I did note that musl would normally work just fine here. Though I appreciate that your other comment goes into much better detail about what was actually going on in the code, as my analysis basically stopped once I understood the high-level flow of what was going on.
Except OpenSSH qould have worked just fine if musl behaved correctly. Instead musl ignored the spec and tried to hack around badly written programs but ended up creating a security hole in a correctly written one!
> $ret = crypt($pw, $hash); $ret == $hash
musl is actually safe against this use. OpenSSL takes a bizarre code path, detects that $hash is too short, calls crypt($pw, $other_string), forgets that it detected that the hash was too short, and checks $hash == return value.
See my other comment.