Re: [PATCH v4 01/14] dmaengine: tegra-apb: Fix use-after-free

[Dmitry Osipenko]

28.01.2020 17:51, Dmitry Osipenko ÐÐÑÐÑ:
> 28.01.2020 17:02, Jon Hunter ÐÐÑÐÑ:
>>
>> On 16/01/2020 20:10, Dmitry Osipenko wrote:
>>> 15.01.2020 12:00, Jon Hunter ÐÐÑÐÑ:
>>>>
>>>> On 14/01/2020 20:33, Dmitry Osipenko wrote:
>>>>> 14.01.2020 18:09, Jon Hunter ÐÐÑÐÑ:
>>>>>>
>>>>>> On 12/01/2020 17:29, Dmitry Osipenko wrote:
>>>>>>> I was doing some experiments with I2C and noticed that Tegra APB DMA
>>>>>>> driver crashes sometime after I2C DMA transfer termination. The crash
>>>>>>> happens because tegra_dma_terminate_all() bails out immediately if pending
>>>>>>> list is empty, thus it doesn't release the half-completed descriptors
>>>>>>> which are getting re-used before ISR tasklet kicks-in.
>>>>>>
>>>>>> Can you elaborate a bit more on how these are getting re-used? What is
>>>>>> the sequence of events which results in the panic? I believe that this
>>>>>> was also reported in the past [0] and so I don't doubt there is an issue
>>>>>> here, but would like to completely understand this.
>>>>>>
>>>>>> Thanks!
>>>>>> Jon
>>>>>>
>>>>>> [0] https://lore.kernel.org/patchwork/patch/675349/
>>>>>>
>>>>>
>>>>> In my case it happens in the touchscreen driver during of the
>>>>> touchscreen's interrupt handling (in a threaded IRQ handler) + CPU is
>>>>> under load and there is other interrupts activity. So what happens here
>>>>> is that the TS driver issues one I2C transfer, which fails with
>>>>> (apparently bogus) timeout (because DMA descriptor is completed and
>>>>> removed from the pending list, but tasklet not executed yet), and then
>>>>> TS immediately issues another I2C transfer that re-uses the
>>>>> yet-incompleted descriptor. That's my understanding.
>>>>
>>>> OK, but what is the exact sequence that it allowing it to re-use the
>>>> incompleted descriptor?
>>>
>>>    TDMA driver                      DMA Client
>>>
>>> 1.
>>>                                     dmaengine_prep()
>>>
>>> 2.
>>>    tegra_dma_desc_get()
>>>    dma_desc = kzalloc()
>>>    ...
>>>    tegra_dma_prep_slave_sg()
>>>    INIT_LIST_HEAD(&dma_desc->tx_list);
>>>    INIT_LIST_HEAD(&dma_desc->cb_node);
>>>    list_add_tail(sgreq->node,
>>>                  dma_desc->tx_list)
>>>
>>> 3.
>>>                                     dma_async_issue_pending()
>>>
>>> 4.
>>>    tegra_dma_tx_submit()
>>>    list_splice_tail_init(dma_desc->tx_list,
>>>                          tdc->pending_sg_req)
>>>
>>> 5.
>>>    tegra_dma_isr()
>>>    ...
>>>    handle_once_dma_done()
>>>    ...
>>>    sgreq = list_first_entry(tdc->pending_sg_req)
>>>    list_del(sgreq->node);
>>>    ...
>>>    list_add_tail(dma_desc->cb_node,
>>>                  tdc->cb_desc);
>>>    list_add_tail(dma_desc->node,
>>>                  tdc->free_dma_desc);
>>
>> Isn't this the problem here, that we have placed this on the free list
>> before we are actually done?
>>
>> It seems to me that there could still be a potential race condition
>> between the ISR and the tasklet running.
> 
> Yes, this should be addressed by the patch #3 "dmaengine: tegra-apb:
> Prevent race conditions of tasklet vs free list".

correction (to avoid confusion): it's actually patch #5, my bad