CVE-2021-47393 is a vulnerability identified in the Linux kernel's hardware monitoring subsystem, specifically within the mlxreg-fan driver module. The issue arises from improper handling of fan speed enforcement via sysfs interface, where the minimum fan speed can be set to values that exceed the nominal maximum speed. This configuration request is intended to allow system administrators or automated processes to enforce fan speeds based on system conditions such as the absence of replaceable units or elevated ambient temperatures. However, when the kernel is compiled with the CONFIG_THERMAL_STATISTICS option enabled, attempts to set the fan speed above the nominal maximum lead to an out-of-bounds memory access during the update of thermal cooling device statistics. The kernel's thermal_cooling_device_stats_update() function does not properly validate the state argument, resulting in a slab-out-of-bounds error detected by Kernel Address Sanitizer (KASAN). This memory corruption can cause kernel crashes (BUG reports) and potentially lead to denial of service or other unpredictable kernel behavior. The vulnerability is triggered by writing to the sysfs interface, which requires appropriate permissions, thus limiting exploitation to local users or processes with elevated privileges. The issue is rooted in the mlxreg-fan driver's failure to return a non-zero error code when the requested fan speed state exceeds the maximum, which would otherwise prevent the invalid statistics update. The detailed kernel stack trace and memory dump confirm the out-of-bounds access and the specific conditions under which the bug manifests. No known exploits are reported in the wild, and no CVSS score has been assigned yet. The vulnerability affects Linux kernel versions identified by the commit hash 65afb4c8e7e4e7e74b28efa1df62da503ca3e7a6 and potentially other versions with similar configurations.

For European organizations, this vulnerability poses a risk primarily to systems running Linux kernels with the mlxreg-fan driver and CONFIG_THERMAL_STATISTICS enabled, which are common in enterprise-grade servers, network equipment, and industrial control systems. Exploitation could lead to kernel crashes, causing denial of service conditions that disrupt critical services and operations. In environments where uptime and reliability are paramount, such as financial institutions, healthcare providers, and manufacturing plants, unexpected system reboots or failures could have significant operational and financial consequences. Although remote exploitation is unlikely due to the need for local write access to sysfs, insider threats or compromised local accounts could leverage this vulnerability to destabilize systems. Additionally, the vulnerability could be used as part of a multi-stage attack to escalate privileges or bypass security controls if combined with other kernel vulnerabilities. The lack of known exploits reduces immediate risk, but the presence of detailed technical information and kernel traces increases the likelihood of future exploit development. Organizations relying on Linux-based infrastructure should assess their exposure, especially those using hardware monitored by mlxreg-fan and thermal statistics features.

To mitigate this vulnerability, European organizations should: 1) Apply the latest Linux kernel patches that address CVE-2021-47393 as soon as they become available from their distribution vendors or kernel maintainers. 2) Temporarily disable the CONFIG_THERMAL_STATISTICS kernel option if feasible, to prevent the vulnerable code path from being executed. 3) Restrict access to the sysfs interface controlling fan speeds (typically under /sys/class/hwmon or /sys/devices) to trusted administrators only, enforcing strict permission controls and auditing access attempts. 4) Monitor system logs for kernel BUG messages or KASAN reports indicating attempts to exploit this vulnerability. 5) Implement host-based intrusion detection systems (HIDS) to detect anomalous writes to thermal control sysfs entries. 6) For critical systems, consider isolating or sandboxing processes that require fan speed adjustments to minimize risk. 7) Engage with hardware vendors to confirm if their devices use mlxreg-fan and request firmware or driver updates if necessary. 8) Maintain a robust patch management and vulnerability scanning program to quickly identify and remediate affected systems.