rust: devres: fix race between concurrent revokers

There is a potential race condition when two paths try to revoke a
Devres concurrently.

The driver core's devres_release_all() calls Revocable::revoke() via the
release callback, while Devres::drop() calls revoke_nosync() on another
CPU.

The revoker that does not claim the is_available swap returns
immediately, but the revoker that did may still be executing
drop_in_place() on the inner data. This can cause a use-after-free when
the other revoker's caller proceeds to drop adjacent resources that
drop_in_place() still references (e.g., Devres<DmaMappedSgt> racing with
SGTable freeing the backing sg_table and pages).

Fix this by adding a Completion. The release callback signals the
Completion after revoke() finishes, and Devres::drop() waits for it when
it loses the is_available swap. This ensures the wrapped object is fully
torn down before Devres::drop() returns.

Cc: stable@vger.kernel.org
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/dri-devel/20260612202841.2577C1F000E9@smtp.kernel.org/
Fixes: 05aa6fb1c2 ("rust: scatterlist: Add abstraction for sg_table")
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20260628174451.2275679-1-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

rust/kernel/devres.rs

@@ -21,7 +21,8 @@ use crate::{
     sync::{
         aref::ARef,
         rcu,
-        Arc, //
+        Arc,
+        Completion, //
     },
     types::{
         CovariantForLt,
@@ -39,6 +40,8 @@ struct Inner<T> {
     node: Opaque<bindings::devres_node>,
     #[pin]
     data: Revocable<T>,
+    #[pin]
+    revocation: Completion,
 }
 
 /// This abstraction is meant to be used by subsystems to containerize [`Device`] bound resources to
@@ -55,6 +58,10 @@ struct Inner<T> {
 /// After the [`Devres`] has been unbound it is not possible to access the encapsulated resource
 /// anymore.
 ///
+/// When a [`Devres`] is dropped, it is guaranteed that `T` has been fully dropped by the time
+/// [`Devres::drop`] returns, even if a concurrent revocation through the release callback is in
+/// progress.
+///
 /// [`Devres`] users should make sure to simply free the corresponding backing resource in `T`'s
 /// [`Drop`] implementation.
 ///
@@ -222,6 +229,7 @@ impl<T: Send + 'static> Devres<T> {
                     };
                 }),
                 data <- Revocable::new(data),
+                revocation <- Completion::new(),
             }),
             GFP_KERNEL,
         )?;
@@ -259,7 +267,9 @@ impl<T: Send + 'static> Devres<T> {
         // SAFETY: `inner` is a valid `Inner<T>` pointer.
         let inner = unsafe { &*inner };
 
-        inner.data.revoke();
+        if inner.data.revoke() {
+            inner.revocation.complete_all();
+        }
     }
 
     #[allow(clippy::missing_safety_doc)]
@@ -363,6 +373,10 @@ impl<T: Send + 'static> Drop for Devres<T> {
                 // this additional reference count.
                 drop(unsafe { Arc::from_raw(Arc::as_ptr(&self.inner)) });
             }
+        } else {
+            // The release callback is concurrently revoking; wait for it to finish
+            // `drop_in_place()` of the wrapped object before returning.
+            self.inner.revocation.wait_for_completion();
         }
     }
 }