Re: [PATCH v3 1/1] x86/fred: Fix INT80 emulation for FRED

[H. Peter Anvin]

On 4/17/24 08:55, Xin Li wrote:
> On 4/17/2024 8:07 AM, H. Peter Anvin wrote:
> > On 4/17/24 04:02, Nikolay Borisov wrote:
> > > On 17.04.24 г. 9:30 ч., Xin Li (Intel) wrote:
> > > > 4) int80_emulation() does a CLEAR_BRANCH_HISTORY, which is likely 
> > > > >     overkill for new x86 CPU implementations that support FRED.
> > >
> > > Well, that's a bit of an overstatement/speculation, because 
> > > clear_branch_history will only be effective if the machine is 
> > > susceptible to the given bug and there isn't a better options (i.e 
> > > using a hardware bit controlling the respective aspect of the CPU).
> > > >
> >
> > It would seem like a huge stretch to expect that a FRED-capable CPU 
> > would not have such a facility. This is a matter of establishing a 
> > baseline for FRED-capable hardware.
> >
> > It would make more sense to me to add it if we turn out to need it; 
> > note that FRED code is currently only enabled on demand, in order to 
> > defend against bit rot until we have physical hardware.
> >
> > Now, if this is still desired, it *probably* belongs better in either 
> > fred_intx()/fred_other() or asm_fred_entrypoint_user, depending on if 
> > this ought to be done for all entries from userspace or only system 
> > calls.
>
> My bad that I didn't make this a good comment, even neglected that 
> clear_branch_history is just a nop on machines w/o such security issues.
>
>
> So how about?
>
> int80_emulation() does a CLEAR_BRANCH_HISTORY, which is IDT-specific.
> While FRED will likely take a different approach when it is needed:

s/when/if it is ever needed/

> it *probably* belongs in either fred_intx()/fred_other() or
> asm_fred_entrypoint_user(), depending on if this ought to be done for 
> all entries from userspace or only system calls.