netdev - Re: [PATCH net-next] xdp: implement xdp_redirect

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Message-ID: <59B043BA.1030001@iogearbox.net>
Date:   Wed, 06 Sep 2017 20:51:38 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     John Fastabend <john.fastabend@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>
CC:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Andy Gospodarek <andy@...yhouse.net>
Subject: Re: [PATCH net-next] xdp: implement xdp_redirect_map for generic
 XDP

On 09/06/2017 08:42 PM, John Fastabend wrote:
> On 09/06/2017 11:18 AM, Jesper Dangaard Brouer wrote:
>> On Wed, 06 Sep 2017 18:24:07 +0200
>> Daniel Borkmann <daniel@...earbox.net> wrote:
>>> On 09/06/2017 05:26 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
>>>
>>> Good point, but ...
>>>
>>> [...]
>>>>    net/core/filter.c |   29 +++++++++++++++++++++++++++++
>>>>    1 file changed, 29 insertions(+)
>>>>
>>>> diff --git a/net/core/filter.c b/net/core/filter.c
>>>> index 5912c738a7b2..6a4745bf2c9f 100644
>>>> --- a/net/core/filter.c
>>>> +++ b/net/core/filter.c
>>>> @@ -2562,6 +2562,32 @@ int xdp_do_redirect(struct net_device *dev, struct xdp_buff *xdp,
>>>>    }
>>>>    EXPORT_SYMBOL_GPL(xdp_do_redirect);
>>>>
>>>> +static int xdp_do_generic_redirect_map(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;
>>>> +	int err;
>>>> +
>>>> +	ri->ifindex = 0;
>>>> +	ri->map = NULL;
>>>> +
>>>> +	fwd = __dev_map_lookup_elem(map, index);
>>>> +	if (!fwd) {
>>>> +		err = -EINVAL;
>>>> +		goto err;
>>>> +	}
>>>> +	skb->dev = fwd;
>>>> +	_trace_xdp_redirect_map(dev, xdp_prog, fwd, map, index);
>>>> +	return 0;
>>>> +err:
>>>> +	_trace_xdp_redirect_map_err(dev, xdp_prog, fwd, map, index, err);
>>>> +	return err;
>>>> +}
>>>> +
>>>>    int xdp_do_generic_redirect(struct net_device *dev, struct sk_buff *skb,
>>>>    			    struct bpf_prog *xdp_prog)
>>>>    {
>>>> @@ -2571,6 +2597,9 @@ int xdp_do_generic_redirect(struct net_device *dev, struct sk_buff *skb,
>>>>    	unsigned int len;
>>>>    	int err = 0;
>>>>
>>>> +	if (ri->map)
>>>> +		return xdp_do_generic_redirect_map(dev, skb, xdp_prog);
>>>
>>> This is not quite correct. Really, the only thing you want
>>> to do here is more or less ...
>>>
>>> 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;
>>>
>>> 	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;
>>> 	}
>>> [...]
>>>
>>> ... such that you have a common path to also do the IFF_UP
>>> and MTU checks that are done here, but otherwise omitted in
>>> your patch.
>>
>> Ah, yes. My patch miss the IFF_UP and MTU check. (I was too inspired by
>> xdp_do_redirect_map).
>>
>>> Otherwise it looks good, but note that it also doesn't really
>>> resolve the issue you mention wrt stale map pointers by the
>>> way.  [...]
>>
>> I actually discovered more cases where we can crash the kernel :-(
>>
>> E.g. driver not supporting XDP_REDIRECT, are still allowed to load an
>> XDP bpf_prog that call bpf_redirect_map() and set the ->map pointer,
>> but it will never call xdp_do_redirect() (which is responsible for
>> clearing/consuming ->map pointer).
>>
>> Another case: You can also call bpf_redirect_map() and then NOT return
>> XDP_REDIRECT (it is obviously strange, but the bpf-helper API allows it).
>
> I think we can cover both these cases with previous suggestion to check
> prog pointers. Working up a patch now.

Yep, they would both be covered.