For reference, crossposting:

I pushed 00c67375b17f4a4cfad53399d1918f2e7eba2c7d to core-updates. Your
patch. Thank you for it. Let's watch for upstream zstd fix also.

I pushed 9feef62b73e284e106717a386624d6da90750a3d to master.

Ubuntu released a patch in the mean time, so while we couldnt make such
patch in a timely manner because the backport was non-trivial and
security-sensitive also didnt want to risk failing to fix the flaw
because I don't have much expertise on it, Ubuntu now has done that
work and we can just use it.

Léo