Shopify: mruby-engine: UAF in MRubyEngine#initialize enables local RCE

Summary Double-init of MRubyEngine frees engine + unmaps mspace, but leaves Ruby DATAPTR dangling. Kernel reuses freed VA via mmapMAPFIXED. Attacker forges memrubyengine struct + mrbstate in reclaimed region, points mrbstate-allocf at libc.system, arranges bytes of mrbstate to also spell a shell...