Re: [PATCH 05/39] x86/entry/32: Unshare NMI return path

[Joerg Roedel <joro@8bytes.org>]

On Thu, Jul 12, 2018 at 01:53:19PM -0700, Andy Lutomirski wrote:
> > On Jul 11, 2018, at 4:29 AM, Joerg Roedel <joro@8bytes.org> wrote:
> > NMI will no longer use most of the shared return path,
> > because NMI needs special handling when the CR3 switches for
> > PTI are added.
> 
> Why?  What would go wrong?
> 
> How many return-to-usermode paths will we have?  64-bit has only one.

In the non-NMI return path we make a decission on whether we return to
user-space or kernel-space and do different things based on that. For
example, when returning to user-space we call
prepare_exit_to_usermode(). With the CR3 switches added later we also
unconditionally switch to user-cr3 when we are in the return-to-user
path.

The NMI return path does not need any of that, as it doesn't call
prepare_exit_to_usermode() even when it returns to user-space. It
doesn't even care where it returns to. It just remembers stack and cr3
on entry in callee-safed registers and restores that on exit. This works
in the NMI path because it is pretty simple and doesn't do any fancy
work on exit.

While working on a previous version I also tried to store stack and cr3
in a callee-safed register and restore that on exit again, but it didn't
work, most likley because something in-between overwrote one of the
registers. I also found it a bit fragile to make make two registers
untouchable in the whole entry-code. It doesn't make future changes
simpler or more robust.

So long story short, the NMI path can be simpler wrt. stack and cr3
handling as the other entry/exit points, and therefore it is handled
differently.

Regards,

	Joerg