KVM: x86: Inject #GP if guest attempts to toggle CR4.LA57 in 64-bit mode

Inject a #GP on MOV CR4 if CR4.LA57 is toggled in 64-bit mode, which is
illegal per Intel's SDM:

  CR4.LA57
    57-bit linear addresses (bit 12 of CR4) ... blah blah blah ...
    This bit cannot be modified in IA-32e mode.

Note, the pseudocode for MOV CR doesn't call out the fault condition,
which is likely why the check was missed during initial development.
This is arguably an SDM bug and will hopefully be fixed in future
release of the SDM.

Fixes: fd8cb433 ("KVM: MMU: Expose the LA57 feature to VM.")
Cc: stable@vger.kernel.org
Reported-by: NSebastien Boeuf <sebastien.boeuf@intel.com>
Signed-off-by: NSean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200703021714.5549-1-sean.j.christopherson@intel.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -975,6 +975,8 @@ int kvm_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
 	if (is_long_mode(vcpu)) {
 		if (!(cr4 & X86_CR4_PAE))
 			return 1;
+		if ((cr4 ^ old_cr4) & X86_CR4_LA57)
+			return 1;
 	} else if (is_paging(vcpu) && (cr4 & X86_CR4_PAE)
 		   && ((cr4 ^ old_cr4) & pdptr_bits)
 		   && !load_pdptrs(vcpu, vcpu->arch.walk_mmu,