CVE-2025-21778 is a vulnerability identified in the Linux kernel related to the tracing subsystem's handling of persistent ring buffers. Specifically, the issue arises when attempting to mmap() (memory map) a trace instance buffer that is attached to reserved memory (reserve_mem). The kernel code attempts to convert virtual addresses to page structures using virt_to_page(), but this function does not support memory allocated via vmap(), which is how persistent ring buffers are implemented. This mismatch causes a kernel crash due to a page fault when the mmap() operation is performed on such buffers. The crash manifests as a BUG in validate_page_before_insert(), leading to an unrecoverable kernel oops and system instability. The vulnerability is mitigated in the current patch by disabling mmap() on persistent ring buffers allocated from reserve_mem, returning an error (-ENODEV) instead of crashing. This issue affects Linux kernel versions around 6.14.0-rc2-test and likely other versions using similar tracing and memory management implementations. The root cause is the improper handling of vmap()'d memory in the tracing ring buffer mmap path, which can be exploited unintentionally by triggering an mmap() call on the affected buffers, causing denial of service through kernel crashes. No known exploits are currently reported in the wild, and no CVSS score has been assigned yet.

For European organizations relying on Linux-based systems, especially those using advanced tracing and debugging features or custom kernel builds with reserve_mem configurations, this vulnerability poses a risk of denial of service. A malicious or unprivileged user with the ability to invoke mmap() on tracing buffers could cause kernel crashes, leading to system instability or downtime. This could impact critical infrastructure, cloud services, and enterprise environments where Linux kernels are widely deployed. The inability to mmap persistent ring buffers safely may also hinder debugging and monitoring capabilities, affecting operational visibility. While the vulnerability does not appear to allow privilege escalation or remote code execution, the resulting kernel panics could disrupt services, impacting availability and potentially leading to data loss or interrupted business processes. Organizations with high availability requirements or those operating in regulated sectors (e.g., finance, healthcare, energy) may face compliance and operational risks if systems are affected.

1. Apply the latest Linux kernel patches that disable mmap() on persistent ring buffers allocated from reserve_mem, as provided by the Linux kernel maintainers. 2. Avoid using reserve_mem for tracing ring buffers until a proper fix allowing safe mmap() is implemented. 3. Restrict access to tracing interfaces and mmap operations to trusted users only, minimizing the risk of accidental or malicious triggering of the vulnerability. 4. Monitor kernel logs for oops or BUG messages related to tracing and mmap operations to detect potential exploitation attempts. 5. For environments requiring persistent ring buffer mmap functionality, consider alternative tracing configurations or wait for upstream fixes that properly handle vmap()'d memory. 6. Implement robust kernel crash recovery and system monitoring to reduce downtime in case of exploitation. 7. Engage with Linux distribution vendors for backported patches and security advisories relevant to deployed kernel versions.