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

[Paolo Bonzini]

On 12/01/21 17:53, Sean Christopherson wrote:
> 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.

Right.

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

They cannot be higher than 52, therefore bits 7:6 are (architecturally) 
always zero.  In other words, I interpret "bit 7:0 contain the number of 
PA bits" as "you need not do an '& 63' yourself", which is basically the 
opposite of "bit 7:6 might be nonzero".  If masking made any difference, 
it would be outside the spec already.

In fact another possibility to avoid UB is to do "& 63" of both s and e 
in rsvd_bits.  This would also be masking bits 7:6 of the CPUID leaf, 
just done differently.

Paolo

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