Hi Andy,
On Mon, Apr 27, 2020 at 10:37:41AM -0700, Andy Lutomirski wrote:
> 1. Use IST for #VC and deal with all the mess that entails.
With the removal of IST shifting I wonder what you would suggest on how
to best implement an NMI-safe IST handler with nesting support.
My current plan is to implement an IST handler which switches itself off
the IST stack as soon as possible, freeing it for re-use.
The flow would be roughly like this upon entering the handler;
build_pt_regs();
RSP = pt_regs->sp;
if (RSP in VC_IST_stack)
error("unallowed nesting")
if (RSP in current_kernel_stack)
RSP = round_down_to_8(RSP)
else
RSP = current_top_of_stack() // non-ist kernel stack
copy_pt_regs(pt_regs, RSP);
switch_stack_to(RSP);
To make this NMI safe, the NMI handler needs some logic too. Upon
entering NMI, it needs to check the return RSP, and if it is in the #VC
IST stack, it must do the above flow by itself and update the return RSP
and RIP. It needs to take into account the case when PT_REGS is not
fully populated on the return side.
Alternativly the NMI handler could safe/restore the contents of the #VC
IST stack or just switch to a special #VC-in-NMI IST stack.
All in all it could get complicated, and imho shift_ist would have been
simpler, but who am I anyway...
Or maybe you have a better idea how to implement this, so I'd like to
hear your opinion first before I spend too many days implementing
something.
Regards,
Joerg
