CVE-2025-39681

The vulnerability is a missing call to resctrl_cpu_detect() in the Hygon vendor-specific BSP init helper, introduced by a prior commit that moved the detection from common code. Without this, boot_cpu_data.x86_cache_occ_scale remains uninitialized, causing a division-by-zero fault in get_rdt_mon_resources() when it calculates mon_l3_config. This affects Hygon CPUs with X86_FEATURE_CQM* support. [1]

Exploitation occurs automatically during early boot on affected systems; no authentication or network access is required. Any boot of a vulnerable kernel leads to a crash, resulting in denial of service. The attacker only needs local access to trigger a reboot, or the system may crash on its own.

The impact is a complete system crash during boot, rendering the system unusable. This can cause downtime for devices such as Siemens SIMATIC CN 4100 that use vulnerable kernel versions. [1]

The fix is to add the missing resctrl_cpu_detect() call in the Hygon BSP init helper. Patches have been applied to the Linux kernel stable branches. [2][3][4] Users should update to patched kernels or backport the fix.