Re: [PATCH 2/2] firmware_loader: fix soundness issue in `request_internal`

[Christian Schrefl <chrisi.schrefl@gmail.com>]

Greetings.

On 07.07.24 2:38 AM, Danilo Krummrich wrote:
> `request_internal` must be called with one of the following function
> pointers: request_firmware(), firmware_request_nowarn(),
> firmware_request_platform() or request_firmware_direct().
> 
> The previous `FwFunc` alias did not guarantee this, which is unsound.
> 
> In order to fix this up, implement `FwFunc` as new type with a
> corresponding type invariant and unsafe `new` function.
> 
> Reported-by: Gary Guo <gary@garyguo.net>
> Closes: https://lore.kernel.org/lkml/20240620143611.7995e0bb@eugeo/
> Signed-off-by: Danilo Krummrich <dakr@redhat.com>
> ---
>  rust/kernel/firmware.rs | 32 ++++++++++++++++++++++++++------
>  1 file changed, 26 insertions(+), 6 deletions(-)
> 
> diff --git a/rust/kernel/firmware.rs b/rust/kernel/firmware.rs
> index 106a928a535e..d765ecc85d38 100644
> --- a/rust/kernel/firmware.rs
> +++ b/rust/kernel/firmware.rs
> @@ -7,11 +7,24 @@
>  use crate::{bindings, device::Device, error::Error, error::Result, str::CStr};
>  use core::ptr::NonNull;
>  
> -// One of the following: `bindings::request_firmware`, `bindings::firmware_request_nowarn`,
> -// `firmware_request_platform`, `bindings::request_firmware_direct`
> -type FwFunc =
> +type RawFwFunc =
>      unsafe extern "C" fn(*mut *const bindings::firmware, *const i8, *mut bindings::device) -> i32;
>  
> +/// # Invariants
> +///
> +/// One of the following: `bindings::request_firmware`, `bindings::firmware_request_nowarn`,
> +/// `firmware_request_platform`, `bindings::request_firmware_direct`
> +struct FwFunc(RawFwFunc);
> +
> +impl FwFunc {
> +    /// # Safety
> +    ///
> +    /// Must be one of the functions listed in the type invariants.
> +    unsafe fn from_raw(func: RawFwFunc) -> Self {
> +        Self(func)
> +    }
Why not write methods that construct each possible FwFunc?
That way the code that needs to know abut this invariant is only inside a single impl block.
Something like:
impl FwFunc {
    fn request_firmware() -> Self {
        // # Safety
        // As per the Type Invariant `bindings::request_firmware` is a valid vaule.
    	FwFunc(bindings::request_firmware)
    }
}

> +}
> +
>  /// Abstraction around a C `struct firmware`.
>  ///
>  /// This is a simple abstraction around the C firmware API. Just like with the C API, firmware can
> @@ -48,7 +61,7 @@ fn request_internal(name: &CStr, dev: &Device, func: FwFunc) -> Result<Self> {
>  
>          // SAFETY: `pfw` is a valid pointer to a NULL initialized `bindings::firmware` pointer.
>          // `name` and `dev` are valid as by their type invariants.
> -        let ret = unsafe { func(pfw as _, name.as_char_ptr(), dev.as_raw()) };
> +        let ret = unsafe { func.0(pfw as _, name.as_char_ptr(), dev.as_raw()) };
>          if ret != 0 {
>              return Err(Error::from_errno(ret));
>          }
> @@ -60,13 +73,20 @@ fn request_internal(name: &CStr, dev: &Device, func: FwFunc) -> Result<Self> {
>  
>      /// Send a firmware request and wait for it. See also `bindings::request_firmware`.
>      pub fn request(name: &CStr, dev: &Device) -> Result<Self> {
> -        Self::request_internal(name, dev, bindings::request_firmware)
> +        // SAFETY: `bindings::request_firmware` is valid by the safety requirement of `FwFunc`.
> +        let func = unsafe { FwFunc::from_raw(bindings::request_firmware) };
> +
> +        Self::request_internal(name, dev, func)
>      }
>  
>      /// Send a request for an optional firmware module. See also
>      /// `bindings::firmware_request_nowarn`.
>      pub fn request_nowarn(name: &CStr, dev: &Device) -> Result<Self> {
> -        Self::request_internal(name, dev, bindings::firmware_request_nowarn)
> +        // SAFETY: `bindings::firmware_request_nowarn` is valid by the safety requirement of
> +        // `FwFunc::new`.
> +        let func = unsafe { FwFunc::from_raw(bindings::firmware_request_nowarn) };
> +
> +        Self::request_internal(name, dev, func)
>      }
>  
>      fn as_raw(&self) -> *mut bindings::firmware {

Cheers,
Christian