The Linux kernel’s tracing subsystem received a targeted security fix for a subtle but real use‑after‑free risk: the trace event verifier previously skipped certain complex pointer formats such as "%*p..", allowing tracepoints to reference memory that might be freed before a trace reader consumes the event. That gap was closed by a set of small, focused commits in the upstream kernel; distributions have since rolled the changes into vendor kernels. The vulnerability — tracked as CVE‑2025‑37938 — is availability‑focused (kernel crashes / oops) but important for any environment that enables tracing, loads event definitions from untrusted modules or userland, or bundles custom tracepoint code into deployed images.
Treat tracepoint and observability configuration as part of your hardened baseline: patch kernels promptly, reduce the attack surface by restricting who can register trace events, and rebuild golden images so the fix becomes persistent across future deployments. The upstream patch and vendor advisories give a clear remediation path — the remaining task for defenders is rapid, methodical rollout and verification.
Appendix: Example commands to get started
Source: MSRC Security Update Guide - Microsoft Security Response Center
Background
What the trace event verifier does
The kernel tracing framework exposes a large set of tracepoints and trace events used by perf, ftrace, BPF, and other observability tooling. These events can contain formatted output — essentially printf‑style format strings paired with event arguments — and the kernel includes a trace event verifier that runs at boot or module load to scan those print formats and validate that any dereferenced pointers will be safe when the event is later read. That verifier is intended to prevent trace events from reading memory that is outside the event payload or memory that may have been freed by the time a reader accesses the event.Why format specifiers matter
The kernel’s extended printk/trace format system supports many pointer‑oriented and helper specifiers (for example, %pS, %pK, and bitmap formats like %pbl). Some specifiers cause the trace subsystem to treat an argument as a reference to data that should be contained in the event buffer, or as a reference that must remain valid after the event is recorded. Those distinctions are critical for the verifier’s safety checks: formats that imply dereferencing or reading pointed‑to memory must be validated so the verifier can ensure the referenced memory is still present when the event is consumed. The kernel documentation explains these format variants and the bitmap-related formats (such as %pb and %*pbl) in detail.The issue (technical summary)
What went wrong
A class of trace event print formats that use the pattern "%p.." was not being covered by the verifier’s checks. When an event’s format contains "%p.." the verifier previously ignored it (or treated it in a way that didn’t trigger the requisite pointer‑liveness checks), leaving the door open for an event to reference memory that:- was allocated when the event fired, but
- was freed before the reader consumed the event.
Why the risk is real even if not broadly exploitable
Unlike some kernel CVEs that enable remote code execution, this defect’s realistic impact is primarily availability: a kernel crash or oops. However, a local attacker (or a container tenant, misconfigured service, or module author) who can register or trigger specially crafted trace events might cause repeated kernel instability or denial of service. The attack requires the ability to introduce or trigger events that use the vulnerable format and to arrange for the pointed‑to memory to be freed before the event reader gets to it — a nontrivial sequence but achievable in permissive or multi‑tenant setups. Distribution and vendor advisories therefore rate the issue as important and assign medium‑to‑high operational priority.What was changed (patch & commit details)
- Upstream kernel commits were authored to teach the trace event verifier about "%p.." variants and to add sample code exercising "%pbl" usage. The main patch was committed by Steven Rostedt and merged into the mainline kernel in the spring‑to‑early‑April 2025 window. Linus’ tree included the change in early April, and the commit landed in stable trees and vendor patch sets used by distributions.
- Distributions incorporated the fix into their kernel packages in subsequent updates: Ubuntu, Debian, SUSE, Amazon Linux and other vendors issued advisories and packages referencing the upstream commit and CVE. Where vendors backported the patch, they released updated kernels; where they could not, they provided guidance for remediation or mitigation. Administrators should consult their distribution’s security advisory to identify the exact package version that contains the upstream commit.
