linux-kernel - Re: [PATCH] spi: spidev: fix possible NULL dereference

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Date:	Fri, 11 Sep 2015 15:11:32 +0300
From:	Jarkko Nikula <jarkko.nikula@...ux.intel.com>
To:	Sudip Mukherjee <sudipm.mukherjee@...il.com>,
	Mark Brown <broonie@...nel.org>
CC:	linux-kernel@...r.kernel.org, linux-spi@...r.kernel.org
Subject: Re: [PATCH] spi: spidev: fix possible NULL dereference

Hi

On 09/10/2015 02:18 PM, Sudip Mukherjee wrote:
> During the last close we are freeing spidev if spidev->spi is NULL, but
> just before checking if spidev->spi is NULL we are dereferencing it.
> Lets add a check there to avoid the NULL dereference.
>
> Signed-off-by: Sudip Mukherjee <sudip@...torindia.org>
> ---
>   drivers/spi/spidev.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/spi/spidev.c b/drivers/spi/spidev.c
> index fba92a5..ef008e5 100644
> --- a/drivers/spi/spidev.c
> +++ b/drivers/spi/spidev.c
> @@ -651,7 +651,8 @@ static int spidev_release(struct inode *inode, struct file *filp)
>   		kfree(spidev->rx_buffer);
>   		spidev->rx_buffer = NULL;
>
> -		spidev->speed_hz = spidev->spi->max_speed_hz;
> +		if (spidev->spi)
> +			spidev->speed_hz = spidev->spi->max_speed_hz;
>
>   		/* ... after we unbound from the underlying device? */
>   		spin_lock_irq(&spidev->spi_lock);

Actually this fixes a *real* NULL dereference case but it is not related 
to normal open/close life cycle of spidev. It requires that spidev is 
kept open when SPI master is removed.

spidev->spi is set to NULL in spidev_remove() and it is called before 
spidev_release() in case master is removed while spidev is in use

It would be good to update this to commit log, add a tag below and Cc 
<stable@...r.kernel.org>

Fixes: 9169051617df ("spi: spidev: Don't mangle max_speed_hz in 
underlying spi device")

You can add these too:

Reviewed-by: Jarkko Nikula <jarkko.nikula@...ux.intel.com>
Tested-by: Jarkko Nikula <jarkko.nikula@...ux.intel.com>
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