Date:	Wed, 28 Jan 2015 15:45:39 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	vyasevic@...hat.com
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	Ben Hutchings <ben@...adent.org.uk>,
	Vladislav Yasevich <vyasevich@...il.com>,
	netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org,
	edumazet@...gle.com
Subject: Re: [PATCH 1/3] ipv6: Select fragment id during UFO/GSO
 segmentation if not set.

Hi,

On Mi, 2015-01-28 at 09:16 -0500, Vlad Yasevich wrote:
> On 01/28/2015 05:34 AM, Hannes Frederic Sowa wrote:
> > Hi,
> > 
> > On Mi, 2015-01-28 at 11:46 +0200, Michael S. Tsirkin wrote:
> >> On Wed, Jan 28, 2015 at 09:25:08AM +0100, Hannes Frederic Sowa wrote:
> >>> Hello,
> >>>
> >>> On Di, 2015-01-27 at 18:08 +0200, Michael S. Tsirkin wrote:
> >>>> On Tue, Jan 27, 2015 at 05:02:31PM +0100, Hannes Frederic Sowa wrote:
> >>>>> On Di, 2015-01-27 at 09:26 -0500, Vlad Yasevich wrote:
> >>>>>> On 01/27/2015 08:47 AM, Hannes Frederic Sowa wrote:
> >>>>>>> On Di, 2015-01-27 at 10:42 +0200, Michael S. Tsirkin wrote:
> >>>>>>>> On Tue, Jan 27, 2015 at 02:47:54AM +0000, Ben Hutchings wrote:
> >>>>>>>>> On Mon, 2015-01-26 at 09:37 -0500, Vladislav Yasevich wrote:
> >>>>>>>>>> If the IPv6 fragment id has not been set and we perform
> >>>>>>>>>> fragmentation due to UFO, select a new fragment id.
> >>>>>>>>>> When we store the fragment id into skb_shinfo, set the bit
> >>>>>>>>>> in the skb so we can re-use the selected id.
> >>>>>>>>>> This preserves the behavior of UFO packets generated on the
> >>>>>>>>>> host and solves the issue of id generation for packet sockets
> >>>>>>>>>> and tap/macvtap devices.
> >>>>>>>>>>
> >>>>>>>>>> This patch moves ipv6_select_ident() back in to the header file.  
> >>>>>>>>>> It also provides the helper function that sets skb_shinfo() frag
> >>>>>>>>>> id and sets the bit.
> >>>>>>>>>>
> >>>>>>>>>> It also makes sure that we select the fragment id when doing
> >>>>>>>>>> just gso validation, since it's possible for the packet to
> >>>>>>>>>> come from an untrusted source (VM) and be forwarded through
> >>>>>>>>>> a UFO enabled device which will expect the fragment id.
> >>>>>>>>>>
> >>>>>>>>>> CC: Eric Dumazet <edumazet@...gle.com>
> >>>>>>>>>> Signed-off-by: Vladislav Yasevich <vyasevic@...hat.com>
> >>>>>>>>>> ---
> >>>>>>>>>>  include/linux/skbuff.h |  3 ++-
> >>>>>>>>>>  include/net/ipv6.h     |  2 ++
> >>>>>>>>>>  net/ipv6/ip6_output.c  |  4 ++--
> >>>>>>>>>>  net/ipv6/output_core.c |  9 ++++++++-
> >>>>>>>>>>  net/ipv6/udp_offload.c | 10 +++++++++-
> >>>>>>>>>>  5 files changed, 23 insertions(+), 5 deletions(-)
> >>>>>>>>>>
> >>>>>>>>>> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> >>>>>>>>>> index 85ab7d7..3ad5203 100644
> >>>>>>>>>> --- a/include/linux/skbuff.h
> >>>>>>>>>> +++ b/include/linux/skbuff.h
