fix: convert to cstrings in PyString::from_object

fixes #5005

This only fixes the API, and adds a test of the API, it does not
deprecate the API or introduce a version which takes `&CStr` directly,
this can be done later.

Author: Dr-Emann

newsfragments/5008.fixed.md

@@ -0,0 +1 @@
+Fix `PyString::from_object`, avoid out of bounds reads by null terminating the `encoding` and `errors` parameters

src/types/string.rs

@@ -10,6 +10,7 @@ use crate::types::PyBytes;
 use crate::IntoPy;
 use crate::{ffi, Bound, Py, PyAny, PyResult, Python};
 use std::borrow::Cow;
+use std::ffi::CString;
 use std::str;
 
 /// Deprecated alias for [`PyString`].
@@ -216,6 +217,8 @@ impl PyString {
         encoding: &str,
         errors: &str,
     ) -> PyResult<Bound<'py, PyString>> {
+        let encoding = CString::new(encoding)?;
+        let errors = CString::new(errors)?;
         unsafe {
             ffi::PyUnicode_FromEncodedObject(
                 src.as_ptr(),
@@ -670,6 +673,18 @@ mod tests {
         })
     }
 
+    #[test]
+    fn test_string_from_object() {
+        Python::with_gil(|py| {
+            let py_bytes = PyBytes::new(py, b"ab\xFFcd");
+
+            let py_string = PyString::from_object(&py_bytes, "utf-8", "ignore").unwrap();
+
+            let result = py_string.to_cow().unwrap();
+            assert_eq!(result, "abcd");
+        });
+    }
+
     #[test]
     #[cfg(not(any(Py_LIMITED_API, PyPy)))]
     fn test_string_data_ucs1() {