The IRQ stack lives in percpu space, so an IRQ handler that overflows it
will overwrite other data structures.

Use vmap() to remap the IRQ stack so that it will have the usual guard
pages that vmap()/vmalloc() allocations have. With this, the kernel will
panic immediately on an IRQ stack overflow.

[ tglx: Move the map code to a proper place and invoke it only when a CPU
  	is about to be brought online. No point in installing the map at
  	early boot for all possible CPUs. Fail the CPU bringup if the vmap()
  	fails as done for all other preparatory stages in CPU hotplug. ]
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Nicolai Stange <nstange@suse.de>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20190414160146.363733568@linutronix.de