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

[Jon Hunter]

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.

Jon

-- 
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