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

[Jon Hunter]

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?

Thanks
Jon

-- 
nvpublic