Question1 1. Memory protection mechanisms provided in the x86 architecture help

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Question1 1. Memory protection mechanisms provided in the x86 architecture help us isolate the trusted computing base (TCB) from untrusted application code. This is achieved because the hardware checks a variety of bits at the time segment selector registers are loaded when the processor executes in the protected mode. These include current privilege level (CPL), descriptor privilege level (DPL) and requestor privilege level (RPL). Answer if the following statements are true or false. Provide a brief explanation to justify your answer. (25 pts., 5 pts. for each part) a. The CPL bits are always the same as the DPL of the currently executing code segment. b. The DPL of a data segment accessible to the currently executing code can be smaller than the CPL. c. If a non-conforming kernel code segment is being executed currently, CPL may be higher than O. d. RPL can be used to ensure that kernel code (e.g., a driver) does not have access to a data segment with DPL = 2 when the code is being executed on behalf of application code with DPL =3. e. A single segment contains all code and data. In this case, it is possible to separate user and kernel code/data using the page protection level mechanism of x86

Explanation / Answer

a) This is true. The cpl bits are always equal to the dpl when the processor is in code segement

b) This is false i may be even to CPL also.

c) Yes it is true.

d) Yes it is true. we can ensure this with the help of a driver.

e) No it is not possible. Usually separate segements are there for user and kernel data.

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