Fix embarassing, basic path traversal attack

Fix the most embarassing kind of path traversal vulnerability
imaginable for such a tool.

You could simply run raou ../../../../tmp/evil_entry

The C version contained various check on the config dir and its
entries which would have prevented this attack. In this port,
the checking functions were deemed unnecessary, as they
did lots of redundant checks too. Unfortunately, I missed this
trivial attack when I decided not to port them.

At the plus side, I found this now myself while sleep-deprived, so
there may be some hope for me after all.

Also, you should not use some non-released software from some
guys git ;-)

src/main.rs

@@ -5,6 +5,7 @@ use std::fs::File;
 use std::io::BufRead;
 use std::io::BufReader;
 use std::io::{Error, ErrorKind};
+use std::fs;
 
 extern crate libc;
 use libc::passwd;
@@ -240,9 +241,16 @@ fn create_execv_args(entry: &Entry, cmdargs: &Vec<String>) -> Vec<*const libc::c
     return args;
 }
 fn exec(entryname: &str, cmdargs: &Vec<String>) -> std::io::Result<()> {
-    let mut filepath: String = String::from("/etc/raou.d/");
-    filepath = filepath + entryname;
+    let basedir: String = String::from("/etc/raou.d/");
+    let filepath: String = basedir.to_string() + entryname;
 
+    let realpath = fs::canonicalize(&filepath)?;
+    if !realpath.starts_with(basedir) {
+        return Err(std::io::Error::new(
+            ErrorKind::InvalidInput,
+            "Specified entry is outside base directory",
+        ));
+    }
     if !std::path::Path::new(&filepath).exists() {
         return Err(std::io::Error::new(
             ErrorKind::NotFound,