Linux Kernel Stmmac RX Checksum Offload Fix CVE-2025-40337

The Linux kernel received a targeted fix for a subtle but consequential networking error: CVE-2025-40337 corrects how the stmmac Ethernet driver handles Rx checksum offload results so that packets for which the hardware reported checksum failures are no longer marked as valid and passed up the network stack.

Background / Overview​

Hardware checksum offload (Rx checksum offload) is a standard performance feature on modern NICs that lets the network interface compute and verify IP/TCP/UDP checksums in hardware to avoid expensive per-packet CPU work. When offload is used, the kernel relies on the NIC to tell it whether a packet’s header and payload checksum are correct; based on that signal the kernel typically sets skb->ip_summed to CHECKSUM_UNNECESSARY so the upper stack can skip software validation.
A logic error in the stmmac driver’s receive routine caused that shortcut to be taken even when the NIC had indicated a checksum failure. In short, corrupt packets could be mislabelled as validated and forwarded into the network stack as if the hardware had approved them. The upstream patch changes the receive decision to consult the hardware’s checksum-failure indicator (the csum_none status flag) and to force the kernel to verify the checksum in software when hardware reports a problem. This correction is recorded in the upstream patch set and has been published to standard CVE trackers.

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What exactly was wrong?​

The technical root cause​

• The stmmac_rx function previously contained logic that set skb->ip_summed = CHECKSUM_UNNECESSARY whenever checksum offloading was enabled (and the frame had a recognized IP ethertype).
• That code did not first check whether the NIC had actually flagged the packet as having a checksum error.
• As a result, when the NIC signalled a checksum failure, the kernel could still mark the buffer as not needing further checksum work — allowing a potentially corrupt or malformed packet to be treated as valid by higher-layer protocols.

The upstream patch adds a simple and correct guard: consult the hardware status bit (csum_none). If that bit is set — indicating the NIC detected a checksum problem — the driver sets skb->ip_summed to CHECKSUM_NONE (or asserts software checksum handling) rather than CHECKSUM_UNNECESSARY. The change is intentionally minimal and surgical to reduce regression risk.

The patch in practical terms​

• File modified: drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
• Code change: an extra condition checks (status & csum_none) before deciding to mark the skb as CHECKSUM_UNNECESSARY; when the condition is true the skb is forced to software-validated mode. The patch itself is two insertions and one deletion — a compact fix.

This is the canonical fix pattern for checksum-offload correctness issues — verify the hardware status before trusting its offload verdict.

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Why this matters: impact and attacker model​

Primary impact: network integrity (and availability risk)​

• The most direct consequence is an integrity risk: corrupt or malformed packets could be passed to the network stack as if they were validated, which may cause application-layer corruption, misparses, or logic errors depending on protocol handling.
• Unlike many kernel CVEs, this is not a direct memory-safety or privilege-escalation bug in itself; it’s a logic correctness defect that can subvert the kernel’s validation assumptions. That makes it dangerous for network-facing services or parsers that assume checksum correctness.

Exploitability and scope​

• Exploitation requires the ability to send specially crafted packets that exercise the stmmac driver’s receive path on an affected host. In other words, an attacker generally needs network access to the interface and the ability to send packets with controlled contents to that interface.
• There is no public evidence (at disclosure) of a working remote-to-local privilege-escalation or remote code execution PoC tied to this specific bug. The principal effect is accepting corrupt packets as valid, which can produce downstream integrity failures or application misbehavior rather than an immediate RCE. Multiple vulnerability trackers list the issue as a medium/medium-high operational severity depending on environment and configuration.

Environments of concern​

• Any Linux system built with the stmmac driver and running the affected kernel code path is potentially exposed.
• Embedded devices, SoC platforms, board support packages and cloud images that include STMicroelectronics or SoC vendors’ stmmac support are the classic risk population. Those kernels are often long-lived and slow to receive updates, increasing practical exposure.
• In mixed vendor/cloud estates (for example, Azure Linux images, WSL kernels, custom marketplace images), an operator must verify whether their specific image builds include the in-tree stmmac driver and whether the vendor supplied a backport or update. Microsoft and cloud vendors often list attested images; operators cannot assume absence of the driver.

