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

[Heiner Kallweit <hkallweit1@gmail.com>]

On 01.09.2023 18:44, Jean Delvare wrote:
> 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.
> 
Shall we go with the patch as-is or do you think it's better to resubmit
w/o the Fixes tag?

>> 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>
>