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 -- Daniel Vetter Mail: daniel@xxxxxxxx Mobile: +41 (0)79 365 57 48 -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
