bpf, mlx4: fix prog refcount in mlx4_en_try_alloc_resources error path

Commit 67f8b1d ("net/mlx4_en: Refactor the XDP forwarding rings
scheme") added a bug in that the prog's reference count is not dropped
in the error path when mlx4_en_try_alloc_resources() is failing from
mlx4_xdp_set().

We previously took bpf_prog_add(prog, priv->rx_ring_num - 1), that we
need to release again. Earlier in the call path, dev_change_xdp_fd()
itself holds a reference to the prog as well (hence the '- 1' in the
bpf_prog_add()), so a simple atomic_sub() is safe to use here. When
an error is propagated, then bpf_prog_put() is called eventually from
dev_change_xdp_fd()

Fixes: 67f8b1d ("net/mlx4_en: Refactor the XDP forwarding rings scheme")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlx4/en_netdev.c

@@ -2747,8 +2747,11 @@ static int mlx4_xdp_set(struct net_device *dev, struct bpf_prog *prog)
 	}
 
 	err = mlx4_en_try_alloc_resources(priv, tmp, &new_prof);
-	if (err)
+	if (err) {
+		if (prog)
+			bpf_prog_sub(prog, priv->rx_ring_num - 1);
 		goto unlock_out;
+	}
 
 	if (priv->port_up) {
 		port_up = 1;

include/linux/bpf.h

@@ -234,6 +234,7 @@ void bpf_register_map_type(struct bpf_map_type_list *tl);
 struct bpf_prog *bpf_prog_get(u32 ufd);
 struct bpf_prog *bpf_prog_get_type(u32 ufd, enum bpf_prog_type type);
 struct bpf_prog *bpf_prog_add(struct bpf_prog *prog, int i);
+void bpf_prog_sub(struct bpf_prog *prog, int i);
 struct bpf_prog *bpf_prog_inc(struct bpf_prog *prog);
 void bpf_prog_put(struct bpf_prog *prog);
 
@@ -303,6 +304,10 @@ static inline struct bpf_prog *bpf_prog_add(struct bpf_prog *prog, int i)
 	return ERR_PTR(-EOPNOTSUPP);
 }
 
+static inline void bpf_prog_sub(struct bpf_prog *prog, int i)
+{
+}
+
 static inline void bpf_prog_put(struct bpf_prog *prog)
 {
 }

kernel/bpf/syscall.c

@@ -680,6 +680,17 @@ struct bpf_prog *bpf_prog_add(struct bpf_prog *prog, int i)
 }
 EXPORT_SYMBOL_GPL(bpf_prog_add);
 
+void bpf_prog_sub(struct bpf_prog *prog, int i)
+{
+	/* Only to be used for undoing previous bpf_prog_add() in some
+	 * error path. We still know that another entity in our call
+	 * path holds a reference to the program, thus atomic_sub() can
+	 * be safely used in such cases!
+	 */
+	WARN_ON(atomic_sub_return(i, &prog->aux->refcnt) == 0);
+}
+EXPORT_SYMBOL_GPL(bpf_prog_sub);
+
 struct bpf_prog *bpf_prog_inc(struct bpf_prog *prog)
 {
 	return bpf_prog_add(prog, 1);