The Linux kernel vulnerability tracked as CVE‑2025‑68289 — described as "usb: gadget: f_eem: Fix memory leak in eem_unwrap" — closes a small but operationally important resource‑management hole in the USB gadget Ethernet Emulation Model (f_eem) function by hardening the error path for usb_ep_queue failures and ensuring all allocated resources are reliably freed.
Background / Overview
USB gadget drivers provide device‑side (peripheral) support inside Linux kernels used on embedded SoCs, development boards, and certain virtualized images. The f_eem gadget implements the USB Ethernet Emulation Model (EEM) function: it receives USB packets from a host and hands them into the kernel networking stack, and vice‑versa. The vulnerable code lives in the f_eem implementation and specifically in the eem_unwrap routine, which processes or unwraps command/packet frames and interacts with endpoint queueing. The immediate technical symptom is a
memory leak — allocations performed during command handling were not always freed if usb_ep_queue (the routine used to enqueue USB transfer requests on an endpoint) failed. Automated memory‑leak tooling (kmemleak) reported unreferenced objects with backtraces pointing into eem_unwrap, which led maintainers to add corrective cleanup logic on the failure path. The fix uses the existing goto‑style error handling to ensure all temporary allocations are freed when a queue operation fails. Why this matters in practice: small, repeated leaks in kernel code are availability problems. On long‑running systems or devices that frequently exercise the gadget path (test rigs, multi‑tenant hosts that permit USB passthrough, embedded appliances), even modest per‑trigger leaks accumulate into slab/kern memory pressure, warnings from KASAN/kmemleak, OOPS messages, and in severe cases out‑of‑memory or service disruption. Distribution trackers and advisory notes for similar kernel gadget fixes consistently treat these defects as availability‑first.
What the public records say (concise timeline and disclosure)
- The CVE entry and corresponding dataset records were published in mid‑December 2025 and describe the same root cause: usb_ep_queue failure not freeing resources in eem_unwrap. The National Vulnerability Database (NVD) summarized the issue and included the kmemleak traces reported by upstream testing.
- Major distribution trackers (Ubuntu, Debian) and open‑source vulnerability databases (OSV, cvefeed, CVEDetails) registered the CVE, mapped it to kernel commits, and flagged affected package versions for each distribution. These records document where upstream stable commits that implement the fix are available and list fixed package versions for distributions that already shipped backports.
- Upstream kernel stable commits addressing the leak were referenced in public trackers (multiple stable‑branch commit IDs). In some environments git.kernel.org access is restricted or blocked, but the stable commits were recorded in vendor and CVE aggregator metadata and subsequently backported into several distribution kernel packages.
Technical anatomy: how eem_unwrap leaked memory
The control flow in brief
- The f_eem code path that unmarshals or "unwraps" an incoming EEM command builds temporary skb(s) and other request structures, then uses usb_ep_alloc_request / usb_ep_queue to schedule a USB transfer on an endpoint.
- If the usb_ep_queue call succeeds, the transfer completes asynchronously and the lifecycle of the allocated objects is managed by the usual completion callbacks.
- If usb_ep_queue fails, however, the older code path did not free all the temporary allocations that had been created for that transfer; the error path missed one or more cleanup steps. Automated memory leak detectors reported unreferenced objects with backtraces that pointed at eem_unwrap allocations and subsequent failed queue attempts.
Evidence from runtime instrumentation
The NVD and distribution advisories include kmemleak snippets showing unreferenced allocations of various sizes (for example 240, 256, 128, 64 bytes) with backtraces that end in eem_unwrap. Those traces pinpoint the leak to heap allocations (kmalloc / skb_clone / usb_ep_alloc_request) created while preparing or queuing the USB transfer. The upstream patch ensures that every branch that can be taken when usb_ep_queue returns an error reaches the code that frees those allocs.
Why goto cleanup remains the practical solution
The maintainers chose to continue using the existing goto‑style cleanup because the function’s error paths are already complex and the code is tightly coupled with kernel USB heap and request lifecycles. Introducing automatic cleanup helpers would require a broader refactor and a higher risk of regression. A simple, explicit cleanup that executes on usb_ep_queue failure is the safe, conservative approach favored for kernel stable‑tree patches.
Impact and exploitability assessment
- Immediate impact: availability / memory pressure. The defect manifests as a growing number of leaked kernel objects when the erroneous branch is repeatedly exercised. This produces KASAN/kmemleak reports, slab growth, and eventually may contribute to service degradation.
