Linux Kernel Fix: usbnet Preemption Safety Patch for CVE-2025-40164

The Linux kernel received a narrowly scoped but important fix for CVE-2025-40164 that corrects a synchronization/context bug in the usbnet driver: Syzbot reported kernel warnings arising from the driver calling smp_processor_id from preemptible code paths, and the upstream remedy adds proper bottom-half protection in usbnet_resume_rx (local_bh_disable/enable to prevent the warning and the associated OOPS trace.

Background​

The report behind CVE-2025-40164 was generated by automated kernel testing (Syzbot) and surfaced as a kernel warning that appears when the usbnet driver calls smp_processor_id while preemption is not safely disabled. The public problem statement and kernel tracking describe a call trace rooted in usbnet_skb_return and usbnet_resume_rx that ends with the kernel complaining about using smp_processor_id from preemptible code — a correctness check that the kernel enforces to guard against subtle CPU-affinity and RCU/softirq ordering mistakes. For clarity:

• usbnet is the generic USB network device driver framework in the Linux kernel used by many USB Ethernet adapters.
• smp_processor_id returns the ID of the current CPU; it must only be called in contexts where preemption is disabled or where the caller can guarantee the CPU cannot migrate the task between CPUs, because the kernel's internal checks will WARN if the call is made while the task is preemptible.
• Softirqs / bottom halves / netif_rx: Historically, network receive processing (netif_rx and related code) runs in softirq or interrupt contexts; mixing assumptions about execution context (process vs softirq) can cause misuse of APIs that read CPU-local state.

Syzbot’s trace shows a user-space process (dhcpcd in the example) triggering the path that leads to the warning; the concrete mitigation committed upstream is to protect usbnet resume/receive code with bottom-half disabling so that calls to CPU-local helpers occur only in a safe context. The change was accepted into the stable trees and mapped to the CVE record.

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Overview of the technical defect​

Why smp_processor_id matters here​

smp_processor_id reads a CPU-local identifier. If the kernel task or context migrates across CPUs while that identifier is in use, subsequent logic that relied on the CPU number may observe inconsistent state. To defend against accidental misuse, the kernel’s debug checks will produce a warning (and a stack trace) when smp_processor_id is called from preemptible code — that is, when the kernel believes preemption may occur and tasks may migrate CPUs.
In usbnet’s case, the code path in question was reached during operations that historically expect to run in softirq/interrupt context (such as resume_rx and netif_rx-related paths). When code paths that may run under normal process context attempt to call CPU-local helpers, the kernel warns. The practical symptom is noisy kernel warnings and potentially an OOPS in debug builds or under aggressive test harnesses such as Syzbot.

What the fix does​

The upstream patch adds explicit bottom-half protection (local_bh_disable/enable) in usbnet_resume_rx, ensuring that the code that calls smp_processor_id executes with bottom halves disabled and therefore in a context where CPU-local reads are safe. This change is intentionally small and defensive: it does not redesign the driver, but it aligns the execution context expectations with the kernel’s built-in sanity checks. The commit was merged into the stable kernel stream and referenced by the CVE tracking entries.

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Affected versions and distribution status​

Public vulnerability trackers and distribution security pages map the upstream commit into stable kernel releases and distribution packages. Debian’s security tracker lists the vulnerability and the package status across releases, and indicates the fix landed in the unstable branch and later stable package versions (for example, the tracker lists a fixed upstream commit and shows which Debian kernel packages were vulnerable or fixed). The Debian tracker indicates that the fix is present in later kernel versions and lists fixed version numbers in certain branches. Independent vulnerability aggregators also recorded the patch and the kernel commit references; these entries show the patch was applied to recent stable branches and offer commit identifiers for cross-checking. The CVE record and aggregators list the same short description and the same root cause (smp_processor_id used in preemptible code), which provides a clear audit trail from Syzbot detection → kernel commit → distribution backports. Practical takeaway:

• If your kernel package pre-dates the upstream stable commit that added local_bh_disable/enable protection in usbnet_resume_rx, your system is likely still vulnerable to the noisy warning or OOPS when the path in question is exercised.
• For Debian users the tracker maps package states for bullseye/bookworm/trixie/etc.; the tracker should be consulted for exact package versions to install.

