KVM: x86: Emulation of MOV-sreg to memory uses incorrect size

In x86, you can only MOV-sreg to memory with either 16-bits or 64-bits size.
In contrast, KVM may write to 32-bits memory on MOV-sreg. This patch fixes KVM
behavior, and sets the destination operand size to two, if the destination is
memory.

When destination is registers, and the operand size is 32-bits, the high
16-bits in modern CPUs is filled with zero.  This is handled correctly.
Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -3207,6 +3207,8 @@ static int em_mov_rm_sreg(struct x86_emulate_ctxt *ctxt)
 		return emulate_ud(ctxt);
 
 	ctxt->dst.val = get_segment_selector(ctxt, ctxt->modrm_reg);
+	if (ctxt->dst.bytes == 4 && ctxt->dst.type == OP_MEM)
+		ctxt->dst.bytes = 2;
 	return X86EMUL_CONTINUE;
 }