CVE-2024-38613: m68k: Fix spinlock race in kernel thread creation

In the Linux kernel, the following vulnerability has been resolved:

m68k: Fix spinlock race in kernel thread creation

Context switching does take care to retain the correct lock owner across the switch from 'prev' to 'next' tasks. This does rely on interrupts remaining disabled for the entire duration of the switch.

This condition is guaranteed for normal process creation and context switching between already running processes, because both 'prev' and 'next' already have interrupts disabled in their saved copies of the status register.

The situation is different for newly created kernel threads. The status register is set to PSS in copythread(), which does leave the IPL at 0. Upon restoring the 'next' thread's status register in switchto() aka resume(), interrupts then become enabled prematurely. resume() then returns via retfromkernelthread() and scheduletail() where run queue lock is released (see finishtaskswitch() and finishlockswitch()).

A timer interrupt calling schedulertick() before the lock is released in finishtaskswitch() will find the lock already taken, with the current task as lock owner. This causes a spinlock recursion warning as reported by Guenter Roeck.

As far as I can ascertain, this race has been opened in commit 533e6903bea0 ("m68k: split retfromfork(), simplify kernelthread()") but I haven't done a detailed study of kernel history so it may well predate that commit.

Interrupts cannot be disabled in the saved status register copy for kernel threads (init will complain about interrupts disabled when finally starting user space). Disable interrupts temporarily when switching the tasks' register sets in resume().

Note that a simple oriw 0x700,%sr after restoring sr is not enough here - this leaves enough of a race for the 'spinlock recursion' warning to still be observed.

Tested on ARAnyM and qemu (Quadra 800 emulation).