CVE-2022-48892 Information

Description

In the Linux kernel the following vulnerability has been resolved:

sched/core: Fix use-after-free bug in dup_user_cpus_ptr()

Since commit 07ec77a1d4e8 (\sched: Allow task CPU affinity to be restricted on asymmetric systems) the setting and clearing of user_cpus_ptr are done under pi_lock for arm64 architecture. However dup_user_cpus_ptr() accesses user_cpus_ptr without any lock protection. Since sched_setaffinity() can be invoked from another process the process being modified may be undergoing fork() at the same time. When racing with the clearing of user_cpus_ptr in __set_cpus_allowed_ptr_locked() it can lead to user-after-free and possibly double-free in arm64 kernel.

Commit 8f9ea86fdf99 (\sched: Always preserve the user requested cpumask) fixes this problem as user_cpus_ptr once set will never be cleared in a task’s lifetime. However this bug was re-introduced in commit 851a723e45d1 (\sched: Always clear user_cpus_ptr in do_set_cpus_allowed()) which allows the clearing of user_cpus_ptr in do_set_cpus_allowed(). This time it will affect all arches.

Fix this bug by always clearing the user_cpus_ptr of the newly cloned/forked task before the copying process starts and check the user_cpus_ptr state of the source task under pi_lock.

Note to stable this patch won’t be applicable to stable releases. Just copy the new dup_user_cpus_ptr() function over.

Reference

https://git.kernel.org/stable/c/b22faa21b6230d5eccd233e1b7e0026a5002b287 https://git.kernel.org/stable/c/7b5cc7fd1789ea5dbb942c9f8207b076d365badc https://git.kernel.org/stable/c/87ca4f9efbd7cc649ff43b87970888f2812945b8