CVE-2021-47634 is a high-severity vulnerability in the Linux kernel's UBI (Unsorted Block Images) subsystem, specifically involving a race condition between two ioctl handlers: ctrl_cdev_ioctl and ubi_cdev_ioctl. The vulnerability arises from improper synchronization between these two code paths, which hold different mutexes (ubi_devices_mutex and ubi->device_mutex respectively), allowing concurrent execution that leads to use-after-free conditions. The root cause is a race between volume attachment and detachment operations within the UBI device management. Specifically, the ctrl_cdev_ioctl function handles ubi_attach and ubi_detach operations, where ubi_detach is protected by reference counting, but ubi_attach's uif_init and uif_close functions can race with ubi_cdev_ioctl's volume creation and removal routines. This results in double free and use-after-free bugs, as device structures may be freed while still being accessed concurrently. The problem was introduced by a commit (714fb87e8bc0) that made devices "available" before they were fully accessible via sysfs, leading to premature access and freeing. The fix involves rolling back this commit and removing the ubi_get_device calls in sysfs attribute show functions to prevent accessing uninitialized device entries. The kernel's kernfs infrastructure now ensures devices are not deleted prematurely, mitigating the race condition. Exploitation of this vulnerability could allow a local attacker with limited privileges (PR:L) to cause memory corruption, leading to privilege escalation or denial of service. The CVSS v3.1 score is 7.8 (high), reflecting high impact on confidentiality, integrity, and availability, with low attack complexity and no user interaction required. No known exploits are reported in the wild yet.

For European organizations, this vulnerability poses a significant risk particularly to servers, embedded devices, and infrastructure running vulnerable Linux kernel versions with UBI support enabled. Exploitation can lead to local privilege escalation, allowing attackers to gain root access or disrupt system availability through kernel crashes or memory corruption. This is critical for sectors relying on Linux-based embedded systems, such as telecommunications, industrial control, automotive, and IoT devices prevalent in Europe. Data centers and cloud providers using Linux kernels with affected versions could face service disruptions or compromise of sensitive data. The vulnerability's local attack vector means insider threats or attackers with limited access could escalate privileges, increasing the risk profile. Given the widespread use of Linux in European IT infrastructure, failure to patch could lead to targeted attacks or lateral movement within networks. The absence of known exploits reduces immediate risk but does not eliminate the urgency for mitigation due to the high severity and potential impact on confidentiality, integrity, and availability.

1. Immediate patching: Apply the latest Linux kernel updates that include the fix for CVE-2021-47634. If vendor-specific kernels are used (e.g., Ubuntu, Red Hat, Debian), ensure their security advisories are followed and patches applied promptly. 2. Kernel version auditing: Identify and inventory all systems running affected kernel versions, especially those with UBI enabled, to prioritize patching. 3. Restrict local access: Limit local user privileges and access to systems where possible to reduce the risk of exploitation by unprivileged users. 4. Monitor kernel logs: Implement monitoring for kernel warnings or crashes related to UBI subsystem operations that could indicate exploitation attempts. 5. Harden device management: Where feasible, disable or restrict UBI usage on systems that do not require it, reducing the attack surface. 6. Use security modules: Employ Linux Security Modules (e.g., SELinux, AppArmor) to enforce strict access controls on device ioctl operations. 7. Incident response readiness: Prepare for potential exploitation by having forensic and recovery procedures in place for affected systems. 8. Vendor coordination: Engage with hardware and OS vendors to ensure timely updates and support for embedded devices that may not receive regular kernel updates.