- Attack vector: local or local‑adjacent. An adversary needs to cause the affected gadget path to execute repeatedly — for example by attaching a malicious USB host to a gadget‑capable device, through USB passthrough in virtualized setups, or by triggering gadget lifecycle events on devices that enable the function. This places most risk on embedded appliances, test benches, developer rigs, and virtualization hosts that allow USB passthrough.
- Privileges: typically low. The path can be reached by events that do not require root, because USB attach/detach and gadget handling are driven by kernel device events tied to physical device interaction. In virtualized environments where guests can present USB devices to hosts, the guest or an untrusted external device may be able to trigger the leak.
- Escalation potential: the leak itself is not a direct code‑execution primitive. Memory leaks are an availability problem by design. Converting a leak into an arbitrary‑write or RCE requires additional, often platform‑specific conditions and heap‑grooming. Public vulnerability records and trackers do not list reliable public PoCs for escalation to RCE at the time of disclosure. Treat claims of privilege escalation from this leak alone as speculative unless accompanied by reproducible exploit code.
Which systems are affected
- Any Linux kernel build that includes the USB gadget f_eem function and predates the stable commits that implement the fix may be vulnerable. Whether a host is actually exposed depends on kernel configuration (CONFIG_USB_GADGET and the f_eem function being built or modular), whether the gadget is enabled on the target hardware, and whether the device’s deployment allows gadget lifecycle events. Desktop and server kernels often do not enable gadget functions by default, while embedded and SoC‑targeted kernels commonly do.
- Distribution package mappings are available in distro trackers; Debian’s tracker lists specific source package versions with vulnerable/fixed status and references the stable kernel commit range. Administrators should consult their distribution’s security tracker for precise version mappings for their release.
Upstream fix and vendor responses
- Upstream: maintainers applied a targeted patch in the stable kernel branch that adds the missing cleanup on usb_ep_queue failure inside eem_unwrap. The patch is intentionally minimal and uses existing goto cleanup labels to free temporary allocations. The upstream commit IDs are recorded in CVE aggregators and distro advisories.
- Distributions: major distributions indexed the CVE and began rolling fixes/backports into their stable kernel packages. Ubuntu published a CVE page mapping the issue and prioritized it as Medium for their typical exposure model; Debian’s tracker shows which releases received fixes and which remained vulnerable pending backports. Administrators must consult their vendor advisories to know the exact fixed package version for their line.
- Vendor/OEM lag: embedded appliance vendors, BSP providers, and OEMs frequently maintain forked kernel trees and may take longer to ship backports. These field devices are often the highest‑risk population because patch cadence is slow and physical access can be common in deployed environments.
Detection and forensic signals
- Kernel logs: dmesg/journalctl may show KASAN or kmemleak output that includes backtraces into eem_unwrap, usb_ep_alloc_request, skb_clone, and related symbols. Watch for repeated kmemleak notifications referencing unreferenced objects originating in uut_unwrap.
- Memory telemetry: rising slab or kernel memory usage correlated to USB gadget attach/detach events is a telltale sign. Use slabtop, /proc/meminfo, and kmemleak (if safe for production) to monitor slow‑growing kernel memory usage that correlates with gadget activity.
- Repro guidance: in a test lab enable kmemleak where feasible, repeatedly exercise the gadget command path (or run a harness that simulates EEM traffic) and inspect kmemleak output for allocations that are never referenced. A patched kernel should not produce the same unreferenced traces under the same trigger.
Recommended remediation and mitigation checklist
- Patch: apply vendor/distribution kernel updates that explicitly reference CVE‑2025‑68289 or include the upstream stable commit(s) that fix the bug. Reboot into the patched kernel to complete remediation. Kernel fixes require rebooting to take effect unless delivered as a reloadable module (rare for gadget core code).
- Validate: after patching, run the same gadget trigger tests in a controlled environment and confirm kmemleak and slab growth are no longer present. Inspect dmesg for absence of earlier backtraces.
- Short‑term mitigations if patching is delayed:
- Disable or blacklist the f_eem gadget module where safe (for modular kernels): add a modprobe blacklist entry and update initramfs if necessary. This avoids exposing the gadget path at the cost of disabling legitimate gadget features.