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Verification: how this was validated​

To keep the record auditable and reproducible, the fix has been validated against two independent and authoritative sources:

• The upstream kernel patch and public net/kernel mailing-list post that contain the change, showing the exact code diff and the author’s rationale. This is the primary, canonical proof of the fix.
• Vendor and CVE trackers (NVD, SUSE, Tenable, CVEDetails/OpenCVE) that indexed the CVE and referenced the git commits or mailing-list patch. These secondary trackers provide the broader mapping into affected release lines and vendor advisories. Cross-referencing these entries confirms the description and the public commit IDs that implement the change.

Where vendor trackers list specific stable commit IDs or package versions, operators should use those identifiers to map vulnerable package versions in their environment rather than relying solely on a CVE summary.

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Detection and triage: how to determine exposure​

Operators should perform a straightforward inventory and triage process to determine exposure quickly.

• Check whether the stmmac driver is present (module or built-in):
• lsmod | grep stmmac
• modinfo stmmac
• grep -i stmmac /boot/config-$(uname -r) (if /boot/config-* is present)
• Identify the running kernel: uname -r
• Inspect kernel configuration for checksum offload and stmmac-related options:
• zcat /proc/config.gz | grep -E 'STMMAC|CHECKSUM|OFFLOAD'
• Confirm whether hardware offload features are enabled for interfaces:
• ethtool -k <iface>

If the stmmac driver is present and the kernel version predates the known fixed commits (as mapped by vendor trackers), treat the host as vulnerable until patched or mitigated. For Microsoft-published images and cloud artifacts, consult the vendor attestation pages and image-update advisories rather than assuming a global vendor-wide statement covers all artifacts.

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Immediate mitigations and recommended remediation​

Short-term mitigation (workaround)​

Disabling Rx checksum offload on affected interfaces eliminates dependence on NIC-reported checksum results and forces software validation in the kernel:

• Disable Rx checksum offload:
• sudo ethtool -K <iface> rx off

This is a pragmatic stopgap that reduces the attack surface: corrupted packets will be validated by the kernel in software and rejected when checksums fail. Note that disabling offloads increases CPU load on busy interfaces and may reduce throughput in high-performance network paths. Use the workaround selectively for affected hosts or interfaces while you plan a full update.

Definitive remediation (apply the fix)​

• Identify vendor-supplied kernel packages that contain the upstream patch or backport and install the vendor-supplied kernel update that explicitly maps to the fix (vendor advisories and distribution security trackers will list fixed package versions).
• Reboot into the updated kernel.
• Validate that the kernel package matches the patched commit or that the package changelog mentions CVE-2025-40337 or the upstream commit IDs.

For cloud images (for example, Azure Linux or other marketplace images) follow the image/vendor bulletins and update to patched images as they become available. Where vendors provide VEX/CSAF attestations or machine-readable advisories, consume those signals to automate triage for fleets.

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Step-by-step remediation checklist​

• Inventory hosts that run Linux kernels with networking drivers:
• uname -r
• lsmod | grep stmmac
• modinfo stmmac
• If stmmac is present and you cannot immediately patch:
• Disable Rx checksum offload on the relevant interfaces: sudo ethtool -K <iface> rx off
• Monitor CPU usage and interface throughput; roll back offload disabling if it creates unacceptable performance problems but prioritize patching.
• Obtain vendor/distro updates:
• Check your distribution security tracker (Ubuntu, Debian, Red Hat, SUSE, etc. and cloud vendor advisories for backported kernel packages that include the fix.
• For custom kernels, merge the upstream patch into your tree or pick the stable kernel update that already includes the change, then build and test.
• Deploy patched kernels in staging, validate networking behavior, and roll the patch to production with controlled reboots.
• After patching, re-enable Rx offload if you previously disabled it and verify operability: ethtool -k <iface>

Numbered and repeatable steps make remediation tractable across large estates and help ensure the fix is applied consistently.

