Re: [PATCH v5 22/34] x86/fred: FRED initialization code

andrew.cooper3
2023-03-22T18:25:43Z

On 22/03/2023 2:22 am, Li, Xin3 wrote:
>> If there is no other concrete reason other than overflowing for assigning NMI and
>> #DB with a stack level > 0, #VE should also be assigned with a stack level > 0, and
>> #BP too. #VE can happen anytime and anywhere, so it is subject to overflowing too.
> With IDT, both #VE and #BP do not use IST, but NMI, #DB, #MC and #DF do.
>
> Let's keep this "secret" logic for now, i.e., not change the stack levels
> for #VE and #BP at this point. We can do "optimization", i.e., change them
> later :).

Fun fact.  #BP used to be IST, and used to share the same IST as #DF.

This was spoiled by CVE-2018-8897 and a MovSS-delayed breakpoint over
INT3, at which point hardware queued both a #BP and #DB on the same IST
stack and lost program state.

There's no need specific need for #BP to be IST to begin with, hence why
making it not-IST was the security fix.

~Andrew