Linux Kernel Patch: Zero Initialize Structs to Fix CVE-2025-40278 Infoleak

The Linux kernel received a surgical fix for CVE-2025-40278 that zero-initializes a local struct used by the traffic-control “ife” action, closing a KMSAN-reported kernel information leak that could otherwise disclose residual stack bytes to unprivileged userspace.

Background / Overview​

CVE-2025-40278 was opened after sanitizer tooling (KMSAN) and syzbot surfaced a kernel information-leak in the net/sched “act_ife” codepath. The bug manifests in tcf_ife_dump, where a local instance of struct tc_ife (named “opt” in public kernel patches) was only partially initialized using designated initializers. Because of C struct padding and designator semantics, some padding bytes remained uninitialized; later, the netlink helper nla_put copied the entire struct into a netlink message, unintentionally exporting the uninitialized padding bytes to userland. The upstream remedy is deliberate and minimal: zero the struct (memset) prior to assigning fields so every byte — including padding — is deterministic before the kernel copies it into the netlink buffer. This change addresses the KMSAN report and prevents the kernel from leaking stack or kernel memory contents via that interface. KMSAN (Kernel Memory Sanitizer) is a dynamic tool used in kernel fuzzing and continuous testing to detect uninitialized reads. It does not necessarily mean the bug was observed in production kernels; rather, KMSAN flags an undefined read that could, under some conditions, reveal kernel memory contents to userspace. KMSAN reports are high-signal: they show the exact allocation and read site, making many of these fixes small, deterministic, and low risk to backport.

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Why this matters: technical anatomy of the leak​

How the leak happened​

• The traffic-control action “ife” exposes configuration and statistics through netlink dump handlers (tcf_ife_dump.
• The handler prepared a compatibility / export structure on the stack and populated some but not all of its bytes with explicit assignments.
• Because of alignment padding inside the struct, designated initializers did not guarantee that every byte of the stack object was initialized.
• The netlink serialization helper (nla_put family) copies the whole struct blob into the netlink message payload. That copy included the padding bytes left uninitialized by the kernel code.
• As a result, kernel stack contents (whatever happened to be in those padding bytes) could be observed by a user-space process that performed a dump via netlink. The sanitizer flagged this as a kernel-infoleak.

Why zeroing is the right fix​

Zero-initializing local marshalling structures before populating fields is a well-established defensive pattern in kernel code because it:

• Eliminates all padding and leftover memory artifacts that might otherwise leak.
• Is a minimal, easily reviewed change that does not alter the semantics of the fields the code intentionally provides to userspace.
• Silences sanitizers such as KMSAN while keeping the happy-path logic intact — this makes the patch safe and straightforward for stable-tree backports.

This pattern has been repeatedly applied across different kernel subsystems when KMSAN or syzbot surfaces similar issues; the community preference is to add an early memset to local structures used for userspace marshalling rather than larger semantic changes.

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Scope, affected code, and reachability​

Affected component and code paths​

• Component: Linux kernel networking scheduling (net/sched), specifically the act_ife action and its netlink dump handler tcf_ife_dump.
• Mechanism: local netlink dump operations that can be triggered by userspace utilities (tc, iproute2 components that query traffic control objects), or management agents that enumerate qdisc/action state.
• Exposure model: local information disclosure — a user or process that can call the appropriate netlink query can receive the struct blob containing any leaked bytes.

Who is most exposed​

• Multi-user and multi-tenant hosts where unprivileged users can interact with netlink-based interfaces (for example, some container runtimes or permissive developer systems).
• Systems that run untrusted workloads and permit unprivileged control-plane queries of kernel state (shared build servers, CI runners, developer workstations).
• Appliances and vendor systems that expose management APIs locally to tenants or operators without strict process isolation.
• Embedded devices that include the same net/sched action and have management agents that process or proxy netlink data.

The practical risk to a single-user, well-hardened desktop is typically lower than the risk for multi-tenant infrastructure; nevertheless, information leaks are powerful reconnaissance primitives and should be treated seriously in shared environments.

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Exploitability and realistic risk assessment​

Immediate exploitability​

At disclosure there is no public proof-of-concept demonstrating automatic escalation from this particular infoleak to remote code execution or privilege escalation. The direct outcome is deterministic information leakage of a few bytes of kernel memory per interaction. On its own, that is not a full RCE vector. Multiple independent vulnerability trackers list this CVE as a KMSAN-detected infoleak and note the lack of any published weaponization.

