netdev - Re: [net-next PATCH v2] udp: Resolve NULL pointer dereference over flow-based vxlan device

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Date:	Thu, 12 May 2016 13:27:10 -0700
From:	Alexander Duyck <alexander.duyck@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Alexander Duyck <aduyck@...antis.com>,
	Netdev <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Thomas Graf <tgraf@...g.ch>, Tom Herbert <tom@...bertland.com>,
	Jiri Benc <jbenc@...hat.com>
Subject: Re: [net-next PATCH v2] udp: Resolve NULL pointer dereference over
 flow-based vxlan device

On Thu, May 12, 2016 at 12:55 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Thu, 2016-05-12 at 12:51 -0700, Alexander Duyck wrote:
>> While testing an OpenStack configuration using VXLANs I saw the following
>> call trace:
>
>> The following trace is seen when receiving a DHCP request over a flow-based
>> VXLAN tunnel.  I believe this is caused by the metadata dst having a NULL
>> dev value and as a result dev_net(dev) is causing a NULL pointer dereference.
>>
>> To resolve this I am replacing the check for skb_dst() with skb_valid_dst()
>> so that we do not attempt to use the metadata dst to retrieve a device in
>> order to determine the network namespace.
>>
>> Fixes: 63058308cd55 ("udp: Add udp6_lib_lookup_skb and udp4_lib_lookup_skb")
>> Signed-off-by: Alexander Duyck <aduyck@...antis.com>
>> ---
>>  net/ipv4/udp.c |    3 ++-
>>  net/ipv6/udp.c |    3 ++-
>>  2 files changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
>> index f67f52ba4809..69aa7ab81933 100644
>> --- a/net/ipv4/udp.c
>> +++ b/net/ipv4/udp.c
>> @@ -114,6 +114,7 @@
>>  #include <net/busy_poll.h>
>>  #include "udp_impl.h"
>>  #include <net/sock_reuseport.h>
>> +#include <net/dst_metadata.h>
>>
>>  struct udp_table udp_table __read_mostly;
>>  EXPORT_SYMBOL(udp_table);
>> @@ -614,7 +615,7 @@ struct sock *udp4_lib_lookup_skb(struct sk_buff *skb,
>>  {
>>       const struct iphdr *iph = ip_hdr(skb);
>>       const struct net_device *dev =
>> -         skb_dst(skb) ? skb_dst(skb)->dev : skb->dev;
>> +             skb_valid_dst(skb) ? skb_dst(skb)->dev : skb->dev;
>
> Looks overly complicated to me.
>
> If this is called from GRO, why don't we simply use skb->dev ?

I'm assuming this was using skb_dst(skb)->dev in order to allow for
use of this function by other callers since the original function
__udp4_lib_lookup_skb was using that.  If we change this then it
reduces the likelihood of the code being reusable by other callers.
In such a case I would probably want to go through and also rename the
functions to make sure they are tagged as being GRO specific.

- Alex