io_uring/tw: serialize ctx->retry_llist with ->uring_lock

The DEFER_TASKRUN local task work paths all run under ctx->uring_lock,
which serializes them with each other and with the rest of the ring's
hot paths. io_move_task_work_from_local() is the exception - it's called
from io_ring_exit_work() on a kworker without holding the lock and from
the iopoll cancelation side right after dropping it.

->work_llist is fine with this, as it's only ever updated via the
expected paths. But the ->retry_llist is updated while runing, and hence
it could potentially race between normal task_work running and the
task-has-exited shutdown path.

Simply grab ->uring_lock while moving the local work to the fallback
list for exit purposes, which nicely serializes it across both the
normal additions and the exit prune path.

Cc: stable@vger.kernel.org
Fixes: f46b9cdb22 ("io_uring: limit local tw done")
Reported-by: Robert Femmer <robert.femmer@x41-dsec.de>
Reported-by: Christian Reitter <invd@inhq.net>
Reported-by: Michael Rodler <michael.rodler@x41-dsec.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/tw.c

@@ -273,8 +273,18 @@ void io_req_task_work_add_remote(struct io_kiocb *req, unsigned flags)
 
 void __cold io_move_task_work_from_local(struct io_ring_ctx *ctx)
 {
-	struct llist_node *node = llist_del_all(&ctx->work_llist);
+	struct llist_node *node;
 
+	/*
+	 * Running the work items may utilize ->retry_llist as a means
+	 * for capping the number of task_work entries run at the same
+	 * time. But that list can potentially race with moving the work
+	 * from here, if the task is exiting. As any normal task_work
+	 * running holds ->uring_lock already, just guard this slow path
+	 * with ->uring_lock to avoid racing on ->retry_llist.
+	 */
+	guard(mutex)(&ctx->uring_lock);
+	node = llist_del_all(&ctx->work_llist);
 	__io_fallback_tw(node, false);
 	node = llist_del_all(&ctx->retry_llist);
 	__io_fallback_tw(node, false);