Commit fc8548c7 authored Jan 30, 2023 by Ilpo Järvinen Committed by Greg Kroah-Hartman Feb 22, 2023

serial: 8250_dma: Fix DMA Rx rearm race



commit 57e9af7831dcf211c5c689c2a6f209f4abdf0bce upstream.

As DMA Rx can be completed from two places, it is possible that DMA Rx
completes before DMA completion callback had a chance to complete it.
Once the previous DMA Rx has been completed, a new one can be started
on the next UART interrupt. The following race is possible
(uart_unlock_and_check_sysrq_irqrestore() replaced with
spin_unlock_irqrestore() for simplicity/clarity):

CPU0					CPU1
					dma_rx_complete()
serial8250_handle_irq()
  spin_lock_irqsave(&port->lock)
  handle_rx_dma()
    serial8250_rx_dma_flush()
      __dma_rx_complete()
        dma->rx_running = 0
        // Complete DMA Rx
  spin_unlock_irqrestore(&port->lock)

serial8250_handle_irq()
  spin_lock_irqsave(&port->lock)
  handle_rx_dma()
    serial8250_rx_dma()
      dma->rx_running = 1
      // Setup a new DMA Rx
  spin_unlock_irqrestore(&port->lock)

					  spin_lock_irqsave(&port->lock)
					  // sees dma->rx_running = 1
					  __dma_rx_complete()
					    dma->rx_running = 0
					    // Incorrectly complete
					    // running DMA Rx

This race seems somewhat theoretical to occur for real but handle it
correctly regardless. Check what is the DMA status before complething
anything in __dma_rx_complete().

Reported-by: Gilles BULOZ <gilles.buloz@kontron.com>
Tested-by: Gilles BULOZ <gilles.buloz@kontron.com>
Fixes: 9ee4b83e ("serial: 8250: Add support for dmaengine")
Cc: stable@vger.kernel.org
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20230130114841.25749-3-ilpo.jarvinen@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

parent 71d6b277

drivers/tty/serial/8250/8250_dma.c

+10 −2

Original line number	Diff line number	Diff line
		@@ -46,15 +46,23 @@ static void __dma_rx_complete(void *param)
		struct uart_8250_dma *dma = p->dma;
		struct tty_port *tty_port = &p->port.state->port;
		struct dma_tx_state state;
		enum dma_status dma_status;
		int count;

		dma->rx_running = 0;
		dmaengine_tx_status(dma->rxchan, dma->rx_cookie, &state);
		/*
		* New DMA Rx can be started during the completion handler before it
		* could acquire port's lock and it might still be ongoing. Don't to
		* anything in such case.
		*/
		dma_status = dmaengine_tx_status(dma->rxchan, dma->rx_cookie, &state);
		if (dma_status == DMA_IN_PROGRESS)
		return;

		count = dma->rx_size - state.residue;

		tty_insert_flip_string(tty_port, dma->rx_buf, count);
		p->port.icount.rx += count;
		dma->rx_running = 0;

		tty_flip_buffer_push(tty_port);
		}