Secure system firmware by disabling read access to firmware ROM
View Patent ↗Systems, methods and software that use a programmable lock bit that disables read access to a firmware ROM until the CPU is reset or enters into a secure CPU mode. The present invention provides for secure system firmware by preventing access to and reading of the contents of the firmware ROM.
1. A system that provides for secure system firmware, comprising:
a central processing unit (CPU);
a firmware read only memory (ROM) coupled to the CPU that stores firmware of the system;
lock logic that programmatically locks an enable/disable configuration bit so that it cannot be changed once it is set;
detect logic that detects read accesses to the firmware ROM and returns a predetermined value instead of the contents of the firmware ROM, or that acts as if the firmware ROM is not present;
unlock logic that unlocks the enable/disable configuration bit so that it can be set once a system reset is detected;
ignore logic that ignores the status of the enable/disable configuration bit when the CPU is in a predetermined operating mode; and
software that asserts the lock bit.
2. The system recited in claim 1 wherein the software asserts the lock bit prior to invoking a non-secure environment.
3. The system recited in claim 2 wherein the non-secure environment comprises a general-purpose operating system.
4. The system recited in claim 1 wherein the configuration bit comprises a plurality of configuration bits.
5. A method that provides for secure firmware in a system having a central processing unit (CPU) and a firmware read only memory (ROM) coupled to the CPU that stores system firmware, comprising:
setting a configuration bit that prevents read access to the firmware ROM unless the system is in a predetermined secure operating mode prior to a time when control of the system is to be transferred from the system firmware to an operating system; and
transferring control of the system from the system firmware to the operating system.
6. The method recited in claim 5 wherein the configuration bit, once set, prevents read access by the CPU to the contents of the firmware ROM until the system is reset.
7. The method recited in claim 5 wherein the configuration bit, once set, prevents read access by the CPU to the contents of the firmware ROM until the system enters a secure operating mode.
8. The method recited in claim 7 wherein the secure operating mode comprises system management mode.
9. The method recited in claim 5 wherein attempts to read the contents of the firmware ROM returns a predetermined value or an undefined value.
10. Apparatus, that provides for secure system firmware in a system including a central processing unit (CPU) and a firmware read only memory (ROM) coupled to the CPU that stores the system firmware, comprising:
detecting means for detecting read access to the firmware ROM and returns a predetermined value instead of the contents of the firmware ROM, or that acts as if the firmware ROM is not present;
unlock logic that unlocks an enable/disable configuration bit so that it can be set once a system reset is detected;
ignore logic that ignores the status of the enable/disable configuration bit when the CPU is in a predetermined operating mode; and
software comprising:
a code segment that executes the firmware when the system is reset;
a code segment that tests and initializes hardware of the system;
a code segment that initializes a run time environment used by an operating system;
a code segment that asserts the configuration bit to prevent access to the firmware ROM unless the system is in a predetermined secure operating mode; and
a code segment that transfers control of the system from the system firmware to the operating system.
11. The apparatus recited in claim 10 wherein the detecting means comprises:
detect logic that detects read accesses to the firmware ROM and returns a predetermined value instead of the contents of the firmware ROM, or that acts as if the firmware ROM is not present.
12. The apparatus recited in claim 10 wherein the detecting means comprises:
a code segment that detects read accesses to the firmware ROM and returns a predetermined value instead of the contents of the firmware ROM, or that acts as if the firmware ROM is not present.
13. The apparatus recited in claim 10 wherein the secure operating mode comprises system management mode.
14. A system, comprising: a processor;
nonvolatile memory, coupled to the processor, including instructions that when executed by the processor, cause the processor to:
lock an enable/disable configuration bit so that it cannot be changed once it is set,
detect read accesses to the nonvolatile memory and return a predetermined value instead of the contents of the nonvolatile memory, or act as if the nonvolatile memory is not present,
unlock the enable/disable configuration bit so that it can be set once a system reset is detected,
ignore the status of the enable/disable configuration bit when the CPU is in a predetermined operating mode, and
assert the lock bit.
15. The system of claim 14 , wherein the instructions cause the processor to assert the lock bit prior to invoking a non-secure environment.
16. The system of claim 15 , wherein the non-secure environment further includes a general-purpose operating system.
17. The system of claim 14 , wherein the configuration bit comprises a plurality of configuration bits.