cachefiles: add missing lock protection when polling

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: cachefiles: add missing lock protection when polling Add missing lock protection in poll routine when iterating xarray, otherwise: Even with RCU read lock held, only the slot of the radix tree is ensured to be pinned there, while the data structure (e.g. struct cachefiles_req) stored in the slot has no such guarantee. The poll routine will iterate the radix tree and dereference cachefiles_req accordingly. Thus RCU read lock is not adequate in this case and spinlock is needed here.

References

• https://git.kernel.org/stable/c/97cfd5e20ddc2e33e16ce369626ce76c9a475fd7
• https://git.kernel.org/stable/c/6bb6bd3dd6f382dfd36220d4b210a0c77c066651
• https://git.kernel.org/stable/c/8eadcab7f3dd809edbe5ae20533ff843dfea3a07
• https://git.kernel.org/stable/c/cf5bb09e742a9cf6349127e868329a8f69b7a014