWORK DMAorINTERRUPT KPANIC 	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=REQUIRED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4. No floor or cap applied.
* Raw paranoid score: 5
* Paranoid score: 5. No floor or cap applied.
* Final recommended bucket: **Actionable Moderate**::

2. Signal breakdown

Conservative positive signals:

* +2 Remote or adjacent network reachable. Attacker controlled RPC over RDMA Read chunk metadata can reach `svc_rdma_read_chunk_range()` when NFS/RDMA service is exposed to the attacker.
* +1 Memory layout invariant restoration. The patch fixes consistency between offset, segment residual length, DMA read segment length, and later `nr_bvec` allocation sizing.
* +1 Availability impact realistic. The u32 underflow can propagate a near UINT_MAX length into `svc_rdma_build_read_segment()` and produce oversized kernel allocations in `svc_rdma_get_rw_ctxt()`.

Paranoid additional signal:

* +1 Weak indirect corruption or invalid structured length concern. The malformed length is consumed as an active RDMA segment length and allocation size, but the supplied evidence supports DoS and invalid sizing rather than arbitrary write or controlled memory corruption.

Negative signals:

* No negative privilege penalty. Host admin privilege is not required by the attacker if the NFS/RDMA service is reachable.
* No race penalty. This is deterministic arithmetic and validation logic, not a timing race.
* No rare hardware penalty. RDMA is deployment specific, but the vulnerability is reachable through the exposed transport in affected deployments.
* No niche subsystem penalty. NFS/RDMA is specialized, but specialization alone is not enough for a negative point.

Non-awarded material signals:

* RealUAF not awarded. No post-lifetime object use is established.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Information disclosure not demonstrated.
* Privilege escalation not demonstrated.
* Concrete repeated release or refcount mutation not present.
* Extra availability was not counted as a second conservative point because the main demonstrated effect is resource pressure from the same oversized allocation path.
* Class A or Class B corruption was not awarded because the patch shows integer underflow and oversized allocation, not a concrete memory corruption primitive.

3. Reachability analysis

The evaluated attacker is a remote or adjacent RDMA client that can connect to an exposed RPC over RDMA or NFS/RDMA server. No local account on the target host is required for that model, so `HostAdminRequired=NO`.

The issue is best treated as adjacent network reachable rather than general Internet reachable, because NFS/RDMA is normally used on storage, cluster, or data center fabrics. A prior privileged administrator must have enabled and exposed the service, but that is a deployment condition, not an attacker privilege requirement once the service is reachable.

Namespace scoped privilege is not relevant to the supplied trigger, so `NamespaceScopedPrivilege=NO`.

The attacker controls Read chunk segment layout and offset or length relationships enough to reach the arithmetic defect. No special race timing is required. RDMA capable infrastructure is required, but the supplied evidence does not justify a physical only or rare hardware penalty.

4. Primitive and severity interpretation

The exact primitive is an integer arithmetic defect in RDMA Read chunk range reconstruction. The vulnerable code can fail to skip a fully consumed segment and can subtract `offset` after `min_t()`, causing a u32 underflow or a wrong per segment DMA length.

This is not an active structured UAF and not fixed stale storage. It is an invalid structured length and resource sizing issue in the RPC over RDMA server path.

The demonstrated consequence is denial of service through oversized kernel allocation or memory pressure. The paranoid interpretation keeps the issue actionable because the underflowed length is consumed by active RDMA segment construction and allocation logic, but the supplied evidence still does not establish arbitrary write, information disclosure, controlled corruption, or privilege escalation.

PrimitiveClass is `CLASS_C` because the demonstrated behavior is primarily DoS and invalid state without an independently useful corruption primitive. `ImportantCandidate=NO` because no Class A primitive, real UAF, controlled overwrite, COW violation, authorization bypass, or LPE chain is established. `AutoDowngradeAllowed=YES` because no unresolved Important grade corruption or boundary bypass primitive remains, although manual review is still required due to network reachability.

Likely operational severity is Actionable Moderate for exposed NFS/RDMA deployments.

5. One-sentence report phrase

An arithmetic underflow in svcrdma Read chunk range handling can let a reachable RDMA client force oversized kernel allocations in the RPC over RDMA server path, causing denial of service without a demonstrated information leak, arbitrary write, or privilege escalation.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue is network or adjacent network reachable in NFS/RDMA deployments and involves attacker influenced integer underflow in DMA length construction, even though the demonstrated impact is DoS only.

7. Call-site confidence

Call-site confidence: medium

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

Patch: svcrdma: Fix offset arithmetic in read_chunk_range
Commit: 6ee4dc7476b3abc92ec1d444533a559f33f59561
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6ee4dc7476b3abc92ec1d444533a559f33f59561

Commit description:

svc_rdma_read_chunk_range() walks a Read chunk's segment list to
build a sub-range starting at byte offset and spanning length bytes
for a Position-Zero or Call chunk. Two arithmetic defects in the
per-segment loop produce wrong DMA lengths and a u32 underflow:

    pcl_for_each_segment(segment, chunk) {
            if (offset > segment->rs_length) {
                    offset -= segment->rs_length;
                    continue;
            }

            dummy.rs_handle = segment->rs_handle;
            dummy.rs_length = min_t(u32, length,
                                    segment->rs_length) - offset;
            dummy.rs_offset = segment->rs_offset + offset;

First, the skip predicate uses '>' instead of '>='. When offset
equals the segment's full rs_length, the segment is fully consumed
and should be skipped, but the loop falls through into the body.
The resulting dummy.rs_length is min_t(u32, length, rs_length) -
rs_length, which underflows to a near-UINT_MAX u32 when length is
smaller than rs_length, or is zero otherwise.

Second, the length formula subtracts offset from the min_t() result
rather than from segment->rs_length before the cap. For offset > 0
the segment's residual is rs_length - offset, not rs_length, so the
cap must be applied to the residual. With the current bracketing,
whenever length is smaller than rs_length - offset the per-segment
length becomes length - offset instead of length, silently dropping
offset bytes from the rebuilt chunk. Combined with the boundary
case above it also enables the u32 underflow path, which propagates
a huge nr_bvec into svc_rdma_build_read_segment() and a multi-MiB
kmalloc_array_node() in svc_rdma_get_rw_ctxt().

Additionally, svc_rdma_read_call_chunk() can invoke this function
with length == 0 when the last Read chunk ends exactly at the end
of the Call chunk. With the corrected >= predicate, every segment
is skipped and the function returns the initial -EINVAL, rejecting
a valid request. Return success immediately when length is zero.
Also break out of the loop once length is fully consumed to avoid
passing zero-length segments to svc_rdma_build_read_segment().

Fix by using '>=' so a fully-consumed segment is skipped, by
moving '- offset' inside min_t() so the cap is applied to the
segment's residual length, by returning success for zero-length
requests, and by stopping iteration when the requested range has
been consumed.

Fixes: d7cc739 ("svcrdma: support multiple Read chunks per RPC")
Cc: stable@vger.kernel.org
Assisted-by: kres (claude-opus-4-7)
Signed-off-by: Chris Mason <clm@meta.com>
Acked-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260526-rpc-kernel-bugs-v1-2-e251306ccca9@oracle.com
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/sunrpc/xprtrdma/svc_rdma_rw.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=6ee4dc7476b3abc92ec1d444533a559f33f59561

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

The original Linux kernel CVE announcement for CVE-2026-89533 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89533

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:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 6.5

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).