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

[Thomas Gleixner]

On Sat, Jun 19 2021 at 20:13, Andy Lutomirski wrote:
> 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.

Bah.

> None of this is NMI-specific. #DB itself has the same problem.

Oh well.

> 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.

That's ugly and TBH in some sense is a breakpoint on the thread stack a
violation of noinstr. I rather see them prevented completely, but yes
that would have to be expanded to pretty much any variable which is
touched in noinstr sections. What a mess.

Thanks,

        tglx