[PATCH v2] rust: drm: fix unsound initialization in drm::Device::new

[Eliot Courtney]

If pinned initialization of drm::Device::Data fails, it calls
drm::Device::release via drm_dev_put. This materializes a reference to
&drm::Device, but it's not fully constructed yet, because initializing
`data` failed. It should not be dropped either. Instead, if pinned
initialization fails, make sure drm::Device::release isn't called.

Fixes: 2e9fdbe5ec7a ("rust: drm: device: drop_in_place() the drm::Device in release()")
Signed-off-by: Eliot Courtney <ecourtney@xxxxxxxxxx>
---
Changes in v2:
- stack allocate `alloc_vtable`
- use const { &vtable } trick to help prevent future issues
- Link to v1: https://patch.msgid.link/20260428-fix-drm-1-v1-1-755057178066@xxxxxxxxxx
---
 rust/kernel/drm/device.rs | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/rust/kernel/drm/device.rs b/rust/kernel/drm/device.rs
index adbafe8db54d..403fc35353c7 100644
--- a/rust/kernel/drm/device.rs
+++ b/rust/kernel/drm/device.rs
@@ -119,13 +119,20 @@ pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<A
         // compatible `Layout`.
         let layout = Kmalloc::aligned_layout(Layout::new::<Self>());
 
+        // Use a temporary vtable without a `release` callback until `data` is initialized, so
+        // init failure can release the DRM device without dropping uninitialized fields.
+        let alloc_vtable = bindings::drm_driver {
+            release: None,
+            ..Self::VTABLE
+        };
+
         // SAFETY:
-        // - `VTABLE`, as a `const` is pinned to the read-only section of the compilation,
+        // - `alloc_vtable` reference remains valid until no longer used,
         // - `dev` is valid by its type invarants,
         let raw_drm: *mut Self = unsafe {
             bindings::__drm_dev_alloc(
                 dev.as_raw(),
-                &Self::VTABLE,
+                &alloc_vtable,
                 layout.size(),
                 mem::offset_of!(Self, dev),
             )
@@ -133,6 +140,10 @@ pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<A
         .cast();
         let raw_drm = NonNull::new(from_err_ptr(raw_drm)?).ok_or(ENOMEM)?;
 
+        // SAFETY: `raw_drm` is a valid pointer to `Self`, given that `__drm_dev_alloc` was
+        // successful.
+        let drm_dev = unsafe { Self::into_drm_device(raw_drm) };
+
         // SAFETY: `raw_drm` is a valid pointer to `Self`.
         let raw_data = unsafe { ptr::addr_of_mut!((*raw_drm.as_ptr()).data) };
 
@@ -140,15 +151,14 @@ pub fn new(dev: &device::Device, data: impl PinInit<T::Data, Error>) -> Result<A
         // - `raw_data` is a valid pointer to uninitialized memory.
         // - `raw_data` will not move until it is dropped.
         unsafe { data.__pinned_init(raw_data) }.inspect_err(|_| {
-            // SAFETY: `raw_drm` is a valid pointer to `Self`, given that `__drm_dev_alloc` was
-            // successful.
-            let drm_dev = unsafe { Self::into_drm_device(raw_drm) };
-
             // SAFETY: `__drm_dev_alloc()` was successful, hence `drm_dev` must be valid and the
             // refcount must be non-zero.
             unsafe { bindings::drm_dev_put(drm_dev) };
         })?;
 
+        // SAFETY: `drm_dev` is still private to this function.
+        unsafe { (*drm_dev).driver = const { &Self::VTABLE } };
+
         // SAFETY: The reference count is one, and now we take ownership of that reference as a
         // `drm::Device`.
         Ok(unsafe { ARef::from_raw(raw_drm) })

---
base-commit: d9a6809478f9815b6455a327aa001737ac7b2c09
change-id: 20260428-fix-drm-1-459b6357de23

Best regards,
--  
Eliot Courtney <ecourtney@xxxxxxxxxx>