[PATCH net-next 1/2] bpf, mlx4: fix prog refcount in mlx4_en_try_alloc_resources error path

[Daniel Borkmann]

Commit 67f8b1dcb9ee ("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.

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 ref to the prog as well, which is then released though
bpf_prog_put() due to the propagated error.

Fixes: 67f8b1dcb9ee ("net/mlx4_en: Refactor the XDP forwarding rings scheme")
Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
---
 drivers/net/ethernet/mellanox/mlx4/en_netdev.c |  5 ++++-
 include/linux/bpf.h                            |  1 +
 kernel/bpf/syscall.c                           | 11 +++++++++++
 3 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/mellanox/mlx4/en_netdev.c b/drivers/net/ethernet/mellanox/mlx4/en_netdev.c
index 0f6225c..4104aec 100644
--- a/drivers/net/ethernet/mellanox/mlx4/en_netdev.c
+++ b/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_add_undo(prog, priv->rx_ring_num - 1);
 		goto unlock_out;
+	}
 
 	if (priv->port_up) {
 		port_up = 1;
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index edcd96d..4f6a4f1 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -234,6 +234,7 @@ u64 bpf_event_output(struct bpf_map *map, u64 flags, void *meta, u64 meta_size,
 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_add_undo(struct bpf_prog *prog, int i);
 struct bpf_prog *bpf_prog_inc(struct bpf_prog *prog);
 void bpf_prog_put(struct bpf_prog *prog);
 
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 228f962..a6e4dd8 100644
--- a/kernel/bpf/syscall.c
+++ b/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_add_undo(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 here!
+	 */
+	atomic_sub(i, &prog->aux->refcnt);
+}
+EXPORT_SYMBOL_GPL(bpf_prog_add_undo);
+
 struct bpf_prog *bpf_prog_inc(struct bpf_prog *prog)
 {
 	return bpf_prog_add(prog, 1);
-- 
1.9.3


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