(PUP-11122) Ensure reg values are null terminated

[joshcooper]

RegQueryValueExW doesn't guarantee the returned buffer for the lpData
parameter is null terminated, so ensure that it is.

When retrieving a registry value of type REG_SZ or REG_EXPAND_SZ extend the
buffer by 1 wchar (2 bytes) so we can always write a wchar null terminator that
is guaranteed not to overwrite user data.

The resulting wchar array is then guaranteed to be properly wchar null
terminated:

\u0041
\u0042
\u0000

which when UTF-16LE encoded will result in the byte array:

\x41 \x00
\x42 \x00
\x00 \x00

If Windows does null terminate, then we will add an additional wchar null
terminator, which won't hurt anything.

The same applies to REG_MULTI_SZ, except it is supposed to have two wchar null
terminators, for a total of 4 bytes. One terminator is for the last string in
the array, and one terminator is for the entire array. For example, if the array
contains the strings ['A', 'B'] and Windows does not null terminate the
lpData buffer, then the resulting UTF-16LE encoded byte array will contain:

\x41 \x00
\x00 \x00
\x42 \x00
\x00 \x00
\x00 \x00

[joshcooper]

This should be backported in 7.x

[joshcooper]

If Windows does correctly terminate the string and we add extra padding, then I wanted to verify it wouldn't cause issues, for example, when converting the REG_MULTI_SZ value to an array. First we convert the UTF-16LE string to UTF-8 in FFI::Pointer#read_wide_string, and then later, we split the UTF-8 string to an array using split(/\0/). This shows how extra padding is ignored and the correct two element array is generated:

irb(main):005:0> WNUL = "\0".encode('UTF-16LE')
=> "\u0000"
...
irb(main):014:0> (["a","b"].map {|s| s.encode('UTF-16LE')}.join(WNUL) + WNUL + WNUL + WNUL + WNUL)
=> "a\u0000b\u0000\u0000\u0000\u0000"
irb(main):015:0> (["a","b"].map {|s| s.encode('UTF-16LE')}.join(WNUL) + WNUL + WNUL + WNUL + WNUL).encode('UTF-8')
=> "a\u0000b\u0000\u0000\u0000\u0000"
irb(main):016:0> (["a","b"].map {|s| s.encode('UTF-16LE')}.join(WNUL) + WNUL + WNUL + WNUL + WNUL).encode('UTF-8').split(/\0/)
=> ["a", "b"]

[github-actions]

Successfully created backport PR for 7.x:

• [Backport 7.x] (PUP-11122) Ensure reg values are null terminated #9219