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

[Danilo Krummrich <dakr@redhat.com>]

On Sun, Jul 07, 2024 at 10:57:31PM +0200, Christian Schrefl wrote:
> 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?

Thanks, this is a good idea indeed, will send a v2.

> 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
>