Re: [PATCH v2] x86/cpu: Fix migration safety with X86_BUG_NULL_SEL

[Sean Christopherson <seanjc@google.com>]

On Mon, Oct 18, 2021, Borislav Petkov wrote:
> On Wed, Oct 13, 2021 at 03:22:30PM +0100, Jane Malalane wrote:
> Totally untested of course.
> 
> static void early_probe_null_seg_clearing_base(struct cpuinfo_x86 *c)
> {
> 	/*
> 	 * A hypervisor may sythesize the bit, but may also hide it
> 	 * for migration safety, so do not probe for model-specific
> 	 * behaviour when virtualised.
> 	 */
> 	if (cpu_has(c, X86_FEATURE_HYPERVISOR))

This isn't correct.  When running as a guest, the intended behavior is to fully
trust the CPUID.0x80000021 bit.  If bit 6 is set, yay, the hypervisor has told
the kernel that it will only ever run on hardware without the bug.  If bit 6 is
clear and HYPERVISOR is true, then the FMS crud can't be trusted because the
kernel _may_ run on affected hardware in the future even if the current underlying
hardware is not affected.

> 		return;
> 
> 	/* Zen3 CPUs advertise Null Selector Clears Base in CPUID. */
> 	if (c->extended_cpuid_level >= 0x80000021 && cpuid_eax(0x80000021) & BIT(6))
> 		return;
> 
> 	/* Zen2 CPUs also have this behaviour, but no CPUID bit. */
> 	if (c->x86 == 0x17 && check_null_seg_clears_base(c))
> 		return;
> 
> 	/* All the remaining ones are affected */
> 	set_cpu_bug(c, X86_BUG_NULL_SEG);
> }

...

> > @@ -1457,8 +1457,6 @@ static void generic_identify(struct cpuinfo_x86 *c)
> >  
> >  	get_model_name(c); /* Default name */
> >  
> > -	detect_null_seg_behavior(c);
> > -
> >  	/*
> >  	 * ESPFIX is a strange bug.  All real CPUs have it.  Paravirt
> >  	 * systems that run Linux at CPL > 0 may or may not have the
> 
> So this function is called on all x86 CPUs. Are you sure others besides
> AMD and Hygon do not have the same issue?
> 
> IOW, I wouldn't remove that call here.

I agree.  If the argument for this patch is that the kernel can be migrated to
older hardware, then it stands to reason that the kernel could also be migrated
to a different CPU vendor entirely.  E.g. start on Intel, migrate to Zen1, kaboom.