Re: [PATCH] rust: macros: fix soundness issue in `module!` macro

[Wedson Almeida Filho]

On Wed, 27 Mar 2024 at 13:04, Benno Lossin <benno.lossin@xxxxxxxxx> wrote:
>
> The `module!` macro creates glue code that are called by C to initialize
> the Rust modules using the `Module::init` function. Part of this glue
> code are the local functions `__init` and `__exit` that are used to
> initialize/destroy the Rust module.
> These functions are safe and also visible to the Rust mod in which the
> `module!` macro is invoked. This means that they can be called by other
> safe Rust code. But since they contain `unsafe` blocks that rely on only
> being called at the right time, this is a soundness issue.
>
> Wrap these generated functions inside of two private modules, this
> guarantees that the public functions cannot be called from the outside.
> Make the safe functions `unsafe` and add SAFETY comments.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Closes: https://github.com/Rust-for-Linux/linux/issues/629
> Fixes: 1fbde52bde73 ("rust: add `macros` crate")
> Signed-off-by: Benno Lossin <benno.lossin@xxxxxxxxx>
> ---
> This patch is best viewed with `git show --ignore-space-change`, since I
> also adjusted the indentation.
>
>  rust/macros/module.rs | 198 ++++++++++++++++++++++++------------------
>  1 file changed, 112 insertions(+), 86 deletions(-)
>
> diff --git a/rust/macros/module.rs b/rust/macros/module.rs
> index 27979e582e4b..16c4921a08f2 100644
> --- a/rust/macros/module.rs
> +++ b/rust/macros/module.rs
> @@ -199,103 +199,129 @@ pub(crate) fn module(ts: TokenStream) -> TokenStream {
>              /// Used by the printing macros, e.g. [`info!`].
>              const __LOG_PREFIX: &[u8] = b\"{name}\\0\";
>
> -            /// The \"Rust loadable module\" mark.
> -            //
> -            // This may be best done another way later on, e.g. as a new modinfo
> -            // key or a new section. For the moment, keep it simple.
> -            #[cfg(MODULE)]
> -            #[doc(hidden)]
> -            #[used]
> -            static __IS_RUST_MODULE: () = ();
> -
> -            static mut __MOD: Option<{type_}> = None;
> -
> -            // SAFETY: `__this_module` is constructed by the kernel at load time and will not be
> -            // 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 _)
> -            }};
> -            #[cfg(not(MODULE))]
> -            static THIS_MODULE: kernel::ThisModule = unsafe {{
> -                kernel::ThisModule::from_ptr(core::ptr::null_mut())
> -            }};
> -
> -            // Loadable modules need to export the `{{init,cleanup}}_module` identifiers.
> -            /// # Safety
> -            ///
> -            /// This function must not be called after module initialization, because it may be
> -            /// freed after that completes.
> -            #[cfg(MODULE)]
> -            #[doc(hidden)]
> -            #[no_mangle]
> -            #[link_section = \".init.text\"]
> -            pub unsafe extern \"C\" fn init_module() -> core::ffi::c_int {{
> -                __init()
> -            }}
> +            // Double nested modules, since then nobody can access the public items inside.
> +            mod __module_init {{
> +                mod __module_init {{
> +                    use super::super::{type_};
> +
> +                    /// The \"Rust loadable module\" mark.
> +                    //
> +                    // This may be best done another way later on, e.g. as a new modinfo
> +                    // key or a new section. For the moment, keep it simple.
> +                    #[cfg(MODULE)]
> +                    #[doc(hidden)]
> +                    #[used]
> +                    static __IS_RUST_MODULE: () = ();
> +
> +                    static mut __MOD: Option<{type_}> = None;
> +
> +                    // SAFETY: `__this_module` is constructed by the kernel at load time and will not be
> +                    // 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 _)
> +                    }};
> +                    #[cfg(not(MODULE))]
> +                    static THIS_MODULE: kernel::ThisModule = unsafe {{
> +                        kernel::ThisModule::from_ptr(core::ptr::null_mut())
> +                    }};
> +
> +                    // Loadable modules need to export the `{{init,cleanup}}_module` identifiers.
> +                    /// # Safety
> +                    ///
> +                    /// This function must not be called after module initialization, because it may be
> +                    /// freed after that completes.
> +                    #[cfg(MODULE)]
> +                    #[doc(hidden)]
> +                    #[no_mangle]
> +                    #[link_section = \".init.text\"]
> +                    pub unsafe extern \"C\" fn init_module() -> core::ffi::c_int {{
> +                        __init()
> +                    }}
>
> -            #[cfg(MODULE)]
> -            #[doc(hidden)]
> -            #[no_mangle]
> -            pub extern \"C\" fn cleanup_module() {{
> -                __exit()
> -            }}
> +                    #[cfg(MODULE)]
> +                    #[doc(hidden)]
> +                    #[no_mangle]
> +                    pub extern \"C\" fn cleanup_module() {{
> +                        __exit()
> +                    }}
>
> -            // Built-in modules are initialized through an initcall pointer
> -            // and the identifiers need to be unique.
> -            #[cfg(not(MODULE))]
> -            #[cfg(not(CONFIG_HAVE_ARCH_PREL32_RELOCATIONS))]
> -            #[doc(hidden)]
> -            #[link_section = \"{initcall_section}\"]
> -            #[used]
> -            pub static __{name}_initcall: extern \"C\" fn() -> core::ffi::c_int = __{name}_init;
> -
> -            #[cfg(not(MODULE))]
> -            #[cfg(CONFIG_HAVE_ARCH_PREL32_RELOCATIONS)]
> -            core::arch::global_asm!(
> -                r#\".section \"{initcall_section}\", \"a\"
> -                __{name}_initcall:
> -                    .long   __{name}_init - .
> -                    .previous
> -                \"#
> -            );
> +                    // Built-in modules are initialized through an initcall pointer
> +                    // and the identifiers need to be unique.
> +                    #[cfg(not(MODULE))]
> +                    #[cfg(not(CONFIG_HAVE_ARCH_PREL32_RELOCATIONS))]
> +                    #[doc(hidden)]
> +                    #[link_section = \"{initcall_section}\"]
> +                    #[used]
> +                    pub static __{name}_initcall: extern \"C\" fn() -> core::ffi::c_int = __{name}_init;
> +
> +                    #[cfg(not(MODULE))]
> +                    #[cfg(CONFIG_HAVE_ARCH_PREL32_RELOCATIONS)]
> +                    core::arch::global_asm!(
> +                        r#\".section \"{initcall_section}\", \"a\"
> +                        __{name}_initcall:
> +                            .long   __{name}_init - .
> +                            .previous
> +                        \"#
> +                    );
> +
> +                    #[cfg(not(MODULE))]
> +                    #[doc(hidden)]
> +                    #[no_mangle]
> +                    pub extern \"C\" fn __{name}_init() -> core::ffi::c_int {{
> +                        __init()
> +                    }}
>
> -            #[cfg(not(MODULE))]
> -            #[doc(hidden)]
> -            #[no_mangle]
> -            pub extern \"C\" fn __{name}_init() -> core::ffi::c_int {{
> -                __init()
> -            }}
> +                    #[cfg(not(MODULE))]
> +                    #[doc(hidden)]
> +                    #[no_mangle]
> +                    pub extern \"C\" fn __{name}_exit() {{
> +                        __exit()
> +                    }}
>
> -            #[cfg(not(MODULE))]
> -            #[doc(hidden)]
> -            #[no_mangle]
> -            pub extern \"C\" fn __{name}_exit() {{
> -                __exit()
> -            }}
> +                    /// # Safety
> +                    ///
> +                    /// This function must
> +                    /// - only be called once,
> +                    /// - not be called concurrently with `__exit`.

