[PATCH v3 4/23] rust: drm: reject cross-device GEM handle creation

The Rust type bounds prove that a GEM object and file use the same
driver implementation, but one driver can own multiple DRM device
instances. drm_gem_handle_create() also requires the object and file
to belong to the same instance.

Expose the owning device pointer within the DRM crate and return
EINVAL for a mismatched instance.

Fixes: c284d3e42338 ("rust: drm: gem: Add GEM object abstraction")

Assisted-by: Claude:claude-opus-5
Signed-off-by: Mike Lothian <[email protected]>
---
 rust/kernel/drm/file.rs    | 7 +++++++
 rust/kernel/drm/gem/mod.rs | 7 +++++++
 2 files changed, 14 insertions(+)

diff --git a/rust/kernel/drm/file.rs b/rust/kernel/drm/file.rs
index 10160601ce5a..0abb7813f011 100644
--- a/rust/kernel/drm/file.rs
+++ b/rust/kernel/drm/file.rs
@@ -45,6 +45,13 @@ pub(super) fn as_raw(&self) -> *mut bindings::drm_file {
         self.0.get()
     }
 
+    /// Return the DRM device that owns this open file.
+    pub(crate) fn device_raw(&self) -> *mut bindings::drm_device {
+        // SAFETY: An open `drm_file` has a valid `minor`, whose `dev` pointer remains valid for
+        // the lifetime of the file.
+        unsafe { (*(*self.as_raw()).minor).dev }
+    }
+
     fn driver_priv(&self) -> *mut T {
         // SAFETY: By the type invariants of `Self`, `self.as_raw()` is always valid.
         unsafe { (*self.as_raw()).driver_priv }.cast()
diff --git a/rust/kernel/drm/gem/mod.rs b/rust/kernel/drm/gem/mod.rs
index 60491e5521e4..334e946833fb 100644
--- a/rust/kernel/drm/gem/mod.rs
+++ b/rust/kernel/drm/gem/mod.rs
@@ -186,6 +186,13 @@ fn create_handle<D, F>(&self, file: &drm::File<F>) -> Result<u32>
         D: drm::Driver<Object = Self, File = F>,
         F: drm::file::DriverFile<Driver = D>,
     {
+        // The associated-type bounds prove a common driver type; separately reject another
+        // instance of that driver before passing the pair to the C API.
+        // SAFETY: `self.as_raw()` is a valid GEM object by the trait invariant.
+        if unsafe { (*self.as_raw()).dev } != file.device_raw() {
+            return Err(EINVAL);
+        }
+
         let mut handle: u32 = 0;
         // SAFETY: The arguments are all valid per the type invariants.
         to_result(unsafe {