- Restrict USB device attachment and USB passthrough in virtualized environments until images are patched. Enforce device allowlists and require operator approval for passthrough.
- Enforce udev rules or system policies that prevent untrusted processes from exercising gadget device nodes.
- Inventory and prioritization: identify hosts and images that compile or load f_eem (check uname -r, kernel config, and lsmod). Prioritize devices with public exposure to untrusted USB attachments (developer machines, lab devices, kiosks, virtualization hosts with passthrough).
- For embedded fleets and OEM‑supplied devices: open vendor support tickets and request backported kernel images if vendor packages are not yet available. Track vendor timelines and apply updated firmware/kernel images as they are produced.
Practical commands and checks (for systems administrators)
- Check kernel version:
- uname -r
- Check if gadget support or f_eem is present in a running system:
- zcat /proc/config.gz | grep -i \"USB_GADGET\"
- lsmod | grep g_ (look for gadget modules such as g_ether, g_eem, g_serial)
- dmesg | grep -i eem
- Confirm package-level fix (example for Debian/Ubuntu):
- apt list --installed linux-image-* | grep $(uname -r)
- Consult distro security tracker for the fixed kernel package name/version and match against installed package changelog.
- Use kmemleak safely in test labs to replicate the traces; avoid enabling kmemleak in sensitive production systems without careful planning, as it can affect performance.
Strengths and notable positives of the remediation
- Minimal and low‑risk: Upstream did a targeted change to error handling rather than a sweeping refactor. Small, defensive fixes are easier to review, backport, and ship in stable kernels without introducing regressions.
- Detectable and auditable: The leak was found by kmemleak/KASAN instrumentation and documented with backtraces, making verification and QA straightforward: re‑run the trace harness and confirm the absence of the reported kmemleak entries.
- Distribution attention: Multiple distributions and trackers recorded the issue and produced advisories or package updates; this coordination shortens time to remediation for many common server and desktop images.
Risks, limitations and remaining concerns
- Vendor lag for embedded devices: Many appliances and vendor kernels are patched on a longer cadence or require vendor‑specific testing; these devices may remain vulnerable in the field longer than mainstream distribution packages. Operations teams should inventory such devices and treat them as high priority.
- Detection noise: Kernel logs and tracer output can be noisy; distinguishing benign device churn from a targeted exploit attempt requires correlation with device attach timestamps and device identifiers. False positives complicate triage.
- No public PoC ≠ no risk: absence of an exploit in the wild does not mean the vulnerability is benign. Memory management bugs are often instrumentalized later in chained exploit techniques; however, escalation claims must be scrutinized and verified with reproducible evidence before raising severity beyond availability.
Operational guidance for mixed Linux/Windows environments
- Windows administrators who host Linux guests, run WSL2, or operate Azure/Marketplace images should treat Linux kernel CVEs like CVE‑2025‑68289 as relevant to their patching cadence: a vulnerable guest or image can produce host instability or be an attack vector in shared infrastructure. Verify whether Microsoft attested images (for example, Azure Linux) include the vulnerable component and whether vendors have published fixes for those images. Microsoft’s product attestation model has been used for other Linux component CVEs and should be consulted where applicable.
- For virtualization hosts: disable untrusted USB passthrough until guest images are confirmed patched. For WSL2: verify the shipped kernel version and coordinate with change windows if a kernel update is required.
Conclusion — practical takeaways
CVE‑2025‑68289 is a classic kernel robustness fix: a predictable memory‑management hole in the f_eem gadget was closed by strengthening error‑path cleanup in eem_unwrap. The immediate operational impact is availability: leaked kernel allocations detected by kmemleak and potential long‑term slab growth on devices that exercise the gadget path. The upstream patch is small, low‑risk, and already recorded in stable kernel commit metadata; distributions have catalogued the CVE and are shipping backports in typical release channels. Administrators should prioritize patching hosts that compile or load gadget code, virtualized hosts that permit USB passthrough, and embedded fleets with vendor kernels. If immediate patching is impractical, disable gadget modules, restrict USB passthrough, and monitor kernel memory telemetry until fixes can be deployed. For operational teams, the practical plan is simple: inventory, patch, reboot, validate — and for embedded/vended images, demand vendor backports or apply tested upstream patches as part of a controlled firmware/kernel update process.
Source: MSRC
Security Update Guide - Microsoft Security Response Center