Subject: Re: [PATCH] net/mlx4_en: Fix bpf_prog_add ref_cnt in mlx4

From: Brenden Blanco <[email-masked]>
Date: Wed, 9 Nov 2016 09:06:15 -0800

On Wed, Nov 09, 2016 at 10:57:32AM +0100, Daniel Borkmann wrote:
> On 11/09/2016 10:45 AM, Zhiyi Sun wrote:
> >On Wed, Nov 09, 2016 at 10:05:31AM +0100, Daniel Borkmann wrote:
> >>On 11/09/2016 08:35 AM, Zhiyi Sun wrote:
> >>>There are rx_ring_num queues. Each queue will load xdp prog. So
> >>>bpf_prog_add() should add rx_ring_num to ref_cnt.
> >>>
> >>>Signed-off-by: Zhiyi Sun <[email-masked]>
> >>
> >>Your analysis looks incorrect to me. Please elaborate in more detail why
> >>you think current code is buggy ...
> >
> >Yes, you are correct. My patch is incorrect. It is not a bug.
> >
> >>Call path is dev_change_xdp_fd(), which does bpf_prog_get_type() on the
> >>fd. This already takes a ref and only drops it in case of error. Thus
> >>in mlx4_xdp_set(), you only need priv->rx_ring_num - 1 refs for the rest
> >>of the rings, so that dropping refs from old_prog makes sure we release
> >>it again. Looks correct to me (maybe a comment would have helped there).
> >
> >I thought mlx4's code is incorrect because in mlx5's driver, function
> >mlx5e_xdp_set() calls a pair of bpf_prog_add/put, the number of add and
> >put to the refs are same. I didn't notice that one "add" has been called in its
> >calller. So, it seems that mlx5's code is incorrect, right?
> 
> Yep, I think the two attached patches are needed.
> 
> The other thing I noticed in mlx5e_create_rq() is that it calls
> bpf_prog_add(rq->xdp_prog, 1) without actually checking for errors.

> From d2bd6b3cd8636716a06b0ea3b1e041e16f87cce0 Mon Sep 17 00:00:00 2001
> Message-Id: <[email-masked]>
> From: Daniel Borkmann <[email-masked]>
> Date: Wed, 9 Nov 2016 10:31:19 +0100
> Subject: [PATCH net-next 1/2] bpf, mlx4: fix prog refcount in mlx4_en_try_alloc_resources error path
> 
> 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 <[email-masked]>
> ---
>  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);
Why not just move the above bpf_prog_add to be below the try_alloc?
Nobody needs those references until all of the resources have been
allocated, and then we can remove the need for bpf_prog_add_undo.
>  		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
> 

> From f0789544432bbb89c53c3b8ac6575d48fed97786 Mon Sep 17 00:00:00 2001
> Message-Id: <[email-masked]>
> In-Reply-To: <[email-masked]>
> References: <[email-masked]>
> From: Daniel Borkmann <[email-masked]>
> Date: Wed, 9 Nov 2016 10:51:26 +0100
> Subject: [PATCH net-next 2/2] bpf, mlx5: fix prog refcount in mlx5e_xdp_set
> 
> dev_change_xdp_fd() already holds a reference, so bpf_prog_add(prog, 1)
> is not correct as it takes one reference too much and will thus leak
> the prog eventually. Also, bpf_prog_add() can fail and is not checked
> for errors here.
> 
> Fixes: 86994156c736 ("net/mlx5e: XDP fast RX drop bpf programs support")
> Signed-off-by: Daniel Borkmann <[email-masked]>
> ---
>  drivers/net/ethernet/mellanox/mlx5/core/en_main.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
> index ba0c774..63309dd 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
> @@ -3121,8 +3121,6 @@ static int mlx5e_xdp_set(struct net_device *netdev, struct bpf_prog *prog)
>  
>  	/* exchange programs */
>  	old_prog = xchg(&priv->xdp_prog, prog);
> -	if (prog)
> -		bpf_prog_add(prog, 1);
There is also another use of bpf_prog_add down below, which does not
check the error return. Same in mlx5e_create_rq.
>  	if (old_prog)
>  		bpf_prog_put(old_prog);
> -- 
> 1.9.3