> >>>>>>>>>> @@ -605,7 +605,8 @@ struct sk_buff {
> >>>>>>>>>>  	__u8			ipvs_property:1;
> >>>>>>>>>>  	__u8			inner_protocol_type:1;
> >>>>>>>>>>  	__u8			remcsum_offload:1;
> >>>>>>>>>> -	/* 3 or 5 bit hole */
> >>>>>>>>>> +	__u8			ufo_fragid_set:1;
> >>>>>>>>> [...]
> >>>>>>>>>
> >>>>>>>>> Doesn't the flag belong in struct skb_shared_info, rather than struct
> >>>>>>>>> sk_buff?  Otherwise this looks fine.
> >>>>>>>>>
> >>>>>>>>> Ben.
> >>>>>>>>
> >>>>>>>> Hmm we seem to be out of tx flags.
> >>>>>>>> Maybe ip6_frag_id == 0 should mean "not set".
> >>>>>>>
> >>>>>>> Maybe that is the best idea. Definitely the ufo_fragid_set bit should
> >>>>>>> move into the skb_shared_info area.
> >>>>>>
> >>>>>> That's what I originally wanted to do, but had to move and grow txflags thus
> >>>>>> skb_shinfo ended up growing.  I wanted to avoid that, so stole an skb flag.
> >>>>>>
> >>>>>> I considered treating fragid == 0 as unset, but a 0 fragid is perfectly valid
> >>>>>> from the protocol perspective and could actually be generated by the id generator
> >>>>>> functions.  This may cause us to call the id generation multiple times.
> >>>>>
> >>>>> Are there plans in the long run to let virtio_net transmit auxiliary
> >>>>> data to the other end so we can clean all of this this up one day?
> >>>>>
> >>>>> I don't like the whole situation: looking into the virtio_net headers
> >>>>> just adding a field for ipv6 fragmentation ids to those small structs
> >>>>> seems bloated, not doing it feels incorrect. :/
> >>>>>
> >>>>> Thoughts?
> >>>>>
> >>>>> Bye,
> >>>>> Hannes
> >>>>
> >>>> I'm not sure - what will be achieved by generating the IDs guest side as
> >>>> opposed to host side?  It's certainly harder to get hold of entropy
> >>>> guest-side.
> >>>
> >>> It is not only about entropy but about uniqueness.  Also fragmentation
> >>> ids should not be discoverable,
> >>
> >> I belive "predictable" is the language used by the IETF draft.
> >>
> >>> so there are several aspects:
> >>>
> >>> I see fragmentation id generation still as security critical:
> >>> When Eric patched the frag id generator in 04ca6973f7c1a0d ("ip: make IP
> >>> identifiers less predictable") I could patch my kernels and use the
> >>> patch regardless of the machine being virtualized or not. It was not
> >>> dependent on the hypervisor.
> >>
> >> And now it's even easier - just patch the hypervisor, and all VMs
> >> automatically benefit.
> > 
> > Sometimes the hypervisor is not under my control. You would need to
> > patch both kernels in your case - non gso frames would still get the
> > fragmentation id generated in the host kernel.
> 
> Why would non-gso frames need a frag id?  We are talking only UDP IPv6
> here, so there is no frag id generation if the packet does't need to
> be fragmented.