Who and what is affected
- Any Linux installation that includes the kernel tracing subsystem and that either:
- enables tracing and loads event definitions that include the "%*p.." pattern, or
- accepts event definitions or modules from untrusted sources (e.g., multi‑tenant nodes, CI runners, developer images), is potentially exposed.
- Embedded devices and appliances that integrate tracing helpers into their kernels and do not follow a fast update cadence can remain vulnerable for months; vendor kernel lags are the common long‑tail risk for this class of defect. Cloud images (Marketplace images or custom golden images) that are built from older kernels can also be affected until images are rebuilt or kernels updated.
- The attack vector is generally local: an unprivileged user capable of loading or triggering a maliciously formatted trace event or a module that defines such events. However, containerized or multi‑tenant environments where tenants can influence tracing instrumentation raise the operational blast radius.
Exploitability, PoC status and real‑world risk
- At disclosure there were no authoritative reports of in‑the‑wild exploitation or published proof‑of‑concept exploit code. Multiple vulnerability databases and trackers documented the issue and its patch but did not show public PoCs. That said, the absence of public proof‑of‑concepts does not imply an absence of practical attack paths: a determined local attacker or a misbehaving module can reproduce the described sequence.
- CVSS and vendor scoring vary by distributor and context; Amazon Linux’s advisory lists a CVSS v3 score around 7.0 for their packaging context, while other vendor trackers model the vector and privileges differently. Operators should prioritize based on exposure: multi‑tenant cloud hosts, developer images, and appliances with tracepoints enabled should get higher urgency than isolated single‑user desktops.
Detection and triage — practical steps
The following steps will help you determine exposure and detect possible hits. These are operational checks, not full forensic recipes.- Inventory kernel versions and tracing config:
- Run uname -a to collect kernel release info.
- Check whether tracing is enabled in the kernel config: grep -i TRACE /boot/config-$(uname -r) or zcat /proc/config.gz | grep -i TRACE.
- Identify whether tracing devices are present: ls /sys/kernel/tracing or ls /sys/kernel/debug/tracing depending on your kernel.
- Check for user_events / loaded trace modules:
- List events: find /sys/kernel/tracing/events -type d.
- Inspect any dynamically registered user_events or modules that install custom events.
- Hunt for crash indicators:
- Search kernel logs for oops/panic/backtrace lines and function names in trace code paths (print_event_fields, trace event parsing). Example: journalctl -k | egrep -i 'trace.*event|oops|BUG:'.
- Testing in a non‑production lab:
- If reproducing or validating the fix, use a test build with KASAN or other sanitizer enabled to quickly highlight use‑after‑free or invalid dereference attempts. This is recommended only in controlled test environments.
- Validate package/changelog presence:
- After installing vendor kernel updates, confirm the package changelog references the upstream commit ID (example commit ea8d7647f9ddf…), or that the vendor advisory explicitly lists CVE‑2025‑37938. Do not assume package version numbers alone guarantee the specific fix — verify the changelog or upstream commit mapping.
Remediation & mitigation
- Primary remediation: install vendor‑provided kernel updates that include the upstream patch. This is the only definitive fix because the verifier behavior is implemented inside kernel code. Vendors have published updates; follow your distro’s standard patch testing and rollout process.
- Short‑term mitigations if you cannot patch immediately:
- Disable tracing on production hosts where it is not required (unmount or restrict access to the tracing sysfs interfaces).
- Restrict which users/accounts can load tracing modules or add tracepoints. Harden container runtimes and CI runners so tenants cannot register arbitrary events into the host tracing subsystem.
- For managed cloud images, replace vulnerable images with updated images provided by the vendor or rebuild custom images with patched kernels.
- Operational hardening:
- Treat tracing and logging infrastructure as sensitive: control who can register or enable tracepoints, and apply the same change control to observability components that you apply to any privileged subsystem.
- Ensure golden images and build pipelines are updated and rebuilt after kernel security updates are applied rather than relying on in‑place fixes alone.
