nbd: partition nbd_read_stat() into nbd_read_reply() and nbd_handle_reply()

Prepare to fix uaf in nbd_read_stat(), no functional changes.
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/20210916093350.1410403-7-yukuai3@huawei.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -700,38 +700,45 @@ static int nbd_send_cmd(struct nbd_device *nbd, struct nbd_cmd *cmd, int index)
 	return 0;
 }
 
-/* NULL returned = something went wrong, inform userspace */
-static struct nbd_cmd *nbd_read_stat(struct nbd_device *nbd, int index)
+static int nbd_read_reply(struct nbd_device *nbd, int index,
+			  struct nbd_reply *reply)
 {
-	struct nbd_config *config = nbd->config;
-	int result;
-	struct nbd_reply reply;
-	struct nbd_cmd *cmd;
-	struct request *req = NULL;
-	u64 handle;
-	u16 hwq;
-	u32 tag;
-	struct kvec iov = {.iov_base = &reply, .iov_len = sizeof(reply)};
+	struct kvec iov = {.iov_base = reply, .iov_len = sizeof(*reply)};
 	struct iov_iter to;
-	int ret = 0;
+	int result;
 
-	reply.magic = 0;
-	iov_iter_kvec(&to, READ, &iov, 1, sizeof(reply));
+	reply->magic = 0;
+	iov_iter_kvec(&to, READ, &iov, 1, sizeof(*reply));
 	result = sock_xmit(nbd, index, 0, &to, MSG_WAITALL, NULL);
 	if (result < 0) {
-		if (!nbd_disconnected(config))
+		if (!nbd_disconnected(nbd->config))
 			dev_err(disk_to_dev(nbd->disk),
 				"Receive control failed (result %d)\n", result);
-		return ERR_PTR(result);
+		return result;
 	}
 
-	if (ntohl(reply.magic) != NBD_REPLY_MAGIC) {
+	if (ntohl(reply->magic) != NBD_REPLY_MAGIC) {
 		dev_err(disk_to_dev(nbd->disk), "Wrong magic (0x%lx)\n",
-				(unsigned long)ntohl(reply.magic));
-		return ERR_PTR(-EPROTO);
+				(unsigned long)ntohl(reply->magic));
+		return -EPROTO;
 	}
 
-	memcpy(&handle, reply.handle, sizeof(handle));
+	return 0;
+}
+
+/* NULL returned = something went wrong, inform userspace */
+static struct nbd_cmd *nbd_handle_reply(struct nbd_device *nbd, int index,
+					struct nbd_reply *reply)
+{
+	int result;
+	struct nbd_cmd *cmd;
+	struct request *req = NULL;
+	u64 handle;
+	u16 hwq;
+	u32 tag;
+	int ret = 0;
+
+	memcpy(&handle, reply->handle, sizeof(handle));
 	tag = nbd_handle_to_tag(handle);
 	hwq = blk_mq_unique_tag_to_hwq(tag);
 	if (hwq < nbd->tag_set.nr_hw_queues)
@@ -774,9 +781,9 @@ static struct nbd_cmd *nbd_read_stat(struct nbd_device *nbd, int index)
 		ret = -ENOENT;
 		goto out;
 	}
-	if (ntohl(reply.error)) {
+	if (ntohl(reply->error)) {
 		dev_err(disk_to_dev(nbd->disk), "Other side returned error (%d)\n",
-			ntohl(reply.error));
+			ntohl(reply->error));
 		cmd->status = BLK_STS_IOERR;
 		goto out;
 	}
@@ -785,6 +792,7 @@ static struct nbd_cmd *nbd_read_stat(struct nbd_device *nbd, int index)
 	if (rq_data_dir(req) != WRITE) {
 		struct req_iterator iter;
 		struct bio_vec bvec;
+		struct iov_iter to;
 
 		rq_for_each_segment(bvec, req, iter) {
 			iov_iter_bvec(&to, READ, &bvec, 1, bvec.bv_len);
@@ -798,7 +806,7 @@ static struct nbd_cmd *nbd_read_stat(struct nbd_device *nbd, int index)
 				 * and let the timeout stuff handle resubmitting
 				 * this request onto another connection.
 				 */
-				if (nbd_disconnected(config)) {
+				if (nbd_disconnected(nbd->config)) {
 					cmd->status = BLK_STS_IOERR;
 					goto out;
 				}
@@ -822,24 +830,30 @@ static void recv_work(struct work_struct *work)
 						     work);
 	struct nbd_device *nbd = args->nbd;
 	struct nbd_config *config = nbd->config;
+	struct nbd_sock *nsock;
 	struct nbd_cmd *cmd;
 	struct request *rq;
 
 	while (1) {
-		cmd = nbd_read_stat(nbd, args->index);
-		if (IS_ERR(cmd)) {
-			struct nbd_sock *nsock = config->socks[args->index];
+		struct nbd_reply reply;
 
-			mutex_lock(&nsock->tx_lock);
-			nbd_mark_nsock_dead(nbd, nsock, 1);
-			mutex_unlock(&nsock->tx_lock);
+		if (nbd_read_reply(nbd, args->index, &reply))
+			break;
+
+		cmd = nbd_handle_reply(nbd, args->index, &reply);
+		if (IS_ERR(cmd))
 			break;
-		}
 
 		rq = blk_mq_rq_from_pdu(cmd);
 		if (likely(!blk_should_fake_timeout(rq->q)))
 			blk_mq_complete_request(rq);
 	}
+
+	nsock = config->socks[args->index];
+	mutex_lock(&nsock->tx_lock);
+	nbd_mark_nsock_dead(nbd, nsock, 1);
+	mutex_unlock(&nsock->tx_lock);
+
 	nbd_config_put(nbd);
 	atomic_dec(&config->recv_threads);
 	wake_up(&config->recv_wq);