linux-kernel - Re: [PATCH v3 05/12] spi: spi-fsl-dspi: Protect against races on dspi->words_in

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Date:   Mon, 16 Mar 2020 12:35:04 +0000 (UTC)
From:   Vladimir Oltean <olteanv@...il.com>
To:     broonie@...nel.org
Cc:     linux-spi@...r.kernel.org, linux-kernel@...r.kernel.org,
        shawnguo@...nel.org, robh+dt@...nel.org, mark.rutland@....com,
        devicetree@...r.kernel.org, eha@...f.com, angelo@...am.it,
        andrew.smirnov@...il.com, gustavo@...eddedor.com, weic@...dia.com,
        mhosny@...dia.com, michael@...le.cc, peng.ma@....com
Subject: Re: [PATCH v3 05/12] spi: spi-fsl-dspi: Protect against races on
 dspi->words_in_flight

Mar 15, 2020 12:44:02 AM Vladimir Oltean <olteanv@...il.com>:

> From: Vladimir Oltean <vladimir.oltean@....com>
>
> dspi->words_in_flight is a variable populated in the *_write functions
> and used in the dspi_fifo_read function. It is also used in
> dspi_fifo_write, immediately after transmission, to update the
> message->actual_length variable used by higher layers such as spi-mem
> for integrity checking.
>
> But it may happen that the IRQ which calls dspi_fifo_read to be
> triggered before the updating of message->actual_length takes place. In
> that case, dspi_fifo_read will decrement dspi->words_in_flight to -1,
> and that will cause an invalid modification of message->actual_length.
>
> Make the simplest fix possible: don't decrement the actual shared
> variable in dspi->words_in_flight from dspi_fifo_read, but actually a
> copy of it which is on stack.
>
> Suggested-by: Michael Walle <michael@...le.cc>
> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>
> ---
> Changes in v4:
> Patch is new.
>
> drivers/spi/spi-fsl-dspi.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/spi/spi-fsl-dspi.c b/drivers/spi/spi-fsl-dspi.c
> index 51224b772680..3ac004aa2abd 100644
> --- a/drivers/spi/spi-fsl-dspi.c
> +++ b/drivers/spi/spi-fsl-dspi.c
> @@ -765,8 +765,10 @@ static u32 dspi_popr_read(struct fsl_dspi *dspi)
>
> static void dspi_fifo_read(struct fsl_dspi *dspi)
> {
> + int num_fifo_entries = dspi->words_in_flight;
> +
> /* Read one FIFO entry and push to rx buffer */
> - while (dspi->words_in_flight--)
> + while (num_fifo_entries--)
> dspi_push_rx(dspi, dspi_popr_read(dspi));
> }
>
> --
> 2.17.1
>
>

Fixes: d59c90a2400f ("spi: spi-fsl-dspi: Convert TCFQ users to XSPI FIFO mode")

Patchwork should know to pick up this tag.

Thanks,
-Vladimir