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Date:   Wed, 13 Jun 2018 12:08:07 +0300
From:   Dan Carpenter <dan.carpenter@oracle.com>
To:     Hugo Lefeuvre <hle@owl.eu.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        devel@driverdev.osuosl.org, linux-kernel@vger.kernel.org,
        kernelnewbies@kernelnewbies.org,
        Marcus Wolf <linux@wolf-entwicklungen.de>
Subject: Re: [PATCH v2] staging: pi433: fix race condition in pi433_ioctl
Message-ID: <20180613090807.4efocuy63kbg4p7r@mwanda>
References: <20180613014741.GC2265@hle-laptop.local>
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On Tue, Jun 12, 2018 at 09:47:41PM -0400, Hugo Lefeuvre wrote:
> In the PI433_IOC_WR_TX_CFG case in pi433_ioctl, instance->tx_cfg is
> modified via
> 
> copy_from_user(&instance->tx_cfg, argp, sizeof(struct pi433_tx_cfg)))
> 
> without any kind of synchronization. In the case where two threads
> would execute this same command concurrently the tx_cfg field might
> enter in an inconsistent state.
> 
> Additionally: if ioctl(PI433_IOC_WR_TX_CFG) and write() execute
> concurrently the tx config might be modified while it is being
> copied to the fifo, resulting in potential data corruption.
> 
> Fix: Get instance->tx_cfg_lock before modifying tx config in the
> PI433_IOC_WR_TX_CFG case in pi433_ioctl.
> 
> Also, do not copy data directly from user space to instance->tx_cfg.
> Instead use a temporary buffer allowing future checks for correctness
> of copied data.
> 
> Signed-off-by: Hugo Lefeuvre <hle@owl.eu.com>
> ---
> Changes in v2:
>     - Use device->tx_fifo_lock instead of introducing a new lock in
>       instance.
>     - Do not copy data directly from user space to instance->tx_cfg,
>       instead use a temporary buffer allowing future checks for
>       correctness of copied data.
> ---
>  drivers/staging/pi433/pi433_if.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/staging/pi433/pi433_if.c b/drivers/staging/pi433/pi433_if.c
> index b061f77dda41..3ec1ed01d04b 100644
> --- a/drivers/staging/pi433/pi433_if.c
> +++ b/drivers/staging/pi433/pi433_if.c
> @@ -880,6 +880,7 @@ pi433_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
>  	int			retval = 0;
>  	struct pi433_instance	*instance;
>  	struct pi433_device	*device;
> +	struct pi433_tx_cfg	tx_cfg_buffer;
>  	void __user *argp = (void __user *)arg;
>  
>  	/* Check type and command number */
> @@ -902,9 +903,15 @@ pi433_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
>  			return -EFAULT;
>  		break;
>  	case PI433_IOC_WR_TX_CFG:
> -		if (copy_from_user(&instance->tx_cfg, argp,
> -				   sizeof(struct pi433_tx_cfg)))
> +		/* do not modify tx config while it is being copied to fifo */
> +		mutex_lock(&device->tx_fifo_lock);
> +		if (copy_from_user(&tx_cfg_buffer, argp,
> +				   sizeof(struct pi433_tx_cfg))) {
> +			mutex_unlock(&device->tx_fifo_lock);
>  			return -EFAULT;
> +		}
> +		memcpy(&instance->tx_cfg, &tx_cfg_buffer, sizeof(struct pi433_tx_cfg));
> +		mutex_unlock(&device->tx_fifo_lock);

The lock is only needed around the memcpy() and that makes the code a
bit simpler as well.

regards,
dan carpenter