Linux ATM CLIP Patch CVE-2025-38459: Fix for Infinite Recursion Crash

A quiet but sharp fix landed in upstream Linux this summer for a long-tail networking subsystem, and it’s the kind of kernel bug that rewards attention from system administrators: the ATM CLIP driver contained an infinite recursive call condition in clip_push that can crash the kernel when triggered more than once by a local ioctl sequence. The flaw, cataloged as CVE-2025-38459, was reported via syzbot, patched upstream, and has already been incorporated into vendor kernels and distribution advisories — meaning administrators who manage Linux hosts (including embedded appliances and network appliances) should evaluate exposure and apply updates or mitigations immediately.

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Background / Overview​

Classical IP over ATM (CLIP) is an older networking component of the Linux kernel that implements RFC‑style IP-over‑ATM behavior; it ships as the net/atm/clip driver and is normally built as a module named clip when enabled in kernel configuration. CLIP supports operations such as ATMARP and control ioctls used to attach sockets to ATM VCCs (virtual channel connections). The recent CVE concerns an interaction between the CLIP ioctl handler and the clip_push code path that is invoked on socket teardown. The vulnerability is not a generic remote code execution bug. It is a local kernel bug that arises when the ATM IOCTL ATMARP_MKIP (used to bind/attach an IP-over-ATM socket) is invoked more than once against the same VCC. In the first call a data-path function pointer is replaced as intended; a poorly guarded second call ends up copying the function pointer into another field, which leads to clip_push calling back into itself (via an overwritten old_push pointer) with a NULL skb. The result reported in the syzbot KASAN dump is uncontrolled recursion and a stack guard page hit — effectively a kernel crash / denial-of-service, with the potential for memory corruption. Upstream maintainers fixed the root cause by preventing the second ATMARP_MKIP invocation on the same vcc (checking vcc->user_back) and adding proper socket locking. Why this matters: even though ATM and CLIP are niche on modern desktop systems, they remain present in kernels, and they are used in certain network appliances, telecom equipment, and some vertical‑market devices. A local user who can access the ATM device or invoke the ioctl (for example, via an unprivileged process using a setuid helper or compromised container that exposes device nodes) could trigger the crash. Distribution maintainers have already pushed fixes; administrators should check and patch or apply alternative mitigations.

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Technical analysis​

The failure mode — how clip_push ends up calling itself​

At a high level the bug is a classic function‑pointer confusion combined with insufficient guarding of repeated ioctl calls:

• The first ioctl(ATMARP_MKIP) path calls clip_mkip, which sets up the CLIP vcc structure and assigns the kernel’s push callback (vcc->push) to the CLIP handler via clip_push. That establishes a chain so that packets sent on the VCC go through clip_push for processing.
• A second ioctl(ATMARP_MKIP) against the same vcc (the bug scenario) copies the vcc->push pointer into clip_vcc->old_push, leaving the structure in an inconsistent state.
• Later, when the socket is closed, vcc_destroy_socket calls the (now-corrupted) clip_push function with a NULL skb. clip_push, expecting to forward to the older push handler saved in clip_vcc->old_push, ends up calling back into clip_push itself because the saved pointer points to the CLIP push function. That creates infinite recursion. The kernel’s stack overrun protections (KASAN or stack guard page) detect the runaway recursion and cause a kernel panic / crash. The syzbot report and kernel OOPS show the repeated clip_push stack frames and a stack guard page hit.

Two important technical takeaways:

• The root cause is not a classic buffer overflow; it is uncontrolled recursion and a corrupted function-pointer chain caused by repeated, unguarded ioctl calls.
• The fix is conservative and defensive: prevent the second ATMARP_MKIP for the same vcc (by checking vcc->user_back) and use lock_sock to avoid races while manipulating vcc internals. Those changes eliminate the sequence that leads to pointer copy and recursion.

Potential consequences beyond crash​

The immediate and verifiable impact is a local denial-of-service — an attacker (or buggy application) that can trigger the sequence will crash the kernel or otherwise cause an OOPS. The NVD assigns a high‑impact rating (C:H/I:H/A:H) in its CVSS vector because, in the worst case, repeated uncontrolled recursion and stack corruption can go beyond a simple crash and lead to memory corruption. The weakness is mapped to CWE‑674 (Uncontrolled Recursion) and CWE‑787 (Out‑of‑bounds Write) in NVD’s analysis; both are consistent with the KASAN backtrace and the pointer-copy failure mode. There is no public proof-of-concept exploit weaponizing this to obtain arbitrary code execution in the wild at the time of writing; the publicly tracked traces are bug‑reports and KASAN splats from syzbot. That said, once a locally triggered crash is possible, escalation pathways sometimes become available in complex ways (for example, via carefully crafted ioctl sequences or use-after-free chains), so the conservative posture is to treat the issue as serious and patch promptly.