What administrators should tell leadership and dev teams
- The vulnerability’s primary impact is stability (availability) rather than immediate remote code execution. That helps prioritize but does not reduce the need to patch in exposed environments. Vendor guidance and distribution updates shipped in mid‑2025; organizations running older kernel images or slow‑moving vendor appliances are the highest risk group.
- Because tracing is often enabled in developer, test, and CI images, and because custom trace definitions may be committed into projects, the triage work should include both kernel patching and an audit of how events are defined and who can register them. This dual approach closes the long tail and reduces the chance of accidental or malicious event registration.
Critical analysis — strengths of the response and remaining concerns
Strengths of the upstream response
- The upstream fix is small, targeted, and surgical: it extends the existing verifier logic rather than redesigning the tracing subsystem. That makes the patch low‑risk and easy to backport into stable branches and vendor kernels. It also included sample code to exercise related formats, which helps vendor and packager validation.
- Multiple distributions quickly mapped the commit into their advisories, enabling operators to rely on standard vendor update workflows. The availability of vendor advisories reduces triage time and helps automation systems map CVE → package quickly.
Residual and systemic risks
- Long‑tail exposure: embedded devices, OEM kernels, and custom‑built images are the biggest lingering risk. These often lag stable kernels and may not receive backports promptly; operators should prioritize inventory and vendor engagement for such assets.
- Observability as an attack surface: this CVE highlights that telemetry and tracing subsystems themselves are attack surfaces. Attackers who can manipulate instrumentation can create reliability or integrity problems, and defenders must consider observability hardening part of their security posture. A single unchecked tracing interface can be the vector for denial‑of‑service or for crafting exploit primitives in more complex chains.
- Verification complexity: because the fix touches subtle parsing/format logic, operators should validate updates in a test lane before mass rollout. Kernel print/trace format behavior is nuanced; misapplied or incomplete patches at the distro level can introduce regressions in observability outputs. Use canary stages to spot any unintended change in trace output parsing or tooling compatibility.
Action checklist (prioritized)
- Inventory: run uname -a, list tracing sysfs and installed kernel package versions across the estate.
- Patch: apply vendor kernel updates that reference CVE‑2025‑37938 or the upstream commit ID and reboot hosts on a staged schedule.
- Restrict: limit who may register trace events or load tracing modules on production hosts.
- Test: validate patched kernels in a staging environment and test observability tooling for regressions.
- Hunt: scan kernel logs for recent oops or suspicious trace‑related warnings and preserve evidence for any suspected incidents.
- Communicate: notify developers and CI owners to avoid shipping or enabling custom trace events from untrusted sources.
Final assessment
CVE‑2025‑37938 is a textbook example of how seemingly minor format parsing edge cases can produce kernel‑level stability hazards. The fix is straightforward and has been incorporated into upstream kernel trees and vendor packages; the operational priority depends on exposure. For cloud providers, multi‑tenant hosts, and environments that permit dynamic tracepoint registration, the urgency is high — patch and validate promptly. For isolated single‑user desktops and systems where tracing is disabled, the practical risk is lower but not zero.Treat tracepoint and observability configuration as part of your hardened baseline: patch kernels promptly, reduce the attack surface by restricting who can register trace events, and rebuild golden images so the fix becomes persistent across future deployments. The upstream patch and vendor advisories give a clear remediation path — the remaining task for defenders is rapid, methodical rollout and verification.
Appendix: Example commands to get started
- Check kernel version:
- uname -sr
- Check whether tracing is present:
- ls /sys/kernel/tracing || ls /sys/kernel/debug/tracing
- find /sys/kernel/tracing/events -type d | head
- Grep kernel logs for trace‑related oops:
- journalctl -k | egrep -i 'trace|trace_event|print_event_fields|oops|BUG:'
- Confirm vendor package includes the fix:
- dnf/changelog or apt changelog for the kernel package and verify the commit ID or CVE in the changelog.
Source: MSRC Security Update Guide - Microsoft Security Response Center
