Description

In the Linux kernel, the following vulnerability has been resolved: sched/deadline: only set free_cpus for online runqueues Commit 16b269436b72 ("sched/deadline: Modify cpudl::free_cpus to reflect rd->online") introduced the cpudl_set/clear_freecpu functions to allow the cpu_dl::free_cpus mask to be manipulated by the deadline scheduler class rq_on/offline callbacks so the mask would also reflect this state. Commit 9659e1eeee28 ("sched/deadline: Remove cpu_active_mask from cpudl_find()") removed the check of the cpu_active_mask to save some processing on the premise that the cpudl::free_cpus mask already reflected the runqueue online state. Unfortunately, there are cases where it is possible for the cpudl_clear function to set the free_cpus bit for a CPU when the deadline runqueue is offline. When this occurs while a CPU is connected to the default root domain the flag may retain the bad state after the CPU has been unplugged. Later, a different CPU that is transitioning through the default root domain may push a deadline task to the powered down CPU when cpudl_find sees its free_cpus bit is set. If this happens the task will not have the opportunity to run. One example is outlined here: https://lore.kernel.org/lkml/20250110233010.2339521-1-opendmb@gmail.com Another occurs when the last deadline task is migrated from a CPU that has an offlined runqueue. The dequeue_task member of the deadline scheduler class will eventually call cpudl_clear and set the free_cpus bit for the CPU. This commit modifies the cpudl_clear function to be aware of the online state of the deadline runqueue so that the free_cpus mask can be updated appropriately. It is no longer necessary to manage the mask outside of the cpudl_set/clear functions so the cpudl_set/clear_freecpu functions are removed. In addition, since the free_cpus mask is now only updated under the cpudl lock the code was changed to use the non-atomic __cpumask functions.

References 6

• git.kernel.org https://git.kernel.org/stable/c/382748c05e58a9f1935f5a653c352422375566ea
• git.kernel.org https://git.kernel.org/stable/c/3ed049fbfb4d75b4e0b8ab54c934f485129d5dc8
• git.kernel.org https://git.kernel.org/stable/c/9019e399684e3cc68c4a3f050e268f74d69c1317
• git.kernel.org https://git.kernel.org/stable/c/91e448e69aca4bb0ba2e998eb3e555644db7322b
• git.kernel.org https://git.kernel.org/stable/c/dbc61834b0412435df21c71410562d933e4eba49
• git.kernel.org https://git.kernel.org/stable/c/fb36846cbcc936954f2ad2bffdff13d16c0be08a

Remediation