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

[Sohil Mehta]

On 1/11/2023 4:37 PM, Dave Hansen wrote:
> 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.
> 
> The only reason that it happens to work is that STAC/CLAC apparently
> actually continue to work even if CR4.SMAP==0.
> 

It seems this is by design. I was trying to experiment with
clearcpuid=smap on an older platform (KBL). I noticed that even if
CR4.SMAP==0 the STAC/CLAC instructions do not fault.

The STAC instruction operation in the SDM also suggests that it may be
intentional. It does *not* list CR4.SMAP=0 as a reason for #UD.

#UD	If the LOCK prefix is used.
	If the CPL > 0.
	If CPUID.(EAX=07H, ECX=0H):EBX.SMAP[bit 20] = 0.


> I'm actually a _bit_ surprised by this, but I bet there's a good reason
> for it.
> 

I would love to find out the actual reasoning behind this. I'll try to
poke some of the architects internally.

> In any case, please just make LASS dependent on SMAP.  It's the right
> thing to do on several levels.

Anyway, the end result still remains the same. We should still make LASS
dependent on SMAP since:

1) The spec says that LASS enforcement only happens when SMAP is enabled.
"A supervisor-mode data access causes a LASS violation only if
supervisor-mode access protection is enabled (because CR4.SMAP = 1)"

2) In the extremely improbably case that LASS is available but SMAP is
not available on the hardware, the STAC and CLAC instructions will fault
due to the missing SMAP CPUID bit.

-Sohil