Converting `NSString` to a String Truncates at Null Bytes

Methods of NSString for conversion to a string may return a partial result.
Since they call CStr::from_ptr on a pointer to the string buffer, the
string is terminated at the first null byte, which might not be the end of the
string.

In addition to the vulnerable functions listed for this issue, the
implementations of Display, PartialEq, PartialOrd, and ToString
for NSString are also affected, since they call those functions.

Impact

Since NSString is commonly used as the type for paths by the Foundation
framework, null byte truncation might allow for easily bypassing file extension
checks. For example, if a file name is provided by a user and validated to have
one of a specific set of extensions, with validation taking place before
truncation, an attacker can add an accepted extension after a null byte (e.g.,
file.exe\0.txt). After truncation, the file name used by the application
would be file.exe.

It would be better to generate unique names for files, instead of using
user-provided names, but not all applications take this approach.

Example:

let string = NSString::from_str("null\0byte");
println!("{}", string);

That example only prints the string “null”.