Re: [PATCH] i2c: i801: fix potential race in i801_block_transaction_byte_by_byte

[Jean Delvare <jdelvare@suse.de>]

Hi Heiner,

On Tue, 29 Aug 2023 08:25:23 +0200, Heiner Kallweit wrote:
> Currently we set SMBHSTCNT_LAST_BYTE only after the host has started
> receiving the last byte. If we get e.g. preempted before setting
> SMBHSTCNT_LAST_BYTE, the host may be finished with receiving the byte
> before SMBHSTCNT_LAST_BYTE is set.
> Therefore change the code to set SMBHSTCNT_LAST_BYTE before writing
> SMBHSTSTS_BYTE_DONE for the byte before the last byte. Now the code
> is also consistent with what we do in i801_isr_byte_done().
> 
> Fixes: efa3cb15ad8b ("i2c-i801: Refactor use of LAST_BYTE in i801_block_transaction_byte_by_byte")

I don't think this is true. This patch refactored the code but didn't
change the logic. The bug existed before already. As far as I see, the
race condition already existed when the kernel switched to git, so
there's no point in having a Fixes statement.

> Reported-by: Jean Delvare <jdelvare@suse.com>
> Cc: stable@vger.kernel.org
> Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
> ---
>  drivers/i2c/busses/i2c-i801.c | 19 +++++++++----------
>  1 file changed, 9 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/i2c/busses/i2c-i801.c b/drivers/i2c/busses/i2c-i801.c
> index 7a0ccc584..8acf09539 100644
> --- a/drivers/i2c/busses/i2c-i801.c
> +++ b/drivers/i2c/busses/i2c-i801.c
> @@ -679,15 +679,11 @@ static int i801_block_transaction_byte_by_byte(struct i801_priv *priv,
>  		return result ? priv->status : -ETIMEDOUT;
>  	}
>  
> -	for (i = 1; i <= len; i++) {
> -		if (i == len && read_write == I2C_SMBUS_READ)
> -			smbcmd |= SMBHSTCNT_LAST_BYTE;
> -		outb_p(smbcmd, SMBHSTCNT(priv));
> -
> -		if (i == 1)
> -			outb_p(inb(SMBHSTCNT(priv)) | SMBHSTCNT_START,
> -			       SMBHSTCNT(priv));
> +	if (len == 1 && read_write == I2C_SMBUS_READ)
> +		smbcmd |= SMBHSTCNT_LAST_BYTE;
> +	outb_p(smbcmd | SMBHSTCNT_START, SMBHSTCNT(priv));
>  
> +	for (i = 1; i <= len; i++) {
>  		status = i801_wait_byte_done(priv);
>  		if (status)
>  			return status;
> @@ -710,9 +706,12 @@ static int i801_block_transaction_byte_by_byte(struct i801_priv *priv,
>  			data->block[0] = len;
>  		}
>  
> -		/* Retrieve/store value in SMBBLKDAT */
> -		if (read_write == I2C_SMBUS_READ)
> +		if (read_write == I2C_SMBUS_READ) {
>  			data->block[i] = inb_p(SMBBLKDAT(priv));
> +			if (i == len - 1)
> +				outb_p(smbcmd | SMBHSTCNT_LAST_BYTE, SMBHSTCNT(priv));
> +		}
> +
>  		if (read_write == I2C_SMBUS_WRITE && i+1 <= len)
>  			outb_p(data->block[i+1], SMBBLKDAT(priv));
>  

Looks good and tested OK.

Reviewed-by: Jean Delvare <jdelvare@suse.de>

-- 
Jean Delvare
SUSE L3 Support