E.g. raw sockets still can generate fragments locally. It is also a
valid setup to have multiple interfaces in one machine, one that is UFO
enabled and one that isn't. In that case, fragmentation id generation
happens on different hosts which I want to avoid.

I haven't looked closely but mismatch of MTUs on interfaces seems like
it could lead to unwanted fragmentation, e.g. see is_skb_forwardable
which is mostly always true for gso frames, so we never stop them on
bridges etc.

> >>> I think that is the same reasoning why we
> >>> don't support TOE.
> >>> If we use one generator in the hypervisor in an openstack alike setting,
> >>> the host deals with quite a lot of overlay networks. A lot of default
> >>> configurations use the same addresses internally, so on the hypervisor
> >>> the frag id generators would interfere by design.
> >>> I could come up with an attack scenario for DNS servers (again :) ):
> >>>
> >>> You are sitting next to a DNS server on the same hypervisor and can send
> >>> packets without source validation (because that is handled later on in
> >>> case of openvswitch when the packet is put into the corresponding
> >>> overlay network). You emit a gso packet with the same source and
> >>> destination addresses as the DNS server would do and would get an
> >>> fragmentation id which is linearly (+ time delta) incremented depending
> >>> on the source and destination address. With such a leak you could start
> >>> trying attack and spoof DNS responses (fragmentation attacks etc.).
> >>> See also details on such kind of attacks in the description of commit
> >>> 04ca6973f7c1a0d.
> >>>
> >>> AFAIK IETF tried with IPv6 to push fragmentation id generation to the
> >>> end hosts, that's also the reason for the introduction of atomic
> >>> fragments (which are now being rolled back ;) ).
> >>>
> >>> Still it is better to generate a frag id on the hypervisor than just
> >>> sending a 0, so I am ok with this change, albeit not happy.
> >>>
> >>> Thanks,
> >>> Hannes
> >>>
> >>
> >> OK so to summarize, identifiers are only re-randomized once per jiffy,
> >> so you worry that within this window, an external observer can discover
> >> past fragment ID values and so predict the future ones.
> >> All that's required is that two paths go through the same box performing
> >> fragmentation.
> >>
> >> Is that a fair summary?
> >>
> >> If yes, we can make this a bit harder by mixing in some
> >> data per input and/or output devices.
> >>
> >> For example, just to give you the idea:
> >>
> >> diff --git a/net/core/dev.c b/net/core/dev.c
> >> index 683d493..4faa7ef 100644
> >> --- a/net/core/dev.c
> >> +++ b/net/core/dev.c
> >> @@ -3625,6 +3625,7 @@ static int __netif_receive_skb_core(struct sk_buff *skb, bool pfmemalloc)
> >>  	trace_netif_receive_skb(skb);
> >>  
> >>  	orig_dev = skb->dev;
> >> +	skb_shinfo(skb)->ip6_frag_id = skb->dev->ifindex;
> >>  
> >>  	skb_reset_network_header(skb);
> >>  	if (!skb_transport_header_was_set(skb))
> >> diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
> >> index ce69a12..819a821 100644
> >> --- a/net/ipv6/ip6_output.c
> >> +++ b/net/ipv6/ip6_output.c
> >> @@ -1092,7 +1092,8 @@ static inline int ip6_ufo_append_data(struct sock *sk,
> >>  				     sizeof(struct frag_hdr)) & ~7;
> >>  	skb_shinfo(skb)->gso_type = SKB_GSO_UDP;
> >>  	ipv6_select_ident(&fhdr, rt);
> >> -	skb_shinfo(skb)->ip6_frag_id = fhdr.identification;
> >> +	skb_shinfo(skb)->ip6_frag_id = jhash_1word(skb_shinfo(skb)->ip6_frag_id,
> >> +						   fhdr.identification);
> >>  
> >>  append:
> >>  	return skb_append_datato_frags(sk, skb, getfrag, from,
> >>
> > 
> > I thought about mixing in the incoming interface identifier into the
> > frag id generation, but that could hurt us badly as soon as a VM has
> > more than one interface to the outside world and uses e.g. ECMP. We need
> > to make sure that those frag ids are unique and the kernel needs to be
> > better than just using a random number generator.
> >
> 
> So the goal behind this series of patches is to restore VM functionality to
> pre-916e4cf46d0204 ("ipv6: reuse ip6_frag_id from ip6_ufo_append_data").

I understand (the patch fixed a NULL ptr deref btw.).

As I said, I don't want to stop this series (hopefully the flag can be
moved into skb_shared_info etc.), would look after that IMHO
(skb flags/IPCB and skb_shared_info have different semantics on
__skb_clone).

I think it is very much worth to try to move the fragmentation id
generation back to the end host and only use this as a fallback.

Bye,
Hannes


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