The NX bit is not just about security

A developer building a bare-metal ARM64 hypervisor for postmarketOS on a MediaTek MT6735 phone tracked down a months-long boot lockup bug caused by speculative instruction fetches to an unmapped bootrom address at 0x0. After ruling out register corruption, icache/dcache coherency, kernel panics, and CPU errata, the fix turned out to be that marking memory as Device only blocks speculative data accesses on ARM - speculative instruction fetches require the memory to be separately marked non-executable (NX). Once the hypervisor's memory mappings were marked non-executable, the phone booted reliably to Android.

Source: https://purplesyringa.moe/blog/guest/the-nx-bit-is-not-just-about-security. 8 Sync News only summarizes and links out; content copyright belongs to the authors and original sources.