rust: module_param: Fix potentially incorrect access of SetOnce<T>

The `set_param` function casts `kernel_param.arg` to `*const SetOnce<T>`
But the Rust module macro in rust/macros/module.rs initializes `arg` with
`#param_name.as_void_ptr()`, and `#param_name` is a `ModuleParamAccess<T>`,
not a `SetOnce<T>`.

ModuleParamAccess<T> has default Rust layout but `set_param` accesses its
first field SetOnce<T> assuming it to be at offset 0. This is not
guaranteed by Rust and could cause type confusion leading to data
corruption.

Fix this by casting `kernel_param.arg` to `ModuleParamAccess<T>` and
then accessing the `value: SetOnce<T>` field.

Fixes: 0b08fc2928 ("rust: introduce module_param module")
Cc: stable@vger.kernel.org
Reported-by: Dylan Zueck <dzueck@uci.edu>
Assisted-by: LLM
Suggested-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: Priya Bala Govindasamy <pgovind2@uci.edu>
Signed-off-by: Petr Pavlu <petr.pavlu@suse.com>

rust/kernel/module_param.rs

@@ -74,8 +74,13 @@ pub trait ModuleParam: Sized + Copy {
     crate::error::from_result(|| {
         let new_value = T::try_from_param_arg(arg)?;
 
-        // SAFETY: By function safety requirements, this access is safe.
-        let container = unsafe { &*((*param).__bindgen_anon_1.arg.cast::<SetOnce<T>>()) };
+        // SAFETY: By function safety requirements, `param` is valid for read.
+        let arg = unsafe { (*param).__bindgen_anon_1.arg };
+        let param_access_ptr = arg.cast::<ModuleParamAccess<T>>();
+        // SAFETY: The `arg` field is initialized with a pointer to a `static ModuleAccessParam` by
+        // the rust module macro. Thus, the pointer is valid for use as a reference.
+        let param_access = unsafe { &*(param_access_ptr) };
+        let container = &param_access.value;
 
         container
             .populate(new_value)