Secure Boot ROM Emulation
Secure boot ROM emulation with locking storage device. A locking storage device is provided by combining a nonvolatile memory device such as flash or EEPROM with one-shot locking logic which write enables at least a portion of the nonvolatile memory device upon power cycling of the overall digital device. This write enable is cleared during the stage 1 bootloader process, thus providing a protected update interval for updating a stage 2 bootloader once per power cycle.
1 . A locking storage device in a digital computing device, the locking storage comprising:
a nonvolatile memory device for storing at least a second stage bootloader for the digital computing device, and
locking logic connected to the nonvolatile memory device for write protecting at least a portion of the nonvolatile memory device holding the second stage bootloader, the locking logic responsive to a first signal from the digital computing device allowing write access to at least a portion of the nonvolatile memory device, and responsive to a second signal for clearing the write access to the nonvolatile memory device, where the first signal is generated once upon powering up the digital computing device.
2 . The locking storage device of claim 1 where a portion of the nonvolatile memory device is protected by the locking logic.
3 . The locking storage device of claim 1 where the entire nonvolatile memory device is protected by the locking logic.
4 . The locking storage device of claim 1 where the nonvolatile memory device stores at least a first second-stage bootloader and a second second-stage bootloader, and the locking logic write protects at least the first second-stage bootloader and the second second-stage bootloader.
5 . A method of using a locking storage device in a digital computing device, the method comprising:
upon powering up the digital device, executing a first stage bootloader stored in nonvolatile memory,
the first stage bootloader checking for the presence of an update to the second stage bootloader stored in the locking storage device, the update stored in a nonvolatile memory device,
validating the update image,
if the update image is successfully validated, copying the update image to the second stage bootloader storage in the locking storage device,
generating the second signal clearing write access to the locking storage device containing the second stage bootloader, and
transferring control to the second stage bootloader.
6 . The method of claim 5 where the first stage bootloader and the second stage bootloader are stored in the same nonvolatile memory device.
7 . The method of claim 5 where the first stage bootloader, the second stage bootloader, and the update image are stored in the same nonvolatile memory device.
8 . The method of claim 5 where the first stage bootloader and the second stage bootloader are stored in the same nonvolatile memory device, the second stage bootloader is stored in locking storage in the nonvolatile memory device, and the first stage bootloader is always write-protected.
9 . The method of claim 5 where the first stage bootloader and the second stage bootloader are stored in the same nonvolatile memory device, and both the first stage bootloader and the second stage bootloader are stored in locking storage.