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Attack surface and real-world impact​

This CVE is a correctness/stability issue rather than a memory-corruption exploit:

• Primary impact: kernel WARN_ON / OOPS and noisy logs; possible driver instability or device removal problems under certain sequences that exercise the usbnet resume/rx path.
• Exploitability: There is no public proof-of-concept demonstrating this bug translates into remote code execution or privilege escalation. The defect arises from incorrect execution context assumptions; converting that into a code-execution primitive would require additional, unrelated vulnerabilities and is not documented at disclosure. This class of bug is therefore best treated as a denial-of-service / stability exposure rather than a straightforward RCE.

Where the risk increases:

• Multi-tenant or shared systems (cloud, VMs) where untrusted workloads can exercise network ioctl paths or otherwise cause MTU changes and similar operations that flow through the path.
• Testbeds, CI runners, or developer/debug kernels with extra debugging options enabled (these often surface WARNs more readily).
• Embedded appliances and long-life devices that ship older kernels and receive delayed backports.

Commercial scanners frequently mark such kernel warnings as higher severity when they detect debug kernels or instrumented builds. That’s a policy choice by the scanners, reflecting operational sensitivity to kernel oopses rather than a change in the underlying exploitability model.

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Detection: how to find if the issue occurred on your systems​

Short diagnostic checklist:

• Scan kernel logs for the characteristic trace and warning string. Look for lines that mention smp_processor_id in a preemptible context or the usbnet call stack (usbnet_skb_return, usbnet_resume_rx, usbnet_change_mtu).
  • Example commands:
    1. journalctl -k | grep -i \"smp_processor_id\"
    2. dmesg | grep -E \"usbnet|smp_processor_id|usbnet_skb_return\"
• Look for OOPS traces that include calls from drivers/net/usb/usbnet.c and check timestamps around network/MTU/sysctl changes or usb device attach/detach events.
• If you have centralized log collection, create an alert rule for the strings above so operator teams can react quickly to reproductions.

Technical verification (developer / packager workflows):

• Inspect the usbnet driver source in your kernel package. Search for local_bh_disable/local_bh_enable around usbnet_resume_rx or for code that previously called smp_processor_id without disabling preemption/bottom halves.
• Confirm the kernel package changelog or security advisory references the upstream stable commit or CVE-2025-40164 explicitly.
• Where vendors provide patches under different commit IDs, match the functional change (bottom-half protection in usbnet_resume_rx) rather than relying solely on version numbers.

If you see the warning in production logs but cannot immediately update kernels, treat the event as an availability incident: collect vmcore/dmesg, preserve logs for post-mortem, and consider operational mitigations below.

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Mitigation and remediation guidance​

Recommended remediation (in order of preference):

1. Apply vendor or distribution kernel updates that include the upstream commit fixing the usbnet resume/rx path.
2. Reboot into the patched kernel; kernel fixes of this type require a kernel image replacement + reboot to fully remediate.
3. Validate remediation by reproducing the original Syzbot-triggering sequence in a test environment or by searching for the warning in logs after the update.

Temporary mitigations when patching is delayed:

• If feasible, unload the usbnet module or disable the affected USB Ethernet adapters temporarily. This removes the code path entirely but may impact network availability.
• Avoid operations that exercise the vulnerable path on unpatched machines — for example, avoid frequent MTU changes, avoid dynamic reconfiguration of the affected USB NICs, and reduce exposure of device control ioctls to untrusted users.
• For cloud images and appliances where vendor kernels lag, contact the vendor for an explicit backport or mitigated image and consider isolating the device behind a firewall until patched.

Operational checklist for rolling updates:

• Inventory hosts that may contain the vulnerable usbnet driver (uname -r, lsmod/modinfo usbnet).
• Prioritize systems by business impact (network-critical appliances first).
• Stage the kernel in a pilot group to test for regressions (this is a small, low-risk change, but test anyway).
• Roll out in waves and reboot hosts into the patched kernel.
• Monitor kernel logs and test network functionality after each wave.