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Operational considerations and risk assessment​

• Performance vs safety: Disabling hardware checksum offload is a reliable mitigation but will increase per-packet CPU work. For high-throughput servers, this can push CPU usage up significantly and may require capacity adjustments during the mitigation window. Make mitigation decisions per-class of host (edge routers and gateways vs development nodes).
• Long-tail devices: Embedded boards, appliances and vendor-supplied images are the long tail of risk. Those images often lag in kernel updates; proactively contact vendors for patches or guidance and consider isolating or replacing vulnerable appliances where vendor updates are unlikely.
• Automation and inventory: Use configuration management and telemetry to find all hosts with stmmac present. Don’t rely exclusively on a single source of truth — cross-check package versions, kernel configs, and module presence. Microsoft and other cloud vendors sometimes publish image attestations that only cover a specific subset of images, so verify each image type in use.
• Detection: There is no straightforward network signature that reliably detects acceptance of corrupt packets. Rely on kernel-level telemetry and packet-capture based validation in test labs to observe incorrect checksum labeling scenarios. If you have packet-capture appliances near affected hosts, sample-to-check whether injected malformed frames are processed despite checksum errors.

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What vendors and trackers say​

• The official NVD/CVE entry summarizes the corrective behavior and lists the vulnerability description. It references upstream commits describing the exact change to stmmac_rx.
• Distribution and enterprise trackers (SUSE, Tenable, CVEDetails/OpenCVE) have mapped the CVE and linked to the kernel commits and patches that implement the fix; these sources are useful for mapping fixed package versions and for operational triage.
• The kernel patch series and mailing-list post include the compact diff and developer rationale, which is the canonical technical record for the fix. Operators and packagers should use the referenced commit IDs when mapping backports.

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Strengths of the upstream fix and residual risks​

Strengths​

• The patch is small and surgical, changing only the decision point that trusted a hardware checksum status. Small fixes of this kind are low risk for regressions and are easy to backport into stable kernel trees.
• The change restores the kernel’s fundamental correctness: do not skip software validation when the hardware reports a checksum failure. That principle reduces opportunities for corrupt packets to be mishandled.

Residual risks and caveats​

• Backport correctness: Downstream backports into distribution-specific stable kernels must be done carefully. Incorrect backports or omission of related context may leave subtle gaps. Always prefer vendor-supplied, tested packages when possible.
• Exposure uncertainty: Whether a given Microsoft image, cloud image, or third-party marketplace image is affected depends on the exact kernel binary used; absence of a global vendor attestation does not mean a given image is safe. Operators must validate the kernel in each image artifact they rely on.
• Operational tradeoffs: The usual tradeoff between performance and security applies to the ethtool workaround. For high-throughput services, plan capacity and testing before disabling offloads widely.

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Recommended timeline for response​

• Within 24 hours:
• Inventory hosts and identify any with stmmac present.
• If a vulnerable host cannot immediately be patched, apply the ethtool workaround on affected interfaces and monitor performance.
• Within 72 hours:
• Acquire, test and stage vendor-supplied kernel updates for staging environments. Validate networking functionality and restart behavior.
• Within one maintenance window:
• Roll patched kernels to production and re-enable offloads (unless vendor guidance recommends keeping offloads disabled temporarily).
• Ongoing:
• Track vendor advisories for any further related fixes or additional CVE mappings and update automation to capture those signals.

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Conclusion​

CVE-2025-40337 is a classic correctness bug in a widely used kernel driver: trusting a hardware offload verdict without checking the hardware’s checksum-failure indication. The fix is compact and straightforward — consult the NIC’s csum_none flag and force software validation when the hardware reports failure — but the operational work remains non-trivial because of the variety of Linux kernels and images in use across clouds, embedded devices, and vendor appliances.
Operators should treat the issue seriously: inventory for stmmac, apply vendor-supplied kernel package updates that include the upstream commit, or use the recommended ethtool workaround while patches are staged. The patch and CVE mapping are publicly recorded in the kernel patch threads and across vendor security trackers; cross-referencing those authoritative records is the correct way to determine exact exposure for any particular kernel build or image.

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Note: the above analysis and remediation guidance were cross-checked against the upstream kernel patch and multiple independent vulnerability trackers to ensure the technical description and operational guidance are accurate and actionable. Where vendor-specific mapping is required (package names and fixed release versions), consult your distribution or image publisher for the canonical fixed package and follow their upgrade procedures.

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Source: MSRC Security Update Guide - Microsoft Security Response Center