Intel IA-32 User Manual

Page 142

4-12 Vol. 3A

PROTECTION

The procedure in code segment D should be able to access data segment E because code
segment D’s CPL is numerically less than the DPL of data segment E. However, the RPL
of segment selector E3 (which the code segment D procedure is using to access data
segment E) is numerically greater than the DPL of data segment E, so access is not
allowed. If the code segment D procedure were to use segment selector E1 or E2 to access
the data segment, access would be allowed.

As demonstrated in the previous examples, the addressable domain of a program or task varies
as its CPL changes. When the CPL is 0, data segments at all privilege levels are accessible; when
the CPL is 1, only data segments at privilege levels 1 through 3 are accessible; when the CPL is
3, only data segments at privilege level 3 are accessible.

The RPL of a segment selector can always override the addressable domain of a program or task.
When properly used, RPLs can prevent problems caused by accidental (or intensional) use of
segment selectors for privileged data segments by less privileged programs or procedures.

It is important to note that the RPL of a segment selector for a data segment is under software
control. For example, an application program running at a CPL of 3 can set the RPL for a data-
segment selector to 0. With the RPL set to 0, only the CPL checks, not the RPL checks, will
provide protection against deliberate, direct attempts to violate privilege-level security for the
data segment. To prevent these types of privilege-level-check violations, a program or procedure
can check access privileges whenever it receives a data-segment selector from another proce-
dure (see Section 4.10.4, “Checking Caller Access Privileges (ARPL Instruction)”).

Figure 4-5. Examples of Accessing Data Segments From Various Privilege Levels

Data

Lowest Privilege

Highest Privilege

Segment E

CPL=1

CPL=3

CPL=0

DPL=2

CPL=2

Segment Sel. E3

RPL=3

Segment Sel. E1

RPL=2

Segment Sel. E2

RPL=1

Code

Segment C

Code

Segment A

Code

Segment B

Code

Segment D