Part 1 of this series described the
copy-on-write (COW) mechanism used to avoid unnecessary copying of pages in
memory, then went into the details of a bug in that mechanism that
could result in the disclosure of sensitive data. A patch written by Linus
Torvalds and merged for the 5.8 kernel appeared to fix that problem without
unfortunate side effects elsewhere in the system. But COW is a complicated
beast and surprises are not uncommon; this particular story was nowhere
near as close to an end as had been thought.

Source: LWN.net – [$] Patching until the COWs come home (part 2)