*-property of Biba model

Question

In Dieter Gollmann's Computer security book, under Biba model, *-property is explained as follows.

If subject s can read (observe) object o, then s can have write access to some other object p only if fo(p)<=fo(o).

I expect that the ultimate idea of this statement must be simply 'no read down' or something like that (according to resources in internet). But I don't understand how this statement express that idea. Can someone please explain?

Answer 1

According to the book you specified, it doesn't explicitly state 'no read down' (Although some sources specify that). In BIBA model the main consideration is integrity. It prevent the contamination of higher levels by lower levels by only allowing information to flow downwards. According to that, if a subject read a specific object-o and tries to write the content to another object-p and fo(p)>=fo(o) that is information flowing upwards. (Sometimes object-o may contain viral data and will be written to a higher level object). So this flow should not be allowed. That is why fo(p)<=fo(o) constraint is there as the integrity *-property.