Re: UBSAN: shift-out-of-bounds in kvm_vcpu_after_set_cpuid

[Sean Christopherson]

On Tue, Jan 12, 2021, Paolo Bonzini wrote:
> On 12/01/21 00:01, Sean Christopherson wrote:
> > > Perhaps cpuid_query_maxphyaddr() should just look at the low 5 bits of
> > > CPUID.80000008H:EAX?
> 
> The low 6 bits I guess---yes, that would make sense and it would have also
> fixed the bug.

No, that wouldn't have fixed this specific bug.  In this case, the issue was
CPUID.80000008H:AL == 0; masking off bits 7:6 wouldn't have changed anything.

And, masking bits 7:6 is architecturally wrong.  Both the SDM and APM state that
bits 7:0 contain the number of PA bits.

KVM could reject guest.MAXPA > host.MAXPA, but arbitrarily dropping bits would
be wrong.