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

[Danilo Krummrich <dakr@redhat.com>]

`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)
+    }
+}
+
 /// 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 {
-- 
2.45.2