[PATCH 1/1] rust: module_param: Fix potentially incorrect access of `SetOnce`

[Priya Bala Govindasamy]

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: 0b08fc292842 ("rust: introduce module_param module")
Reported-by: Dylan Zueck <dzu...@uci.edu>
Assisted-by: LLM
Signed-off-by: Priya Bala Govindasamy <pgovi...@uci.edu>
---
 rust/kernel/module_param.rs | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/module_param.rs b/rust/kernel/module_param.rs
index f9a14765a926..7ae87ffe1d6b 100644
--- a/rust/kernel/module_param.rs
+++ b/rust/kernel/module_param.rs
@@ -75,7 +75,9 @@ pub trait ModuleParam: Sized + Copy {
         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>>()) };
+        let param_access = unsafe { &*((*param)
+            .__bindgen_anon_1.arg.cast::<ModuleParamAccess<T>>()) };
+        let container = &param_access.value;
 
         container
             .populate(new_value)
-- 
2.34.1