io_uring/rsrc: ignore dummy_ubuf for buffer cloning

For placeholder buffers, &dummy_ubuf is assigned which is a static
value. When buffers are attempted cloned, don't attempt to grab a
reference to it, as we both don't need it and it'll actively fail as
dummy_ubuf doesn't have a valid reference count setup.

Link: https://lore.kernel.org/io-uring/Zw8dkUzsxQ5LgAJL@ly-workstation/Reported-by: Lai, Yi <yi1.lai@linux.intel.com>
Fixes: 7cc2a6ea ("io_uring: add IORING_REGISTER_COPY_BUFFERS method")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/rsrc.c

@@ -1176,7 +1176,8 @@ static int io_clone_buffers(struct io_ring_ctx *ctx, struct io_ring_ctx *src_ctx
 	for (i = 0; i < nbufs; i++) {
 		struct io_mapped_ubuf *src = src_ctx->user_bufs[i];
 
-		refcount_inc(&src->refs);
+		if (src != &dummy_ubuf)
+			refcount_inc(&src->refs);
 		user_bufs[i] = src;
 	}