CVE-2022-48892

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 tasks 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 wont be applicable to stable releases. Just copy the new dup_user_cpus_ptr() function over.

Weakness

The product calls free() twice on the same memory address, potentially leading to modification of unexpected memory locations.

Affected Software

Potential Mitigations

References

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