I don't think the second item is needed here, it really is a
requirement on `__exit`.

> +                    unsafe fn __init() -> core::ffi::c_int {{
> +                        match <{type_} as kernel::Module>::init(&THIS_MODULE) {{
> +                            Ok(m) => {{
> +                                // SAFETY:
> +                                // no data race, since `__MOD` can only be accessed by this module and
> +                                // there only `__init` and `__exit` access it. These functions are only
> +                                // called once and `__exit` cannot be called before or during `__init`.
> +                                unsafe {{
> +                                    __MOD = Some(m);
> +                                }}
> +                                return 0;
> +                            }}
> +                            Err(e) => {{
> +                                return e.to_errno();
> +                            }}
> +                        }}
> +                    }}
>
> -            fn __init() -> core::ffi::c_int {{
> -                match <{type_} as kernel::Module>::init(&THIS_MODULE) {{
> -                    Ok(m) => {{
> +                    /// # Safety
> +                    ///
> +                    /// This function must
> +                    /// - only be called once,
> +                    /// - be called after `__init`,
> +                    /// - not be called concurrently with `__init`.

The second item is incomplete: it must be called after `__init` *succeeds*.

With that added (which is a different precondition), I think the third
item can be dropped because if you have to wait to see whether
`__init` succeeded or failed before you can call `__exit`, then
certainly you cannot call it concurrently with `__init`.

> +                    unsafe fn __exit() {{
> +                        // SAFETY:
> +                        // no data race, since `__MOD` can only be accessed by this module and there
> +                        // only `__init` and `__exit` access it. These functions are only called once
> +                        // and `__init` was already called.
>                          unsafe {{
> -                            __MOD = Some(m);
> +                            // Invokes `drop()` on `__MOD`, which should be used for cleanup.
> +                            __MOD = None;
>                          }}
> -                        return 0;
>                      }}
> -                    Err(e) => {{
> -                        return e.to_errno();
> -                    }}
> -                }}
> -            }}
>
> -            fn __exit() {{
> -                unsafe {{
> -                    // Invokes `drop()` on `__MOD`, which should be used for cleanup.
> -                    __MOD = None;
> +                    {modinfo}
>                  }}
>              }}
> -
> -            {modinfo}
>          ",
>          type_ = info.type_,
>          name = info.name,
>
> base-commit: 4cece764965020c22cff7665b18a012006359095
> --
> 2.44.0
>
>