Data processor with a privileged state firewall and method therefore
First Claim
1. A data processor with a privileged state firewall, comprising:
- a central processing unit for executing a plurality of instructions and accessing memory in response to an execution of said plurality of instructions, said central processing unit characterized as operating in a first mode and a second mode which is at a lower privilege level than said first mode;
when in said first mode said central processing unit allows access to a protected system resource by one of said plurality of instructions;
when in said second mode said central processing unit inhibits access to said protected system resource by any of said plurality of instructions; and
a firewall circuit coupled to said central processing unit, for recognizing when said central processing unit changes from said second mode to said first mode, and for providing a firewall exception trigger signal if said central processing unit remains in said first mode continuously for a predetermined time.
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Accused Products
Abstract
A data processor (20) includes a firewall circuit (50) that monitors privilege level changes or transitions between privilege modes, such as from user mode and user space into supervisory or privileged mode and operating system space. The firewall circuit starts a timer (54) whenever a central processing unit (22) enters supervisor mode. If the timer (54) determines the passage of a predetermined time while the central processing unit remains continuously in supervisory mode without re-entering user mode, a predefined security policy is invoked. For example, the security policy may require at this point that the data processor (20) is to be reset. Different timer (54) time-out values and different security policies can be set for different types of privilege level changes. In one embodiment, a default time-out value provides protection for multiple types of privilege level changes.
164 Citations
21 Claims
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1. A data processor with a privileged state firewall, comprising:
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a central processing unit for executing a plurality of instructions and accessing memory in response to an execution of said plurality of instructions, said central processing unit characterized as operating in a first mode and a second mode which is at a lower privilege level than said first mode; when in said first mode said central processing unit allows access to a protected system resource by one of said plurality of instructions; when in said second mode said central processing unit inhibits access to said protected system resource by any of said plurality of instructions; and a firewall circuit coupled to said central processing unit, for recognizing when said central processing unit changes from said second mode to said first mode, and for providing a firewall exception trigger signal if said central processing unit remains in said first mode continuously for a predetermined time. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11)
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12. A method for providing a privileged state firewall in a data processor, comprising the steps of:
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recognizing when a central processing unit enters a first mode which is at a higher privilege level than a second mode, said central processing unit allowing access to a protected system resource by one of a plurality of instructions in said first mode and inhibiting access to said protected system resource by any of said plurality of instructions when in said second mode; measuring a passage of time while said central processing unit is continuously in said first mode; determining when said passage of time exceeds a predetermined time; and activating a firewall exception trigger signal in response to said step of determining. - View Dependent Claims (13, 14, 15, 16, 17)
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18. In a data processor comprising a central processing unit for executing a plurality of instructions and accessing memory in response to an execution of said plurality of instructions, said central processing unit characterized as operating in a first mode and a second mode which is at a lower privilege level than said first mode, and a timer for counting a predetermined number of periods of a clock signal and providing an output signal in response thereto, the improvement wherein:
the timer is part of a firewall circuit and the firewall circuit enables a counting of clock signals by said timer when said central processing unit changes from said second mode to said first mode, and resets said timer when said central processing unit changes from said first mode to said second mode. - View Dependent Claims (19, 20, 21)
Specification