KVM: x86: fix emulation of "movzbl %bpl, %eax"

When I was looking at RHEL5.9's failure to start with
unrestricted_guest=0/emulate_invalid_guest_state=1, I got it working with a
slightly older tree than kvm.git.  I now debugged the remaining failure,
which was introduced by commit 660696d1 (KVM: X86 emulator: fix
source operand decoding for 8bit mov[zs]x instructions, 2013-04-24)
introduced a similar mis-emulation to the one in commit 8acb4207 (KVM:
fix sil/dil/bpl/spl in the mod/rm fields, 2013-05-30).  The incorrect
decoding occurs in 8-bit movzx/movsx instructions whose 8-bit operand
is sil/dil/bpl/spl.

Needless to say, "movzbl %bpl, %eax" does occur in RHEL5.9's decompression
prolog, just a handful of instructions before finally giving control to
the decompressed vmlinux and getting out of the invalid guest state.

Because OpMem8 bypasses decode_modrm, the same handling of the REX prefix
must be applied to OpMem8.

Reported-by: Michele Baldessari <michele@redhat.com>
Cc: stable@vger.kernel.org
Cc: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>

arch/x86/kvm/emulate.c

@@ -4117,7 +4117,10 @@ static int decode_operand(struct x86_emulate_ctxt *ctxt, struct operand *op,
 	case OpMem8:
 		ctxt->memop.bytes = 1;
 		if (ctxt->memop.type == OP_REG) {
-			ctxt->memop.addr.reg = decode_register(ctxt, ctxt->modrm_rm, 1);
+			int highbyte_regs = ctxt->rex_prefix == 0;
+
+			ctxt->memop.addr.reg = decode_register(ctxt, ctxt->modrm_rm,
+					       highbyte_regs);
 			fetch_register_operand(&ctxt->memop);
 		}
 		goto mem_common;