Re: [PATCH] kfifo: add memory barrier in kfifo to prevent data loss

["xiaoguangrong(Xiao Guangrong)" <xiaoguangrong@tencent.com>]

On 12/12/18 8:50 AM, Kees Cook wrote:
> On Mon, Dec 10, 2018 at 7:41 PM <yulei.kernel@gmail.com> wrote:
>>
>> From: Yulei Zhang <yuleixzhang@tencent.com>
>>
>> Early this year we spot there may be two issues in kernel
>> kfifo.
>>
>> One is reported by Xiao Guangrong to linux kernel.
>> https://lkml.org/lkml/2018/5/11/58
>> In current kfifo implementation there are missing memory
>> barrier in the read side, so that without proper barrier
>> between reading the kfifo->in and fetching the data there
>> is potential ordering issue.
>>
>> Beside that, there is another potential issue in kfifo,
>> please consider the following case:
>> at the beginning
>> ring->size = 4
>> ring->out = 0
>> ring->in = 4
>>
>>      Consumer                        Producer
>> ---------------                  --------------
>> index = ring->out; /* index == 0 */
>> ring->out++; /* ring->out == 1 */
>> < Re-Order >
>>                                   out = ring->out;
>>                                   if (ring->in - out >= ring->mask)
>>                                       return -EFULL;
>>                                   /* see the ring is not full */
>>                                   index = ring->in & ring->mask;
>>                                   /* index == 0 */
>>                                   ring->data[index] = new_data;
>> 　　　　　　　　　　　　　　　　 ring->in++;
>>
>> data = ring->data[index];
>> /* you will find the old data is overwritten by the new_data */
>>
>> In order to avoid the issue:
>> 1) for the consumer, we should read the ring->data[] out before
>> updating ring->out
>> 2) for the producer, we should read ring->out before updating
>> ring->data[]
>>
>> So in this patch we introduce the following four functions which
>> are wrapped with proper memory barrier and keep in pairs to make
>> sure the in and out index are fetched and updated in order to avoid
>> data loss.
>>
>> kfifo_read_index_in()
>> kfifo_write_index_in()
>> kfifo_read_index_out()
>> kfifo_write_index_out()
>>
>> Signed-off-by: Yulei Zhang <yuleixzhang@tencent.com>
>> Signed-off-by: Guangrong Xiao <xiaoguangrong@tencent.com>
> 
> I've added some more people to CC that might want to see this. Thanks
> for sending this!

Hi,

Ping... could anyone have a look? ;)

Thanks!