rust: block: require Sync for Operations::QueueData

The queue data installed in a `GenDisk` is stored in the request queue and
handed back to the driver as a shared borrow through the `queue_rq` and
`commit_rqs` callbacks. Both callbacks obtain that borrow via
`ForeignOwnable::borrow` and may execute concurrently on several CPUs,
since the block layer runs one hardware queue per CPU. That means a shared
reference to the same queue data can be live on multiple threads at once,
which is only sound when the referent is `Sync`.

The initial `GenDisk` private data support omitted this bound, so a
driver could install a non-`Sync` type as queue data and then access
it concurrently from multiple CPUs without synchronization. Add a
`Sync` bound to the `QueueData` associated type to rule that out.

Fixes: 90d952fac8 ("rust: block: add `GenDisk` private data support")
Cc: stable@vger.kernel.org
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://msgid.link/20260608-queue-data-sync-v1-1-0efff051aaf3@kernel.org
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://patch.msgid.link/20260929-rust-block-for-v7-4-rc1-b4-v1-9-642a4c1aebd7@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

rust/kernel/block/mq/operations.rs

@@ -36,7 +36,7 @@
 pub trait Operations: Sized {
     /// Data associated with the `struct request_queue` that is allocated for
     /// the `GenDisk` associated with this `Operations` implementation.
-    type QueueData: ForeignOwnable;
+    type QueueData: ForeignOwnable + Sync;
 
     /// Called by the kernel to queue a request with the driver. If `is_last` is
     /// `false`, the driver is allowed to defer committing the request.