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

[Sean Christopherson <seanjc@google.com>]

On Mon, Jan 11, 2021, Jim Mattson wrote:
> It looks like userspace can possibly induce this by providing guest
> CPUID information with a "physical address width" of 64 in leaf
> 0x80000008.

It was actually the opposite, where userspace provides '0' and caused '63 - 0 + 1'
to overflow.  KVM controls the upper bound, and rsvd_bits() explicitly handles
'end < start', so an absurdly large maxpa is handled correctly.

Aleady fixed by Paolo in commit 2f80d502d627 ("KVM: x86: fix shift out of bounds
reported by UBSAN").

> Perhaps cpuid_query_maxphyaddr() should just look at the low 5 bits of
> CPUID.80000008H:EAX? Better would be to return an error for
> out-of-range values, but I understand that the kvm community's stance
> is that, in general, guest CPUID information should not be validated
> by kvm.

And rob Paolo of his crazy^Wbrilliant bit math shenanigans? :-D