ublk: read any SQE values upfront

Since SQE memory is shared with userspace, we should only be reading it
once. We cannot read it multiple times, particularly when it's read once
for validation and then read again for the actual use.

ublk_ch_uring_cmd() is safe when called as a retry operation, as the
memory backing is stable at that point. But for normal issue, we want
to ensure that we only read ublksrv_io_cmd once. Wrap the function in
a helper that reads the value into an on-stack copy of the struct.

Cc: [email protected] # 6.0+
Reviewed-by: Ming Lei <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

Author: axboe

drivers/block/ublk_drv.c

@@ -1261,9 +1261,10 @@ static void ublk_handle_need_get_data(struct ublk_device *ub, int q_id,
 	ublk_queue_cmd(ubq, req);
 }
 
-static int ublk_ch_uring_cmd(struct io_uring_cmd *cmd, unsigned int issue_flags)
+static int __ublk_ch_uring_cmd(struct io_uring_cmd *cmd,
+			       unsigned int issue_flags,
+			       struct ublksrv_io_cmd *ub_cmd)
 {
-	struct ublksrv_io_cmd *ub_cmd = (struct ublksrv_io_cmd *)cmd->cmd;
 	struct ublk_device *ub = cmd->file->private_data;
 	struct ublk_queue *ubq;
 	struct ublk_io *io;
@@ -1362,6 +1363,23 @@ static int ublk_ch_uring_cmd(struct io_uring_cmd *cmd, unsigned int issue_flags)
 	return -EIOCBQUEUED;
 }
 
+static int ublk_ch_uring_cmd(struct io_uring_cmd *cmd, unsigned int issue_flags)
+{
+	struct ublksrv_io_cmd *ub_src = (struct ublksrv_io_cmd *) cmd->cmd;
+	struct ublksrv_io_cmd ub_cmd;
+
+	/*
+	 * Not necessary for async retry, but let's keep it simple and always
+	 * copy the values to avoid any potential reuse.
+	 */
+	ub_cmd.q_id = READ_ONCE(ub_src->q_id);
+	ub_cmd.tag = READ_ONCE(ub_src->tag);
+	ub_cmd.result = READ_ONCE(ub_src->result);
+	ub_cmd.addr = READ_ONCE(ub_src->addr);
+
+	return __ublk_ch_uring_cmd(cmd, issue_flags, &ub_cmd);
+}
+
 static const struct file_operations ublk_ch_fops = {
 	.owner = THIS_MODULE,
 	.open = ublk_ch_open,