Linux Kernel CVE-2025-40042: Race in kprobe Init Triggers Kernel Oops

A newly published Linux-kernel vulnerability, tracked as CVE-2025-40042, fixes a race condition in kprobe initialization that can lead to a NULL-pointer dereference and a kernel crash — a local, availability-focused defect that has been upstreamed into the kernel stable trees and is being absorbed by downstream distributors and vendors.

Background​

kprobes are a long-standing Linux tracing mechanism that allow dynamic instrumentation of kernel code. They are used by developers, observability tooling, and performance profilers to insert callbacks at runtime without recompiling or rebooting. Because kprobes operate in kernel context and are triggered synchronously during program execution, any defect in their initialization or dispatch paths can have immediate, host-wide consequences.
The specific issue fixed by CVE-2025-40042 arises during the intersection of two initialization paths: the code that registers and enables kprobes, and the code that initializes per-event perf structures used by kprobe profiling. Under certain interleavings on SMP (multi-CPU) systems, one CPU can see that kprobe functionality is enabled while another CPU has not yet completed the initialization that populates the per-CPU perf data pointer; the result is a dereference of a NULL pointer inside the kprobe dispatch/perf handling path and an oops or panic. The public advisory text and stack trace excerpts make this sequence explicit.

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What happened, in plain terms​

• During kprobe/perf setup, code A assigns a per-event perf list pointer (call->perf_events) and then enables the tracing class (class->reg), which signals that kprobe profiling is available.
• If CPU1 observes the kprobe-enabled flag (class->reg) before CPU0 has finished assigning call->perf_events, CPU1 can enter the profiling handler (kprobe_perf_func) and attempt to read a NULL perf_events pointer.
• That dereference results in an instruction- or memory-access fault in kernel context, producing a kernel oops and often a crash or panic on the host. The published stack traces show the faulting frames and registers pointing at kprobe_perf_func and the perf-related fields.

This is a classic initialization-order race: enabling a feature before its required per-CPU data structures are guaranteed visible to all CPUs. The fix upstream is focused on pairing or ordering the initialization steps so that either the registration cannot be visible until perf_events is initialized, or the dispatch path checks for presence before dereferencing.

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Why this matters to system administrators and engineers​

• Availability impact: This defect causes kernel oopses/panics — the primary risk is denial-of-service (DoS) for the host running the vulnerable kernel. It does not, in its published form, constitute a remote code-execution primitive.
• Local attack vector: An attacker must be able to trigger the kprobe dispatch path on the target system. That generally requires local code execution or the ability to load or provoke kprobes/perf events on the machine. In many environments, unprivileged users or misconfigured tooling can exercise these paths.
• Embedded/legacy exposure: Vendor kernels (Android, embedded Linux forks) and long-lived appliance images that lag upstream are the most likely to remain vulnerable for the longest time, because upstream commits must be backported by vendors or distributors. This is the usual risk profile for small, correctness-only kernel fixes.

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Technical anatomy: the stack trace and root cause​

The published kernel log snippets include a classical kernel oops trace showing:

• Faulting function: kprobe_perf_func
• Caller: kprobe_dispatcher
• Context: an attempt to read call->perf_events via this_cpu_ptr(call->perf_events) where call refers to a trace_event_call structure used by the kprobe tracing class
• Symptom: call->perf_events appears uninitialized (NULL) on the CPU that takes the trace

The advisory and vulnerability descriptions reconstruct the race as:

• CPU0: assign call->perf_events = some_list
• CPU0: call class->reg() to enable kprobe profiling
• CPU1: immediately sees the enabled flag and enters kprobe_dispatcher
• CPU1: calls kprobe_perf_func, which reads this_cpu_ptr(call->perf_events) and dereferences NULL

A straightforward defensive pattern fixes this: ensure that either the registration is serialized with the perf_events assignment, or add a protective check in the dispatch/profiling path to treat a NULL perf_events as “not yet ready” and bail out safely. The upstream patch implements that defensive pairing so the kprobe dispatcher cannot dereference an uninitialized perf pointer.

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Severity, exploitability and real-world risk​

• Public trackers and the NVD describe this vulnerability as a kernel NULL-pointer dereference caused by a race during initialization; the primary impact is availability. This is reflected in community CVSS interpretations that prioritize Denial‑of‑Service.
• There is no public evidence that this specific defect has been weaponized into a remote code-execution or privilege-escalation exploit. However, kernel faults that can be triggered locally remain a high-priority item for defenders because they can be used to crash hosts and might, when combined with other flaws, accelerate exploitation. The published advisories treat it as a local DoS robustness fix rather than an elevation-of-privilege vector.
• Attackability depends on local controls: systems that permit untrusted users to load or influence kprobe/perf behavior (development hosts, shared servers, container hosts with permissive capabilities) are more exposed. Administrators should assume that any host that allows local users to interact with kprobes or perf could be abused to trigger the fault.

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Upstream fix and vendor handling​

The upstream kernel maintainers applied a focused, low-risk patch in the tracing/kprobe initialization code. The change is intentionally small: it pairs or reorders initialization steps so that the perf_events pointer is visible before kprobe profiling registration is enabled OR the dispatcher protects itself from dereferencing an uninitialized pointer.
Distributors and vendors are expected to absorb the commit into their stable trees and push fixed kernel packages. Several public vulnerability databases (NVD, Debian/OSV, cvefeed) have already published entries for CVE-2025-40042 and reference the upstream commits and the stack-trace evidence. Administrators should watch their distribution security trackers and vendor advisories for specific kernel package versions that include the backport.

