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

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Message-ID: <CAH5fLgjmUOLb63xgv85GLt92vr6ghSbp1T_vKjPK143061SeMw@mail.gmail.com>
Date: Thu, 29 Aug 2024 11:25:29 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org, 
	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>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...sung.com>, 
	Trevor Gross <tmgross@...ch.edu>
Subject: Re: [RFC PATCH] rust: Provide correct provenance when constructing THIS_MODULE

On Wed, Aug 28, 2024 at 8:02 PM Boqun Feng <boqun.feng@...il.com> 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>

Thanks. This probably didn't need to be an RFC.

Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>