CVE-2026-23380: 
Linux Kernel vulnerability analysis and mitigation

CVE-2026-23380 is a reference counting flaw in the Linux kernel's tracing subsystem, specifically within the tracing_buffers_mmap_close() function. When a process forks, the child copies the parent's Virtual Memory Areas (VMAs) without incrementing the user_mapped reference count; when both processes exit, the close function is called twice, triggering a WARN_ON kernel assertion on the second call. Affected kernel versions include 6.10, 6.10.1–6.12.76, 6.13–6.18.16, 6.19–6.19.6, and 7.0-rc1 through 7.0-rc2. It carries a CVSS v3.1 base score of 5.5 (Medium) (Red Hat Advisory, Red Hat Bugzilla).

The root cause is classified as CWE-911 (Improper Update of Reference Count) and CWE-617 (Reachable Assertion). Normally, the mapped memory region has VM_DONTCOPY set, which prevents child processes from inheriting the VMA on fork — but this flag is only a hint and can be overridden by an application calling madvise(MADVISE_DOFORK). Once VM_DONTCOPY is cleared, a fork causes the child to inherit the VMA without incrementing user_mapped; when both parent and child exit, tracing_buffers_mmap_close() is invoked twice, and the second invocation finds user_mapped == 0, returning -ENODEV and firing a WARN_ON. The fix increments the user_mapped reference count in the VMA's open callback without re-mapping the pages (Red Hat Bugzilla, Kernel Patch).

Successful exploitation results in a denial-of-service condition through kernel assertion failures that destabilize the kernel tracing subsystem. An unprivileged local attacker with low-privilege access can trigger kernel WARN_ON messages, potentially causing system instability or disruption of tracing functionality. There is no impact on confidentiality or integrity — only availability is affected (Red Hat Advisory).

1. Gain local access: Obtain a low-privileged local user account on a system running a vulnerable Linux kernel version (6.10, 6.10.1–6.12.76, 6.13–6.18.16, 6.19–6.19.6, or 7.0-rc1/rc2).
2. Open a tracing buffer: Open a file descriptor to a kernel tracing buffer (e.g., /sys/kernel/debug/tracing/trace_pipe_raw or similar) and mmap() it into the process's address space.
3. Override VM_DONTCOPY: Call madvise(addr, length, MADV_DOFORK) on the mapped tracing buffer region to reset the VM_DONTCOPY flag, allowing the VMA to be inherited by child processes on fork.
4. Fork the process: Call fork() so the child process inherits the parent's VMA for the tracing buffer without incrementing the user_mapped reference count.
5. Trigger double-close: Allow both the parent and child processes to exit (or close the mapping), causing tracing_buffers_mmap_close() to be called twice — the second call finds user_mapped == 0, triggers WARN_ON, and returns -ENODEV, resulting in a denial-of-service condition (Red Hat Bugzilla).

• Logs: Kernel log messages (via dmesg or /var/log/kern.log) containing WARN_ON or WARNING: entries referencing tracing_buffers_mmap_close or the tracing subsystem; error codes -ENODEV appearing in tracing-related kernel stack traces.
• Process Behavior: Processes making madvise(MADV_DOFORK) syscalls on memory regions associated with kernel tracing buffer file descriptors, followed immediately by fork() calls — detectable via strace or auditd syscall monitoring.
• File System: Unusual or repeated access to /sys/kernel/debug/tracing/ buffer files (e.g., trace_pipe_raw, per_cpu/cpuX/trace_pipe_raw) by non-root processes.

Apply the upstream kernel patches available for the following fixed versions: 6.12.77, 6.18.17, 6.19.7, and 7.0-rc3 or later. Patches are available from the Linux stable tree with commit hashes 91f3e8d84c89, b0f269ba6fef, cdd96641b642, and e39bb9e02b68. Distribution-specific updates are available from Red Hat, Amazon Linux (ALAS2023-2026-1543, ALAS2023-2026-1596), SUSE (SUSE-SU-2026:22048-1), and others. As a temporary workaround for systems that cannot be patched immediately, restrict unprivileged access to kernel tracing debugfs interfaces and monitor for kernel warning messages related to tracing_buffers_mmap_close (Red Hat Advisory, Red Hat Bugzilla).