CVE-2021-47318: 
Linux Kernel vulnerability analysis and mitigation

CVE-2021-47318 affects the Linux kernel and involves a use-after-free vulnerability in the arch_topology subsystem. The issue occurs when topology_scale_freq_tick() (called from scheduler_tick()) may use a pointer to 'struct scale_freq_data' that was previously cleared by topology_clear_scale_freq_source(), without proper protection (Kernel Git).

The vulnerability stems from a race condition where topology_scale_freq_tick() could access a pointer that was previously cleared by topology_clear_scale_freq_source(). The issue was introduced by commit 01e055c120a4 which allowed multiple entities to provide sched_freq_tick() callback. Since topology_scale_freq_tick() is called from the scheduler tick, adding direct locking was not a viable solution (Kernel Git).

This vulnerability could lead to a use-after-free condition, potentially resulting in system crashes or memory corruption when the previously cleared scale_freq_data is accessed. The issue affects the kernel's ability to safely manage CPU topology scaling frequency data (Kernel Git).

The vulnerability requires specific timing conditions to be met between the scheduler tick and topology operations. It primarily affects systems that utilize CPU frequency scaling and topology management features (Kernel Git).

The issue was fixed by implementing RCU (Read-Copy-Update) mechanism to ensure race-free updates. The fix uses synchronize_rcu() to guarantee that all RCU critical sections that started before it is called will finish before it returns, preventing use-after-free scenarios. The patch was tested by Vincent Guittot and Qian Cai, and reviewed by Ionela Voinescu (Kernel Git).