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Who is affected?​

Kernel and configuration surface​

• The vulnerability is in the net/atm/clip driver. Systems whose kernel was compiled with CONFIG_ATM and CONFIG_ATM_CLIP enabled — especially those that include the clip module — are in the exposed population. Many mainstream kernels include the ATM subsystem as modular code, even on builds where it is rarely used. Admins should not assume absence simply because they don’t use ATM day-to-day.
• NVD and downstream advisories list the weakness as upstream (kernel.org) and note that vendor kernels and various distribution packages have had patches applied. Distributions that issue long-term support kernels or backports will have vendor advisories; several mainstream distributors already published updates or advisory entries after the upstream fix.

Typical affected deployments​

• Network appliances, embedded Linux devices, home router firmwares or vendor images that still expose ATM/CLIP support.
• Virtualized guests or containers running kernels that have ATM modules loaded or available, especially if device nodes are passed through, or privileged helpers run on behalf of containerized apps.
• Desktop and server Linux installs: most modern desktop/server use-cases do not use ATM, but the kernel may still include the CLIP driver as a module; the module could be auto-loaded if a user-mode process issues ATM ioctls.

In short: the population is smaller than for a widely-exposed networking service vulnerability, but it is not negligible — especially in telecom, embedded, or vendor device fleets.

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Detection: how to know whether a host is vulnerable​

1. Check if the CLIP (ATM) modules are present or loaded:
   • Run: lsmod | grep -i atm
   • If the module is loaded, you’ll see entries such as clip, atm, or atm_*. If no module is loaded, the driver may still be available as a module on disk.
2. Inspect your kernel configuration for ATM/CLIP:
   • If /proc/config.gz is available: zcat /proc/config.gz | grep -i 'ATM_CLIP'
   • Alternatively, look for CONFIG_ATM and CONFIG_ATM_CLIP in your kernel config file (often in /boot/config-$(uname -r).
   • If CONFIG_ATM_CLIP=y or =m, the kernel supports CLIP (either built-in or modular).
3. Check for ATM device or proc entries:
   • Look under /proc/net for atm-related entries (for example, /proc/net/atm) or kernel logs referencing atm devices. If your distribution ships ATM subsystems, these sysfs/proc entries can reveal runtime usage.
4. Confirm vendor/distro advisory status:
   • Check your distribution’s security tracker or the kernel CVE pages for whether your installed kernel version or package has a backport applied. Many distros listed NVD/Advisory entries and vendor advisories after the upstream patch.

If the module is not present and the kernel was not built with CLIP support, the host is not vulnerable to this specific issue. But for fleets, an inventory across images and versions is recommended.

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Mitigation and recommended actions​

These are prioritized for security teams and administrators responsible for Linux hosts. The primary action is to patch the kernel via your distribution's normal channels. Secondary actions are short-term mitigations if immediate patching is not possible.

1) Apply vendor / distribution kernel updates (preferred)​

• Install the kernel update or security patch issued by your distribution. Vendor packages and security advisories have already been published for multiple major distros; if you are running a mainstream distribution, use the package manager (apt, yum/dnf, zypper, etc. to upgrade to the fixed kernel and reboot to load the updated kernel. This is the only permanent fix for the vulnerable code path.

Example (high-level):

1. Update package lists: apt update || yum check-update
2. Install kernel updates: apt upgrade linux-image- || yum update kernel
3. Reboot to the new kernel.

Keep in mind: vendors may backport the remediation into an older kernel branch (no version bump in the kernel numbering required), so check advisory text rather than only kernel version numbers.

2) Temporary mitigation: unload or blacklist ATM/CLIP modules​

If ATM/CLIP is not in use on the host and you cannot immediately apply a kernel patch, you can mitigate the risk by preventing the CLIP driver from being loaded:

• To check loaded modules: lsmod | grep clip
• To remove the module (if currently unused): sudo modprobe -r clip atm
  • Note: modprobe -r fails if the module is in use; check lsof / proc references before removing.
• To permanently block module auto-loading: put a blacklist entry into /etc/modprobe.d/blacklist-atm.conf:
  • blacklist clip
  • blacklist atm
• After blacklisting, update initramfs (if necessary) and reboot to ensure the modules are not available.

This is a pragmatic approach for systems that do not need ATM and where removing the module does not break functionality. Use caution on appliances that rely on ATM features.

3) Constrain access to device nodes and privileged helpers​

Because the bug is local and triggered via ioctl, reducing the set of users who can issue ATM ioctls reduces risk exposure:

• Ensure /dev entries related to ATM devices are owned by root and have restrictive permissions.
• Audit any setuid or helper binaries that may interact with ATM device nodes; remove unnecessary privileges.
• For containerized environments, avoid mapping ATM device nodes into containers or granting CAP_NET_ADMIN to untrusted containers.

4) Monitoring and incident response​

• Watch for kernel OOPS/KASAN reports referencing clip_push, clip_mkip, net/atm/clip.c or for unusual kernel panic stack traces pointing to net/atm. Those are indicators of attempted or accidental triggering.
• Capture memory/dump artifacts for analysis if you see repeated OOPS that match the syzbot splat — they help analysts verify whether an exploit attempt targeted this vector.

