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 -- 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
