Re: [RFC PATCH 1/4] x86/entry/nmi: Switch to the entry stack before switching to the thread stack

[Andy Lutomirski]

On Sat, Jun 19, 2021, at 3:51 PM, Thomas Gleixner wrote:
> On Tue, Jun 01 2021 at 14:52, Lai Jiangshan wrote:
> > From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> >
> > Current kernel has no code to enforce data breakpoint not on the thread
> > stack.  If there is any data breakpoint on the top area of the thread
> > stack, there might be problem.
> 
> And because the kernel does not prevent data breakpoints on the thread
> stack we need to do more complicated things in the already horrible
> entry code instead of just doing the obvious and preventing data
> breakpoints on the thread stack?

Preventing breakpoints on the thread stack is a bit messy: it’s possible for a breakpoint to be set before the address in question is allocated for the thread stack.

None of this is NMI-specific. #DB itself has the same problem.  We could plausibly solve it differently by disarming breakpoints in the entry asm before switching stacks. I’m not sure how much I like that approach.

> 
> Confused.
> 
> Thanks,
> 
>         tglx
>