[PATCH] dmabuf: fix dmabuf file poll uaf issue

[Charan Teja Kalla]

Hi TJ,

Seems I have got answers from [1], where it is agreed upon epoll() is
the source of issue.

Thanks a lot for the discussion.

[1] https://lore.kernel.org/lkml/0000000000002d631f0615918f1e@google.com/

Thanks
Charan

On 5/5/2024 9:50 PM, Charan Teja Kalla wrote:
> Thanks T.J for the reply!!
> 
> On 5/4/2024 4:43 AM, T.J. Mercier wrote:
>> It looks like a similar conclusion about epoll was reached at:
>> https://lore.kernel.org/all/a87d7ef8-2c59-4dc5-ba0a-b821d1effc72@amd.com/
>>
> I am unaware of this discussion. Thanks...
> 
>> I agree with Christian that it should not be possible for the file to
>> be freed while inside dma_buf_poll. Aside from causing problems in
>> dma_buf_poll, ep_item_poll itself would have issues dereferencing the
>> freed file pointer.
>>
> Not sure about my understanding: ep_item_poll() always call the ->poll()
> interface with a stable 'struct file' because of ep->mtx. This lock
> ensures that:
>    a) If eventpoll_release_file() get the ep->mtx first, ->poll()
> corresponds to the epitem(target file) will never be called, because it
> is removed from the rdlist.
> 
>    b) If ep_send_events() get the ep->mtx() first, ->poll() will get
> called with a stable 'struct file', __but the refcount(->f_count) of a
> file can be zero__. I am saying that this is stable because the 'struct
> file' contents are still valid till we are in ->poll().
> 
> Can you/Christian help me with what I am missing here to say that
> ->poll() is receiving stale 'struct file*', please?
> 
> And, If you are convinced with above, I think, It should have been the
> responsibility of ->poll() implementation to have taken refcount on a
> file that is going to be still valid even after ->poll() exits. Incase
> of dma_buf_poll() implementation, it took the refcount on a file that is
> not going to be valid once the dma_buf_poll() exits(because of mentioned
> race with the freeing of the 'struct file*').
> 
> So, in dma_buf_poll(), Should we be using atomic_long_inc_not_zero()
> based implementation to take the refcount on a file?
> 
> Thanks,
> Charan