From mboxrd@z Thu Jan  1 00:00:00 1970
From: Daniel Borkmann <daniel@iogearbox.net>
Subject: Re: [PATCH net] udp: prevent bugcheck if filter truncates packet
 too much
Date: Sat, 09 Jul 2016 11:48:49 +0200
Message-ID: <5780C881.1040609@iogearbox.net>
References: <20160708155233.C78AAA0ECC@unicorn.suse.cz> <1468020700.30694.38.camel@edumazet-glaptop3.roam.corp.google.com> <20160709002016.GA27494@ast-mbp.thefacebook.com>
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Cc: Michal Kubecek <mkubecek@suse.cz>,
	Willem de Bruijn <willemb@google.com>,
	"David S. Miller" <davem@davemloft.net>,
	samanthakumar <samanthakumar@google.com>,
	Marco Grassi <marco.gra@gmail.com>, netdev@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
	James Morris <jmorris@namei.org>,
	Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
	Patrick McHardy <kaber@trash.net>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Eric Dumazet <eric.dumazet@gmail.com>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <20160709002016.GA27494@ast-mbp.thefacebook.com>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: netdev.vger.kernel.org

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.