CVE-2021-47635 is a vulnerability in the Linux kernel's UBIFS (UBI File System) implementation related to improper reference count management when setting the PG_private flag on pages. The issue arises because UBIFS fails to increment the page's reference count after marking it as private, violating the kernel's memory management rules. Normally, when a page is marked private (PG_private flag set), its reference count must be incremented to reflect the additional ownership. The kernel's page migration and compaction routines rely on accurate reference counts to determine if pages are in use and safe to migrate. Due to the missing increment, the page migration process incorrectly assumes the page is unused and proceeds to migrate it concurrently with other processes accessing the page. This leads to a race condition causing a kernel BUG triggered by a negative reference count, resulting in a kernel panic or system crash. The vulnerability is triggered during typical filesystem write and read operations involving page cache management and page migration. The root cause is a logic error in UBIFS's handling of page reference counts during write operations, specifically after setting the private flag without incrementing the refcount. This bug can cause system instability and denial of service due to kernel crashes. There is no indication of remote code execution or privilege escalation directly from this flaw, but the kernel panic can disrupt system availability. The vulnerability affects Linux kernel versions containing the faulty UBIFS code prior to the patch. No known exploits are reported in the wild as of the publication date. The vulnerability requires local access to trigger, as it involves filesystem operations and kernel memory management. No authentication or user interaction beyond normal filesystem usage is needed once local access is obtained. The issue is technical and specific to the UBIFS filesystem implementation in Linux kernels, impacting systems that use UBIFS, typically embedded devices or systems with flash storage using UBI volumes.

For European organizations, the primary impact of CVE-2021-47635 is on the availability and stability of Linux-based systems utilizing the UBIFS filesystem, commonly found in embedded devices, IoT devices, and specialized industrial equipment. A kernel panic caused by this vulnerability can lead to unexpected system reboots or downtime, disrupting critical operations. Organizations relying on embedded Linux systems in sectors such as manufacturing, telecommunications, automotive, or critical infrastructure could face operational interruptions. Although the vulnerability does not directly compromise confidentiality or integrity, denial of service conditions can affect service continuity and reliability. In environments where embedded Linux devices are used for monitoring, control, or data acquisition, such disruptions could have cascading effects on business processes or safety systems. The lack of known exploits reduces immediate risk, but unpatched systems remain vulnerable to accidental or malicious triggering of the bug. European organizations with supply chains or products incorporating affected Linux kernels should prioritize patching to maintain system stability. The impact is more pronounced in sectors with high reliance on embedded Linux devices running UBIFS, especially where remote or automated recovery is limited.

1. Apply the official Linux kernel patches that fix the reference count handling in UBIFS as soon as they become available from trusted sources or Linux distributions. 2. Identify and inventory all systems using UBIFS filesystems, particularly embedded devices and specialized equipment, to assess exposure. 3. For devices where kernel updates are not feasible, consider isolating or restricting access to limit triggering conditions, such as controlling write operations to UBIFS volumes. 4. Implement monitoring to detect kernel panics or unexpected reboots indicative of this issue. 5. For development and testing environments, reproduce the issue to validate patch effectiveness and ensure system stability post-update. 6. Engage with device vendors or suppliers to confirm patch availability and deployment plans for embedded products. 7. Where possible, transition critical systems to filesystems without this vulnerability or to Linux kernel versions confirmed to have the fix. 8. Establish robust backup and recovery procedures to minimize downtime impact in case of crashes. 9. Educate system administrators and embedded device operators about the symptoms and risks associated with this vulnerability to enable prompt response.