Re: [PATCH] epoll: try to be a _bit_ better about file lifetimes

[Kees Cook <keescook@chromium.org>]

On Sat, May 04, 2024 at 12:03:18AM +0100, Al Viro wrote:
> On Fri, May 03, 2024 at 03:46:25PM -0700, Kees Cook wrote:
> > On Fri, May 03, 2024 at 02:52:38PM -0700, Linus Torvalds wrote:
> > > That means that the file will be released - and it means that you have
> > > violated all the refcounting rules for poll().
> > 
> > I feel like I've been looking at this too long. I think I see another
> > problem here, but with dmabuf even when epoll is fixed:
> > 
> > dma_buf_poll()
> > 	get_file(dmabuf->file)		/* f_count + 1 */
> > 	dma_buf_poll_add_cb()
> > 		dma_resv_for_each_fence ...
> > 			dma_fence_add_callback(fence, ..., dma_buf_poll_cb)
> > 
> > dma_buf_poll_cb()
> > 	...
> >         fput(dmabuf->file);		/* f_count - 1 ... for each fence */
> > 
> > Isn't it possible to call dma_buf_poll_cb() (and therefore fput())
> > multiple times if there is more than 1 fence? Perhaps I've missed a
> > place where a single struct dma_resv will only ever signal 1 fence? But
> > looking through dma_fence_signal_timestamp_locked(), I don't see
> > anything about resv nor somehow looking into other fence cb_list
> > contents...
> 
> At a guess,
>                 r = dma_fence_add_callback(fence, &dcb->cb, dma_buf_poll_cb);
> 		if (!r)
> 			return true;
> 
> prevents that - it returns 0 on success and -E... on error;
> insertion into the list happens only when it's returning 0,
> so...

Yes; thank you. I *have* been looking at it all too long. :)


The last related thing is the drivers/gpu/drm/vmwgfx/ttm_object.c case:

/**
 * get_dma_buf_unless_doomed - get a dma_buf reference if possible.
 *
 * @dmabuf: Non-refcounted pointer to a struct dma-buf.
 *
 * Obtain a file reference from a lookup structure that doesn't refcount
 * the file, but synchronizes with its release method to make sure it
 * has
 * not been freed yet. See for example kref_get_unless_zero
 * documentation.
 * Returns true if refcounting succeeds, false otherwise.
 *
 * Nobody really wants this as a public API yet, so let it mature here
 * for some time...
 */
static bool __must_check get_dma_buf_unless_doomed(struct dma_buf *dmabuf)
{
        return atomic_long_inc_not_zero(&dmabuf->file->f_count) != 0L;
}

If we end up adding epi_fget(), we'll have 2 cases of using
"atomic_long_inc_not_zero" for f_count. Do we need some kind of blessed
helper to live in file.h or something, with appropriate comments?

-- 
Kees Cook