Overwrite detection for kernel text and read-only data

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Overwrite detection for kernel text and read-only data
Tim Bird, Sony Mobile


In embedded systems, drivers or other subsystems can easily (mistakenly) overwrite kernel text or kernel read-only area. It can be very difficult to debug who is overwriting kernel.

It would be nice to have some mechanism to detect kernel overwriting or corruption by setting a write-protect attribute in page tables for kernel text or read-only data. x86 already has this feature in the form of CONFIG_DEBUG_RODATA. This project would consist of providing support for this feature in ARM, as well as possibly creating exceptions for code which needs to modify the kernel text at runtime, such as Kprobe or ftrace.

A significant difficulty is that the kernel memory for ARM is currently mapped by section (that is, using 1MB sections). This means that small memory areas cannot be individually re-mapped RO on page boundaries. If the kernel has code which must be writable for some reason, then with that current mapping, at least a 1MB section would be used for that writable code. Likely, the linking sections of the source would have to be modified to support RO/RW attributes, to coalesce the sections into correct categories.

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