misc: fastrpc: fix double refcounting on dmabuf

dma buf refcount has to be done by the driver which is going to use the fd.
This driver already does refcount on the dmabuf fd if its actively using it
but also does an additional refcounting via extra ioctl.
This additional refcount can lead to memory leak in cases where the
applications fail to call the ioctl to decrement the refcount.

So remove this extra refcount in the ioctl

More info of dma buf usage at drivers/dma-buf/dma-buf.c

Reported-by: Mayank Chopra <mak.chopra@codeaurora.org>
Reported-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
Tested-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20190829092926.12037-5-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/fastrpc.c

@@ -1198,26 +1198,6 @@ static int fastrpc_device_open(struct inode *inode, struct file *filp)
 	return 0;
 }
 
-static int fastrpc_dmabuf_free(struct fastrpc_user *fl, char __user *argp)
-{
-	struct dma_buf *buf;
-	int info;
-
-	if (copy_from_user(&info, argp, sizeof(info)))
-		return -EFAULT;
-
-	buf = dma_buf_get(info);
-	if (IS_ERR_OR_NULL(buf))
-		return -EINVAL;
-	/*
-	 * one for the last get and other for the ALLOC_DMA_BUFF ioctl
-	 */
-	dma_buf_put(buf);
-	dma_buf_put(buf);
-
-	return 0;
-}
-
 static int fastrpc_dmabuf_alloc(struct fastrpc_user *fl, char __user *argp)
 {
 	struct fastrpc_alloc_dma_buf bp;
@@ -1253,8 +1233,6 @@ static int fastrpc_dmabuf_alloc(struct fastrpc_user *fl, char __user *argp)
 		return -EFAULT;
 	}
 
-	get_dma_buf(buf->dmabuf);
-
 	return 0;
 }
 
@@ -1322,9 +1300,6 @@ static long fastrpc_device_ioctl(struct file *file, unsigned int cmd,
 	case FASTRPC_IOCTL_INIT_CREATE:
 		err = fastrpc_init_create_process(fl, argp);
 		break;
-	case FASTRPC_IOCTL_FREE_DMA_BUFF:
-		err = fastrpc_dmabuf_free(fl, argp);
-		break;
 	case FASTRPC_IOCTL_ALLOC_DMA_BUFF:
 		err = fastrpc_dmabuf_alloc(fl, argp);
 		break;