selftests/xsk: Avoid use-after-free on ctx

The put lowers the reference count to 0 and frees ctx, reading it
afterwards is invalid. Move the put after the uses and determine the
last use by the reference count being 1.

Fixes: 39e940d4 ("selftests/xsk: Destroy BPF resources only when ctx refcount drops to 0")
Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/20220901202645.1463552-1-irogers@google.com

tools/testing/selftests/bpf/xsk.c

@@ -1237,15 +1237,15 @@ void xsk_socket__delete(struct xsk_socket *xsk)
 	ctx = xsk->ctx;
 	umem = ctx->umem;
 
-	xsk_put_ctx(ctx, true);
-
-	if (!ctx->refcount) {
+	if (ctx->refcount == 1) {
 		xsk_delete_bpf_maps(xsk);
 		close(ctx->prog_fd);
 		if (ctx->has_bpf_link)
 			close(ctx->link_fd);
 	}
 
+	xsk_put_ctx(ctx, true);
+
 	err = xsk_get_mmap_offsets(xsk->fd, &off);
 	if (!err) {
 		if (xsk->rx) {