[$] Patching until the COWs come home (part 2)

Part 1 of this series described thecopy-on-write (COW) mechanism used to avoid unnecessary copying of pages inmemory, then went into the details of a bug in that mechanism thatcould result in the disclosure of sensitive data. A patch written by LinusTorvalds and merged for the 5.8 kernel appeared to fix that problem withoutunfortunate side effects elsewhere in the system. But COW is a complicatedbeast and surprises are not uncommon; this particular story was nowherenear as close to an end as had been thought.