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Vendor and distribution considerations​

The patch went upstream into the stable kernel branches and was tracked by multiple vulnerability databases and distribution trackers. Debian’s tracker lists package status across releases and shows fixed upstream commit mapping; independent aggregators also reference kernel commit IDs for the change. That mapping is the authoritative route for packagers and vendors to decide which kernel package version to ship as fixed. Important operational caveats:

• Vendor/OEM lag: Embedded devices and vendor-supplied kernel images often trail upstream kernels by months or years. Those images frequently require vendor-provided backports rather than upstream merges. Until vendors ship a patched image, those devices remain at risk of the stability/warn condition and should be treated as higher-priority for isolation or replacement. This long-tail problem is a recurring theme across many kernel CVEs.
• Cloud images: Marketplace images and provider-supplied kernels may or may not include the fix at the time it lands upstream; operators of cloud-hosted VMs should verify both the guest kernel image and the hypervisor host kernel where relevant.

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Why this fix matters (analysis)​

On the surface, CVE-2025-40164 is not an eye-catching remote code-execution vulnerability. It is a correctness and context-safety patch: the driver assumed an execution context that wasn't always true, and the kernel's debug checks flagged that mismatch. But there are several reasons this class of change is important:

• Operational stability: Kernel warnings and OOPSes can cause service interruptions, reboots, and noisy logs that complicate incident response. Fixing the underlying context assumptions reduces the risk of transient crashes.
• Fuzzing and automation: Automated test harnesses like Syzbot are specifically valuable because they find corner cases that human review misses. When those harnesses report a warning, patching reduces false positives and prevents future regressions under automated load.
• Defensive correctness: Small context and synchronization mismatches are fertile ground for more complex chains in unlikely but possible scenarios. Removing the warning reduces attack surface for future, unrelated exploitation attempts that sometimes combine multiple small bugs. Several vendor advisories and tracking entries emphasize that while this is primarily a stability fix, maintaining kernel correctness is an important security posture.

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Risks, limitations, and unverifiable claims​

• There is no public proof-of-exploit turning CVE-2025-40164 into privilege escalation or RCE at disclosure. Published trackers and advisories explicitly classify this as a warning/stability fix rather than an immediate exploit primitive. That assertion is corroborated by multiple independent trackers. Still, one must treat “no known exploitation” as provisional: absence of public exploit code does not guarantee the vulnerability could not be abused in a complex chain in future. Where an organization’s risk model strongly penalizes any kernel instability, treat this as higher priority.
• Cross-vendor mapping may vary. Different distributions and vendors produce backports under different kernel package versions and with different commit numbers. Verifying the actual presence of the functional change (bottom-half protection in usbnet_resume_rx) in the supplied kernel source/changelog is the only robust way to confirm a package is patched. Relying purely on package version numbers without commit-level confirmation can be misleading.
• Some aggregator entries include metadata (EPSS, CVSS) at different times. Those metrics are often assigned by third parties and can fluctuate; they should be used to prioritize remediation but not as the sole authority on technical impact.

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Practical hunting and remediation playbook (concise)​

1. Identify candidates:
   • uname -r ; modinfo usbnet ; rpm -q --changelog kernel-package | grep -i usbnet
2. Detect occurrences:
   • journalctl -k --no-pager | egrep -i \"smp_processor_id|usbnet_skb_return|usbnet_resume_rx\"
3. Verify patch presence:
   • Check kernel source: grep -n \"usbnet_resume_rx\" -n drivers/net/usb/usbnet.c and look for local_bh_disable/enable.
   • Confirm distribution changelog / advisory mentions CVE-2025-40164 or the equivalent upstream commit.
4. Remediate:
   • Apply vendor/distribution kernel update, reboot, and validate logs clear.
5. Interim controls:
   • Unload usbnet or disable affected devices; isolate unpatched appliances; restrict untrusted users’ ability to perform network device ioctls.

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Conclusion​

CVE-2025-40164 is a compact, well-understood kernel correctness fix: Syzbot found a context-safety issue where usbnet used smp_processor_id in code paths that could be preemptible, and upstream maintainers addressed the problem by adding explicit bottom-half protection in usbnet_resume_rx. The concrete risk is primarily availability and noisy kernel warnings rather than direct remote code execution, and the upstream patch is small and straightforward to backport. Operators should prioritize kernel updates according to business impact, verify vendor backports for appliances and embedded images, and monitor kernel logs for the characteristic trace strings until every affected image in their estate has been upgraded.

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Source: MSRC Security Update Guide - Microsoft Security Response Center