linux-kernel - Re: [RFC PATCH] rust: Provide correct provenance when constructing THIS

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Message-ID: <8df8ce27-b3b2-4cf2-926d-424456b0c7d2@proton.me>
Date: Sat, 31 Aug 2024 20:48:10 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Boqun Feng <boqun.feng@...il.com>, rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Andreas Hindborg <a.hindborg@...sung.com>, Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>
Subject: Re: [RFC PATCH] rust: Provide correct provenance when constructing THIS_MODULE

On 28.08.24 20:01, Boqun Feng wrote:
> Currently while defining `THIS_MODULE` symbol in `module!()`, the
> pointer used to contruct `ThisModule` is derived from an immutable
> reference of `__this_module`, which means the pointer doesn't have
> the provenance for writting, and that means any write to that pointer
> is UB regardless of data races or not. However, the usage of
> `THIS_MODULE` includes passing this pointer to functions that may write
> to it (probably in unsafe code), and this will create soundness issues.
> 
> One way to fix this is using `addr_of_mut!()` but that requires the
> unstable feature "const_mut_refs". So instead of `addr_of_mut()!`,
> an extern static `Opaque` is used here: since `Opaque<T>` is transparent
> to `T`, an extern static `Opaque` will just wrap the C symbol (defined
> in a C compile unit) in an `Opaque`, which provides a pointer with
> writable provenance via `Opaque::get()`. This fix the potential UBs
> because of pointer provenance unmatched.
> 
> Reported-by: Alice Ryhl <aliceryhl@...gle.com>
> Signed-off-by: Boqun Feng <boqun.feng@...il.com>

Reviewed-by: Benno Lossin <benno.lossin@...ton.me>

---
Cheers,
Benno

> ---
>  rust/macros/module.rs | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/macros/module.rs b/rust/macros/module.rs
> index 571ffa2e189c..aef3b132f32b 100644
> --- a/rust/macros/module.rs
> +++ b/rust/macros/module.rs
> @@ -217,7 +217,11 @@ pub(crate) fn module(ts: TokenStream) -> TokenStream {
>              // freed until the module is unloaded.
>              #[cfg(MODULE)]
>              static THIS_MODULE: kernel::ThisModule = unsafe {{
> -                kernel::ThisModule::from_ptr(&kernel::bindings::__this_module as *const _ as *mut _)
> +                extern \"C\" {{
> +                    static __this_module: kernel::types::Opaque<kernel::bindings::module>;
> +                }}
> +
> +                kernel::ThisModule::from_ptr(__this_module.get())
>              }};
>              #[cfg(not(MODULE))]
>              static THIS_MODULE: kernel::ThisModule = unsafe {{
> --
> 2.45.2
>