Subject: Re: [V2 PATCH net-next 1/2] xdp: implement xdp_redirect_map for generic XDP

From: Daniel Borkmann <[email-masked]>
Date: Thu, 07 Sep 2017 16:09:56 +0200

On 09/07/2017 02:33 PM, Jesper Dangaard Brouer wrote:
> Using bpf_redirect_map is allowed for generic XDP programs, but the
> appropriate map lookup was never performed in xdp_do_generic_redirect().
>
> Instead the map-index is directly used as the ifindex.  For the
> xdp_redirect_map sample in SKB-mode '-S', this resulted in trying
> sending on ifindex 0 which isn't valid, resulting in getting SKB
> packets dropped.  Thus, the reported performance numbers are wrong in
> commit 24251c264798 ("samples/bpf: add option for native and skb mode
> for redirect apps") for the 'xdp_redirect_map -S' case.
>
> It might seem innocent this was lacking, but it can actually crash the
> kernel.  The potential crash is caused by not consuming redirect_info->map.
> The bpf_redirect_map helper will set this_cpu_ptr(&redirect_info)->map
> pointer, which will survive even after unloading the xdp bpf_prog and
> deallocating the devmap data-structure.  This leaves a dead map
> pointer around.  The kernel will crash when loading the xdp_redirect
> sample (in native XDP mode) as it doesn't reset map (via bpf_redirect)
> and returns XDP_REDIRECT, which will cause it to dereference the map
> pointer.
>
> Fixes: 6103aa96ec07 ("net: implement XDP_REDIRECT for xdp generic")
> Fixes: 24251c264798 ("samples/bpf: add option for native and skb mode for redirect apps")
> Signed-off-by: Jesper Dangaard Brouer <[email-masked]>
> ---
>   include/trace/events/xdp.h |    4 ++--
>   net/core/filter.c          |   14 +++++++++++---
>   2 files changed, 13 insertions(+), 5 deletions(-)
>
> diff --git a/include/trace/events/xdp.h b/include/trace/events/xdp.h
> index 862575ac8da9..4e16c43fba10 100644
> --- a/include/trace/events/xdp.h
> +++ b/include/trace/events/xdp.h
> @@ -138,11 +138,11 @@ DEFINE_EVENT_PRINT(xdp_redirect_template, xdp_redirect_map_err,
>
>   #define _trace_xdp_redirect_map(dev, xdp, fwd, map, idx)		\
>   	 trace_xdp_redirect_map(dev, xdp, fwd ? fwd->ifindex : 0,	\
> -				0, map, idx);
> +				0, map, idx)
>
>   #define _trace_xdp_redirect_map_err(dev, xdp, fwd, map, idx, err)	\
>   	 trace_xdp_redirect_map_err(dev, xdp, fwd ? fwd->ifindex : 0,	\
> -				    err, map, idx);
> +				    err, map, idx)
>
>   #endif /* _TRACE_XDP_H */
>
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 5912c738a7b2..3767470cab6c 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -2566,13 +2566,19 @@ int xdp_do_generic_redirect(struct net_device *dev, struct sk_buff *skb,
>   			    struct bpf_prog *xdp_prog)
>   {
>   	struct redirect_info *ri = this_cpu_ptr(&redirect_info);
> +	struct bpf_map *map = ri->map;
>   	u32 index = ri->ifindex;
>   	struct net_device *fwd;
>   	unsigned int len;
>   	int err = 0;
>
> -	fwd = dev_get_by_index_rcu(dev_net(dev), index);
>   	ri->ifindex = 0;
> +	ri->map = NULL;
> +
> +	if (map)
> +		fwd = __dev_map_lookup_elem(map, index);
> +	else
> +		fwd = dev_get_by_index_rcu(dev_net(dev), index);
>   	if (unlikely(!fwd)) {
>   		err = -EINVAL;
>   		goto err;
> @@ -2590,10 +2596,12 @@ int xdp_do_generic_redirect(struct net_device *dev, struct sk_buff *skb,
>   	}
>
>   	skb->dev = fwd;

Looks much better above, thanks!

> -	_trace_xdp_redirect(dev, xdp_prog, index);
> +	map ? _trace_xdp_redirect_map(dev, xdp_prog, fwd, map, index)
> +		: _trace_xdp_redirect(dev, xdp_prog, index);

Could we rather make this in a way such that when the two
tracepoints are disabled and thus patched out, that we can
also omit the extra conditional which has no purpose then?
Perhaps just a consolidated _trace_xdp_generic_redirect_map()
would be better to avoid this altogether given we have twice
the same anyway, here and in err path.

Thanks,
Daniel

>   	return 0;
>   err:
> -	_trace_xdp_redirect_err(dev, xdp_prog, index, err);
> +	map ? _trace_xdp_redirect_map_err(dev, xdp_prog, fwd, map, index, err)
> +		: _trace_xdp_redirect_err(dev, xdp_prog, index, err);
>   	return err;
>   }
>   EXPORT_SYMBOL_GPL(xdp_do_generic_redirect);
>