Check CPUID instruction in Stage0 #VC handler (project-oak#4921)

We want to make sure that the instruction pointer in a #VC exception really pointed to a CPUID instruction since it is the only #VC exception type we support.

Ref b/330197837

stage0_bin/src/asm/boot.s

@@ -43,7 +43,10 @@ gp_handler:
 vc_handler:
  pop %ebx # get the error code
  cmp $0x72, %ebx # is this about CPUID?
- jne 2f # if not, skip ahead
+ jne 2f # if not, skip ahead and crash
+ mov (%esp), %ebx # get the instruction pointer
+ cmpw $0xa20f, (%ebx) # was this really a CPUID instruction?
+ jne 2f # if not it might be injected by the hypervisor, skip ahead and crash
  cmp $0x0, %ecx # are we asked for a CPUID subleaf?
  jne 2f # if yes, skip ahead, as we don't support subleaves
  # Use the GHCB MSR protocol to request one page of CPUID information. The protocol itself is