Why the leak is still important​

• Kernel address-space layout randomization (KASLR) and similar mitigations can be weakened by even small, repeated leaks of kernel pointers or predictable bytes; attackers often combine an infoleak with another bug (for example, a write primitive or use-after-free) to build a full exploit chain.
• In multi-tenant environments an unprivileged guest or container that can make many netlink queries might be able to aggregate leaked bytes to infer kernel layout or parse recognizable structures.
• Even if a leak yields only non-pointer bytes, those bytes can reveal object IDs, handles, or parts of in-kernel data that facilitate follow-on attacks or fingerprinting.

Therefore, while this CVE alone does not equate to immediate remote compromise, it raises the attack surface for local adversaries and should be remediated accordingly.

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The patch, upstream response, and vendor timelines​

What the upstream commit does​

• The upstream kernel commit changes tcf_ife_dump to zero-initialize the struct tc_ife instance (commonly with a memset or declaring it as a zeroed initializer) before assigning fields.
• This ensures all bytes — including padding — are deterministic and prevents nla_put from exporting undefined memory contents.
• The change is intentionally small and safe for stable-tree backports; it does not alter protocol semantics or the fields intentionally provided to userspace.

Upstream and stable-tree references for this CVE are present in public tracking pages and OSV entries; the patch has been merged into the stable branches for backporting by distributions. Operators should expect distribution kernel packages to carry those stable commits in short order.

Vendor and distribution mapping​

• Public vulnerability trackers (NVD, OSV, major distribution security trackers) list CVE-2025-40278 and map it to upstream stable commits and backports. Administrators should consult their vendor’s kernel advisories for the exact package-level fix and the backport policy for their distribution.
• Because this is a small, surgical fix, vendors can usually backport and ship it quickly; nevertheless, timelines vary between mainstream distributions, appliance vendors, and OEM kernel builds.
• Special-case devices — embedded appliances, routers, and OEM images — are often the longest tail. If such systems are under management, request explicit backport schedules from the vendor or plan compensating controls.

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Detection, verification, and patch validation​

How to confirm whether a host is vulnerable or patched​

• Inventory kernels:
• On a running Linux host, capture kernel version with uname -r and query the package changelog for security entries referencing CVE-2025-40278 or the upstream commit IDs. Distributors often include the upstream commit ID in package changelogs for kernel packages.
• Search kernel source or packaged modules:
• If you maintain custom kernels, search your tree for tcf_ife_dump and inspect whether the local struct initialization includes a memset or zeroing before population.
• Review vendor advisories:
• Consult your distribution’s security tracker (Debian, Ubuntu, Red Hat, SUSE, etc. for explicit mapping to fixed package versions.
• Repro testing in controlled environment:
• In a lab or non-production environment, test the relevant netlink dump operation (for example by using tc with the appropriate action loaded) and monitor kernel logs for KMSAN or sanitizer warnings — note that KMSAN-enabled builds are required to reproduce the sanitizer trace precisely.

If you are uncertain whether a vendor package contains the exact commit, request the vendor’s changelog or verify that the package includes the upstream stable commit hash shown in public trackers.

Signatures and telemetry to watch for​

• KMSAN warnings in kernel logs (visible only on sanitizer-enabled builds); these show the callstack and the exact uninitialized read site.
• Unusual or unexpected netlink responses containing non-deterministic padding bytes are difficult to detect in production without instrumentation, but validation in a controlled KMSAN-enabled environment will reproduce the original syzbot report.
• Look for package-manager audit entries and kernel changelogs that mention the fix; those are the most practical operational signals.

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Remediation guidance and rollout priorities​

Immediate steps for operations teams​

• Apply vendor-supplied kernel updates that include the stable commit for CVE-2025-40278 as soon as vendor packages are available.
• If you build and maintain your own kernels, pull the upstream stable commit or apply the one-line defensive change (memset the struct) and rebuild your kernel packages; then test and deploy using your normal kernel rollout process.
• For hosts where applying a kernel update is delayed, reduce the local attack surface:
• Restrict who can query or manipulate traffic-control configuration and netlink interfaces (e.g., limit access to root or a small admin group).
• Harden container and tenant policies so unprivileged containers cannot execute netlink queries for traffic-control objects.
• Centralize and gate management-plane operations through trusted agents rather than exposing raw netlink access to unprivileged processes.

