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?

Regards,
Tomasz Stanislawski



On 03/16/2012 05:04 PM, Rob Clark wrote:

From: Rob Clark <redacted>

Works in a similar way to get_file(), and is needed in cases such as
when the exporter needs to also keep a reference to the dmabuf (that
is later released with a dma_buf_put()), and possibly other similar
cases.

Signed-off-by: Rob Clark <redacted>
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