What does the simple security (ss) property mean in the Bell-LaPadula model?

A.
No read up

B.
No write down

C.
No read down

D.
No write up

Explanation:
Three main rules are used and enforced in the Bell-LaPadula model:
The simple security (SS) rule, the *-property (star property) rule, and the strong star property rule. The simple
security rule states that a subject at a given security level cannot read data that reside at a higher security level.
The *-property rule (star property rule) states that a subject in a given security level cannot write information to
a lower security level. The simple security rule is referred to as the “no read up” rule, and the *-property rule is
referred to as the “no write down” rule.
The third rule, the strong star property rule, states that a subject that has read and write capabilities can only
perform those functions at the same security level; nothing higher and nothing lower. So, for a subject to be
able to read and write to an object, the clearance and classification must be equal.
Incorrect Answers:
B: The simple security rule is referred to as the “no read up” rule, not the “no write down” rule. The *-property
rule is referred to as the “no write down” rule.
C: The simple security rule is referred to as the “no read up” rule, not the “no read down” rule.
D: The simple security rule is referred to as the “no read up” rule, not the “no write up” rule.

Harris, Shon, All In One CISSP Exam Guide, 6th Edition, McGraw-Hill, New York, 2013, pp. 369-370