Re: [PATCH net] udp: prevent bugcheck if filter truncates packet too much

[Daniel Borkmann <daniel@iogearbox.net>]

On 07/09/2016 02:20 AM, Alexei Starovoitov wrote:
> On Sat, Jul 09, 2016 at 01:31:40AM +0200, Eric Dumazet wrote:
>> On Fri, 2016-07-08 at 17:52 +0200, Michal Kubecek wrote:
>>> If socket filter truncates an udp packet below the length of UDP header
>>> in udpv6_queue_rcv_skb() or udp_queue_rcv_skb(), it will trigger a
>>> BUG_ON in skb_pull_rcsum(). This BUG_ON (and therefore a system crash if
>>> kernel is configured that way) can be easily enforced by an unprivileged
>>> user which was reported as CVE-2016-6162. For a reproducer, see
>>> http://seclists.org/oss-sec/2016/q3/8
>>>
>>> Fixes: e6afc8ace6dd ("udp: remove headers from UDP packets before queueing")
>>> Reported-by: Marco Grassi <marco.gra@gmail.com>
>>> Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
>>> ---
>>>   net/ipv4/udp.c | 2 ++
>>>   net/ipv6/udp.c | 2 ++
>>>   2 files changed, 4 insertions(+)
>>>
>>> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
>>> index ca5e8ea29538..4aed8fc23d32 100644
>>> --- a/net/ipv4/udp.c
>>> +++ b/net/ipv4/udp.c
>>> @@ -1583,6 +1583,8 @@ int udp_queue_rcv_skb(struct sock *sk, struct sk_buff *skb)
>>>
>>>   	if (sk_filter(sk, skb))
>>>   		goto drop;
>>> +	if (unlikely(skb->len < sizeof(struct udphdr)))
>>> +		goto drop;
>>>
>>>   	udp_csum_pull_header(skb);
>>>   	if (sk_rcvqueues_full(sk, sk->sk_rcvbuf)) {
>>> diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
>>> index 005dc82c2138..acc09705618b 100644
>>> --- a/net/ipv6/udp.c
>>> +++ b/net/ipv6/udp.c
>>> @@ -620,6 +620,8 @@ int udpv6_queue_rcv_skb(struct sock *sk, struct sk_buff *skb)
>>>
>>>   	if (sk_filter(sk, skb))
>>>   		goto drop;
>>> +	if (unlikely(skb->len < sizeof(struct udphdr)))
>>> +		goto drop;
>>>
>>>   	udp_csum_pull_header(skb);
>>>   	if (sk_rcvqueues_full(sk, sk->sk_rcvbuf)) {
>>
>>
>> Arg :(
>>
>> Acked-by: Eric Dumazet <edumazet@google.com>
>
> this is incomplete fix. Please do not apply. See discussion at security@kernel

Ohh well, didn't see it earlier before starting the discussion at security@...

I'm okay if we take this for now as a quick band aid and find a better way how
to deal with the underlying issue long-term so that it's /guaranteed/ that it
doesn't bite us any further in such fragile ways.