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Timeline and disclosures​

• The issue was reported into the kernel workflow after syzbot produced a KASAN splat showing the uncontrolled recursion in clip_push. The upstream kernel trees received patches that check vcc->user_back and add lock_sock where necessary to prevent the double-ioctl sequence and races that lead to corrupted pointers. Upstream fix references and stable backports were published shortly afterward; downstream vendors (distributions and enterprise kernel maintainers) began issuing advisories and packages to address the CVE. The NVD entry and multiple vendor advisories record the same technical narrative and the remediation steps.
• To date there are no widely reported public exploit code samples weaponizing CVE‑2025‑38459. The public materials are the syzbot splat and upstream kernel patch commits plus distribution advisories. Nevertheless, the possibility of developing a local exploit from a pointer‑corruption / recursion bug means treating the issue with urgency for exposed systems.

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Practical checklist for administrators (quick actions)​

1. Inventory: Identify hosts with CONFIG_ATM_CLIP enabled or clip/atm modules present. (lsmod, /proc/config.gz)
2. Patch: Apply vendor-supplied kernel updates and reboot. This is the definitive fix.
3. Mitigate (if urgent): If patching is delayed and ATM/CLIP is not needed, unload modules and blacklist them to prevent auto-load.
4. Lock down: Restrict access to ATM device nodes and remove unnecessary privileges from binaries that could call ATM ioctls.
5. Monitor: Track kernel logs for clip_push/atm OOPS messages and react to any repeated crashes with forensics and patch verification.

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Why Windows enthusiasts should care​

Even on a Windows‑focused forum, CVEs like CVE‑2025‑38459 deserve attention for several practical reasons:

• Many organizations run mixed OS environments; network appliances, gateways, and virtualized network functions that sit at perimeter or transit points may run Linux kernels with niche drivers enabled — a crash there can disrupt services that Windows hosts depend on. Understanding the scope reduces operational surprises.
• The remediation pattern is familiar: patch promptly, remove unused kernel surface, and restrict device access. Those same operational security triage steps apply across platforms.
• Embedded or consumer networking devices running vendor Linux can be collateral damage: routers and vendor boxes often conceal kernel config choices that expose unusual subsystems. A Windows admin may find that their network connectivity problems or appliance reboots correlate to a vendor kernel that needed the fix. The fix’s presence in vendor advisories is the signal to push firmware or kernel updates for connected devices.

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Critical perspective — strengths of the response and residual risks​

What was done well:

• The bug was detected via automated kernel fuzzing infrastructure (syzbot), demonstrating the value of automated coverage for low-traffic subsystems. The syzbot KASAN report produced actionable stack traces that allowed maintainers to craft a minimal, effective patch.
• Upstream developers applied a focused fix — a guard that prevents the sequence causing corruption and added locking — rather than a heavy-handed removal of functionality. This reduces the risk of regressions in other ATM flows.
• Distributions and vendors rapidly worked to produce backports and advisories so that operators can receive fixes through normal update channels.

Remaining or potential risks:

• Local-only exposure reduces remote exploitation risk, but local attack surfaces are still significant in multi-tenant, containerized, or shared-host environments. Untrusted containers or unprivileged user contexts that gain access to device nodes are realistic attack vectors.
• The ATM/CLIP code path is not frequently exercised in many environments, so some operators may not realize the presence of the driver until an incident occurs. That means inventory and visibility remain a weak point for many organizations.
• Absence of a public exploit does not imply the flaw is innocuous. The combination of uncontrolled recursion and pointer confusion leaves room for sophisticated exploitation attempts that might, on certain kernels or architectures, escalate to privilege gains — though such exploitation would be non-trivial and highly platform-dependent.

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Final takeaways​

• CVE‑2025‑38459 is a local kernel denial-of-service / pointer-corruption issue in the Linux ATM CLIP driver (net/atm/clip). It was detected by syzbot, patched upstream, and has been backported by major vendors and distributions. Administrators should treat systems with ATM/CLIP enabled as potentially vulnerable.
• The fastest, safest mitigation is to apply your distribution’s kernel/security updates and reboot. If that cannot be done immediately and CLIP is not needed, unload and blacklist the ATM/CLIP modules to remove the attack surface. Inventory hosts and restrict access to ATM device nodes to reduce local risk.
• For mixed environments and appliances, check vendor firmware advisories as well — appliance vendors often backport kernel fixes into firmware or kernel packages that are distributed on a different cadence than mainstream distributions. Confirm updates and schedule reboots as required.

This is a textbook example of why even niche kernel subsystems must be monitored and patched: a tiny function-pointer misstep in an uncommon driver can produce crisp, reproducible kernel crashes — and those crashes can have outsized operational impact in the right environment.

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Source: MSRC Security Update Guide - Microsoft Security Response Center