netdev - RE: [PATCH net-next 30/33] xdp: clear grow memory in bpf_xdp_adjust

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Date:   Sun, 26 Apr 2020 22:26:54 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>, sameehj@...zon.com
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, zorik@...zon.com, akiyano@...zon.com,
        gtzalik@...zon.com,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        John Fastabend <john.fastabend@...il.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        David Ahern <dsahern@...il.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Saeed Mahameed <saeedm@...lanox.com>,
        steffen.klassert@...unet.com
Subject: RE: [PATCH net-next 30/33] xdp: clear grow memory in
 bpf_xdp_adjust_tail()

Jesper Dangaard Brouer wrote:
> Clearing memory of tail when grow happens, because it is too easy
> to write a XDP_PASS program that extend the tail, which expose
> this memory to users that can run tcpdump.
> 
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
> ---

Hi Jesper, Thanks for the series any idea what the cost of doing
this is? If you have some data I would be curious to know a
baseline measurment, a grow with memset, then a grow with memset.
I'm guess this can be relatively expensive?

>  net/core/filter.c |    4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 5e9c387f74eb..889d96a690c2 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -3442,6 +3442,10 @@ BPF_CALL_2(bpf_xdp_adjust_tail, struct xdp_buff *, xdp, int, offset)
>  	if (unlikely(data_end < xdp->data + ETH_HLEN))
>  		return -EINVAL;
>  
> +	/* Clear memory area on grow, can contain uninit kernel memory */
> +	if (offset > 0)
> +		memset(xdp->data_end, 0, offset);
> +
>  	xdp->data_end = data_end;
>  
>  	return 0;
> 
>