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Practical remediation guidance (immediate and long-term)​

The authoritative remediation path is to install a kernel package that includes the upstream fix and then reboot hosts to run the updated kernel. That is the only way to make the kprobe code in kernel memory reflect the corrected behavior.
Short checklist:

• Inventory affected hosts:
• Use configuration management, package inventories, or uname -r and kernel config checks to find systems that run kernels vulnerable to CVE-2025-40042.
• Track vendor/distro advisories:
• Monitor your distribution’s CVE/security tracker for explicit package versions that include the fix; look for changelog entries referencing the kprobe/trace fix or CVE-2025-40042.
• Apply patched packages and reboot:
• Install the patched kernel packages from your vendor/distribution and schedule reboots per normal maintenance windows.
• For embedded devices and vendor kernels:
• Contact vendors for firmware/kernel images that include the backport; if vendors do not supply updates promptly, escalate and consider compensating controls (see below).

Short-term mitigations if patching is delayed (practical, conservative):

• Restrict local untrusted code execution. Enforce least privilege and reduce the set of users who can run code that might trigger trace/kprobe events.
• Harden capability assignments and container runtime policies so unprivileged or containerized workloads cannot create kprobes or perf events. On many distributions, creation of some perf events or kprobes requires CAP_SYS_ADMIN or equivalent privileges — remove unnecessary privilege where possible.
• Isolate machines that are heavily used for development (where dynamic tracing is common) from production or multi-tenant hosts until patched.
• Monitor logs and triage kernel oopses aggressively; if you see the kprobe stack trace (kprobe_perf_func / kprobe_dispatcher) prioritize patching that host.

Note: There is no safe “configuration knob” that universally disables kprobes without functional impact; many administrator workflows rely on tracers. The correct fix is a kernel update.

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Detection and forensic indicators​

A crashed host vulnerable to this race will typically show kernel oops output similar to the published trace. Detection and triage steps include:

• Search dmesg/journalctl for “Unable to handle kernel paging request” near traces that reference kprobe_perf_func or kprobe_dispatcher.
• Look for stack traces that include the trace_kprobe.c symbols and the sample register dumps captured in the advisories. Those patterns are highly indicative of the specific NULL dereference described in the CVE entry.
• Add log rules to your SIEM to flag repeated kernel oopses that reference kprobe or perf symbols — those hosts should be prioritized for patching.

Example detection signatures to hunt for (paraphrased patterns):

• “kprobe_perf_func” AND “kprobe_dispatcher” in dmesg/journal
• “Unable to handle kernel paging request” near a kprobe symbol
• Repeated DRM or trace subsystem oopses correlated with local writes or debug/tracing activity

These are practical, high-yield signals because the race yields a deterministic dereference in the dispatch path.

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Developer and maintainer lessons​

• Initialization ordering matters: making a feature visible before its required per-CPU or per-object data structures are fully initialized is a recurring bug class in concurrent kernels. The fix for CVE-2025-40042 addresses precisely that mistake.
• Defensive programming in dispatch paths is inexpensive and valuable: simple presence checks or paired synchronization often convert catastrophic faults into clean error returns.
• Small, surgical patches are preferred: the upstream change is intentionally minimal to avoid regressions while restoring correctness. That makes it straightforward for distributions and vendors to backport the fix into stable kernel branches without wide behavioral changes. Community discussion around similar kernel fixes shows this approach consistently reduces regression risk.

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Risk matrix and prioritization guidance​

• High priority to patch: multi-tenant servers, developer hosts, CI/CD runners, and any machine that allows untrusted or multi-user tracing. These environments provide the easiest local vectors to trigger the race.
• Medium priority: single-user desktops used for development where unprivileged tracing is common.
• Lower priority: control-plane hosts that are fully locked down with no local untrusted process execution and where kprobe/perf access is tightly controlled.

The remediation window should be determined by exposure: if hosts are reachable by many local users or are used for developer workloads, treat this as a high-priority kernel update.

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What remains uncertain or unverifiable​

• There is no corroborated public proof-of-concept (PoC) exploit that demonstrates remote or privilege-escalating behavior stemming from this race in the published advisories. Public trackers characterize the impact as availability-focused, and the defect requires a local trigger. Treat claims of remote exploitation or full compromise as unverified unless a reliable PoC or exploit writeup appears.
• The exact set of downstream package versions that include the backport will vary by distribution and vendor; administrators must consult their distribution security trackers for authoritative fixed versions. The upstream commit is present in kernel.org, but verification for each vendor package is required.

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Quick reference: what to do now​

• Identify all hosts running Linux kernels that predate the kprobe fix.
• Monitor your distribution security tracker and apply the fixed kernel package as soon as it is available for your platform.
• Reboot patched hosts to ensure the updated kernel is active.
• If you cannot patch immediately:
• Limit untrusted local execution and tighten capability grants that permit tracing/perf usage.
• Isolate developer or perf-heavy hosts.
• Watch kernel logs for the kprobe-specific oops signatures and escalate any matches for immediate patching.

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Conclusion​

CVE-2025-40042 is a surgical but important kernel robustness fix: a race between kprobe activation and perf-event initialization could leave a profiling pointer unset on some CPUs and allow a kprobe dispatch to dereference NULL, producing a kernel oops. The impact is primarily availability and local exploitability; the upstream kernel fix limits the exposure and is the correct remediation. Administrators should prioritize applying the corrected kernel packages from their distributors or vendors, rebooting affected systems, and using log-hunting and privilege-hardening as interim mitigations where necessary. Timely patching remains the most effective protection.

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Source: MSRC Security Update Guide - Microsoft Security Response Center