Re: [PATCH] dma-buf: add get_dma_buf()

[Tomasz Stanislawski <t.stanislaws@samsung.com>]

On 05/22/2012 04:32 PM, Daniel Vetter wrote:
> On Tue, May 22, 2012 at 03:47:12PM +0200, Tomasz Stanislawski wrote:
>> Hi,
>> I think I discovered an interesting issue with dma_buf.
>> I found out that dma_buf_fd does not increase reference
>> count for dma_buf::file. This leads to potential kernel
>> crash triggered by user space. Please, take a look on
>> the scenario below:
>>
>> The applications spawns two thread. One of them is exporting DMABUF.
>>
>>       Thread I         |   Thread II       | Comments
>> -----------------------+-------------------+-----------------------------------
>> dbuf = dma_buf_export  |                   | dma_buf is creates, refcount is 1
>> fd = dma_buf_fd(dbuf)  |                   | assume fd is set to 42, refcount is still 1
>>                        |      close(42)    | The file descriptor is closed asynchronously, dbuf's refcount drops to 0
>>                        |  dma_buf_release  | dbuf structure is freed, dbuf becomes a dangling pointer
>> int size = dbuf->size; |                   | the dbuf is dereferenced, causing a kernel crash
>> -----------------------+-------------------+-----------------------------------
>>
>> I think that the problem could be fixed in two ways.
>> a) forcing driver developer to call get_dma_buf just before calling dma_buf_fd.
>> b) increasing dma_buf->file's reference count at dma_buf_fd
>>
>> I prefer solution (b) because it prevents symmetry between dma_buf_fd and close.
>> I mean that dma_buf_fd increases reference count, close decreases it.
>>
>> What is your opinion about the issue?
> 
> I guess most exporters would like to hang onto the exported dma_buf a bit
> and hence need a reference (e.g. to cache the dma_buf as long as the
> underlying buffer object exists). So I guess we can change the semantics
> of dma_buf_fd from transferring the reference you currently have (and
> hence forbidding any further access by the caller) to grabbing a reference
> of it's on for the fd that is created.
> -Daniel

Hi Daniel,
Would it be simpler, safer and more intuitive if dma_buf_fd increased
dmabuf->file's reference counter?

Regards,
Tomasz Stanislawski