Re: [PATCH 3/7] x86/cpu: Disable kernel LASS when patching kernel alternatives

[Chen, Yian]

On 1/11/2023 4:37 PM, Dave Hansen wrote:
> On 1/11/23 16:27, Chen, Yian wrote:
> > > It seems we are implicitly relying on the on stac() and clac()
> > > calls that are added for SMAP. Have you tried running with SMAP
> > > disabled i.e "clearcpuid=smap"?
> > Yes, I tested with clearcpuid=smap.
> It works by accident, then.
>
> clearcpuid=smap means that the kernel should be running as if
> CPUID.(EAX=07H,ECX=0H):EBX.SMAP[bit 20]==0.  STAC/CLAC should #UD in
> that case.
It could be something wrong in my Simics simulation environment.

> The only reason that it happens to work is that STAC/CLAC apparently
> actually continue to work even if CR4.SMAP==0.
>
> I'm actually a _bit_ surprised by this, but I bet there's a good reason
> for it.
>
> In any case, please just make LASS dependent on SMAP.  It's the right
> thing to do on several levels.
Sure, I will add the dependency.