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

[Heikki Krogerus]

On Mon, Jan 30, 2023 at 01:48:40PM +0200, Ilpo Järvinen wrote:
> __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>

FWIW:

Acked-by: Heikki Krogerus <heikki.krogerus@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

-- 
heikki