[PATCH 1/2] serial: 8250_dma: Fix DMA Rx completion race

[Ilpo Järvinen]

__dma_rx_complete() is called from two places:
  - Through the DMA completion callback dma_rx_complete()
  - From serial8250_rx_dma_flush() after IIR_RLSI or IIR_RX_TIMEOUT
The former does not hold port's lock during __dma_rx_complete() which
allows these two to race and potentially insert the same data twice.

Extend port's lock coverage in dma_rx_complete() to prevent the race
and check if the DMA Rx is still pending completion before calling
into __dma_rx_complete().

Reported-by: Gilles BULOZ <gilles.buloz@xxxxxxxxxxx>
Tested-by: Gilles BULOZ <gilles.buloz@xxxxxxxxxxx>
Fixes: 9ee4b83e51f7 ("serial: 8250: Add support for dmaengine")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
---
 drivers/tty/serial/8250/8250_dma.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_dma.c b/drivers/tty/serial/8250/8250_dma.c
index 37d6af2ec427..5594883a96f8 100644
--- a/drivers/tty/serial/8250/8250_dma.c
+++ b/drivers/tty/serial/8250/8250_dma.c
@@ -62,9 +62,14 @@ static void dma_rx_complete(void *param)
 	struct uart_8250_dma *dma = p->dma;
 	unsigned long flags;
 
-	__dma_rx_complete(p);
-
 	spin_lock_irqsave(&p->port.lock, flags);
+	if (dma->rx_running)
+		__dma_rx_complete(p);
+
+	/*
+	 * Cannot be combined with the previous check because __dma_rx_complete()
+	 * changes dma->rx_running.
+	 */
 	if (!dma->rx_running && (serial_lsr_in(p) & UART_LSR_DR))
 		p->dma->rx_dma(p);
 	spin_unlock_irqrestore(&p->port.lock, flags);
-- 
2.30.2