io_uring: fix io_sqe_files_unregister() hangs

io_sqe_files_unregister() uninterruptibly waits for enqueued ref nodes,
however requests keeping them may never complete, e.g. because of some
userspace dependency. Make sure it's interruptible otherwise it would
hang forever.

Cc: stable@vger.kernel.org # 5.6+
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -992,6 +992,10 @@ enum io_mem_account {
 	ACCT_PINNED,
 };
 
+static void destroy_fixed_file_ref_node(struct fixed_file_ref_node *ref_node);
+static struct fixed_file_ref_node *alloc_fixed_file_ref_node(
+			struct io_ring_ctx *ctx);
+
 static void __io_complete_rw(struct io_kiocb *req, long res, long res2,
 			     struct io_comp_state *cs);
 static void io_cqring_fill_event(struct io_kiocb *req, long res);
@@ -7244,11 +7248,15 @@ static void io_sqe_files_set_node(struct fixed_file_data *file_data,
 static int io_sqe_files_unregister(struct io_ring_ctx *ctx)
 {
 	struct fixed_file_data *data = ctx->file_data;
-	struct fixed_file_ref_node *ref_node = NULL;
+	struct fixed_file_ref_node *backup_node, *ref_node = NULL;
 	unsigned nr_tables, i;
+	int ret;
 
 	if (!data)
 		return -ENXIO;
+	backup_node = alloc_fixed_file_ref_node(ctx);
+	if (!backup_node)
+		return -ENOMEM;
 
 	spin_lock_bh(&data->lock);
 	ref_node = data->node;
@@ -7260,7 +7268,18 @@ static int io_sqe_files_unregister(struct io_ring_ctx *ctx)
 
 	/* wait for all refs nodes to complete */
 	flush_delayed_work(&ctx->file_put_work);
-	wait_for_completion(&data->done);
+	do {
+		ret = wait_for_completion_interruptible(&data->done);
+		if (!ret)
+			break;
+		ret = io_run_task_work_sig();
+		if (ret < 0) {
+			percpu_ref_resurrect(&data->refs);
+			reinit_completion(&data->done);
+			io_sqe_files_set_node(data, backup_node);
+			return ret;
+		}
+	} while (1);
 
 	__io_sqe_files_unregister(ctx);
 	nr_tables = DIV_ROUND_UP(ctx->nr_user_files, IORING_MAX_FILES_TABLE);
@@ -7271,6 +7290,7 @@ static int io_sqe_files_unregister(struct io_ring_ctx *ctx)
 	kfree(data);
 	ctx->file_data = NULL;
 	ctx->nr_user_files = 0;
+	destroy_fixed_file_ref_node(backup_node);
 	return 0;
 }