Subject: Re: [PATCH net-next] xdp: implement xdp_redirect_map for generic XDP

From: John Fastabend <[email-masked]>
Date: Wed, 06 Sep 2017 11:42:18 -0700

On 09/06/2017 11:18 AM, Jesper Dangaard Brouer wrote:
> On Wed, 06 Sep 2017 18:24:07 +0200
> Daniel Borkmann <[email-masked]> 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.