Contributed by rueda on from the ROPper-stopper dept.
Implement MAP_STACK option for mmap(). Synchronous faults (pagefault and syscall) confirm the stack register points at MAP_STACK memory, otherwise SIGSEGV is delivered. sigaltstack() and pthread_attr_setstack() are modified to create a MAP_STACK sub-region which satisfies alignment requirements. Observe that MAP_STACK can only be set/cleared by mmap(), which zeroes the contents of the region -- there is no mprotect() equivalent operation, so there is no MAP_STACK-adding gadget. This opportunistic software-emulation of a stack protection bit makes stack-pivot operations during ROPchain fragile (kind of like removing a tool from the toolbox).
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