[PATCH 0/4] x86: fix free_thread_info() with uninitalized thread_info

[Akinobu Mita <akinobu.mita@gmail.com>]

This patch series fixes a problem described below. The actual fix is only
made by the patch 3/4. The rest of patches help it make simple and there
is no actual behavioral change.

x86 arch specific free_thread_info() accesses thread_info->task to call
free_thread_xstate(). But the thread_info may not be initialized yet.
So invalid pointer derefence may happen in free_thread_xstate().

It happens in the following scenario in dup_task_struct()

1. call alloc_task_struct() to allocate empty task_struct
2. call alloc_thread_info() to allocate empty thread_info
3. call arch_dup_task_struct()

x86 arch specific arch_dup_task_struct() copies task_struct from source
task_struct. it also allocates empty xstate and copy from source if
source task_struct has ->thread.xstate.

If the xstate allocation failed, arch_dup_task_struct() returns error.

4. call free_thread_info() to deallocate thread_info

x86 arch specific free_thread_info() calls free_thread_xstate() with
thread_info->task. But the thread_info is not initialized yet.