Prioritization matrix​

• Highest priority:
• Multi-tenant hosts, CI/build farms, shared virtualization hosts, and VDI infrastructure where many untrusted users or tenants share kernel resources.
• Medium priority:
• Desktop images in enterprise fleets that run developer tooling or allow unprivileged management-plane access.
• Lower priority (but still relevant):
• Single-user locked-down workstations where unprivileged processes cannot access the netlink interfaces required to trigger tcf_ife_dump.

Rollout checklist (practical steps)​

• Inventory kernels and images that include net/sched/act_ife (package and config review).
• Confirm vendor advisories and package versions that include the fix.
• Stage patched kernels in a test ring; run representative networking workloads.
• Deploy patched kernels to production in waves, monitoring kernel logs and netlink behavior.
• Maintain rollback plans and retain pre-patch logs for comparison.

This is a small, low-regression patch, so the rollout risk is low relative to more invasive kernel changes; nevertheless, standard kernel deployment hygiene (testing, rolling updates, monitoring) applies.

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Wider context: why KMSAN-surfaced bugs are common and what they reveal​

KMSAN and syzbot have become highly effective at uncovering subtle initialization or marshalling bugs in the kernel that would otherwise remain latent. These flaws often share a common pattern:

• Structures used for userspace marshalling or cross-ABI compat are partially populated by code paths that assume certain fields are always written, but C padding or alternative code paths can leave bytes untouched.
• Netlink and ioctl-style interfaces frequently copy binary blobs into userspace, so any uninitialized bytes in the in-kernel structure can be exfiltrated directly.
• The fixes are usually straightforward (memset or kzalloc) and low-risk, but they matter because even small leaks erode defense-in-depth. The community routinely prefers small defensive fixes so stable branches can receive backports quickly.

Because these patterns repeat across subsystems, operators and kernel developers increasingly adopt systematic defensive practices: always zero local marshalling buffers, use explicit field-by-field put helpers where possible, and validate sanitizer reports as high-priority correctness issues.

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Critical analysis: strengths of the upstream approach and residual risks​

Strengths​

• The upstream fix is minimal, well-scoped, and follows a conservative risk-reduction strategy: zero initialization prevents leakage without changing protocol semantics, reducing regression risk.
• KMSAN-provided evidence gives high confidence about the existence and location of the defect, which simplifies verification and patch validation.
• The change is easy to review and backport, which typically speeds vendor response and distribution packaging.

Residual and systemic risks​

• Inventory lag: operators running vendor kernels or appliances may not receive the patch immediately. Embedded devices and OEM kernels remain the longest tail.
• Chained exploitation: even a small infoleak can be useful when combined with other issues. In threat models where local code execution is possible, the leak increases attacker leverage.
• Observability: in production (non-sanitized) builds, confirming that the system was leaking specific kernel bytes retrospectively is difficult without pre-patch logs or forensic captures. This makes detection after-the-fact problematic for many operators.

Given these residuals, remediation should be prioritized in multi-tenant and high-assurance environments even for seemingly minor KMSAN-detected infoleaks.

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Practical detection and hunting playbook​

• Short-term: check your package manager and vendor security bulletins for kernel updates that reference CVE-2025-40278 or the upstream stable commit. If present, stage and deploy.
• If you cannot patch immediately:
• Restrict access to management/netlink interfaces that enumerate traffic-control actions.
• Audit containers and unprivileged processes with netlink access; remove or restrict capabilities that could perform tcf_ife_dump style queries.
• For forensic validation:
• If you suspect prior leakage, capture kernel crashlogs, dmesg, and any management-plane logs and compare with sanitized reproductions in a lab with KMSAN-enabled builds.
• In enterprise detection pipelines, add a check for the specific package-changelog or upstream commit referenced in vendor advisories to automate patch detection.

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Conclusion​

CVE-2025-40278 is a textbook KMSAN-discovered kernel information leak solved by a minimal defensive change: zero the local struct used for netlink marshalling before populating fields. The fix is safe, stable, and straightforward to backport, but the operational imperative remains: apply vendor kernel updates promptly, prioritize multi-tenant and shared hosts, and reduce unprivileged access to kernel management interfaces until patches are in place. While the CVE is not a documented RCE in isolation, information leaks are powerful enablers in complex exploit chains — treating this as a correctness fix with medium operational priority in shared environments is the prudent posture.

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Source: MSRC Security Update Guide - Microsoft Security Response Center