Date:	Wed, 28 Jan 2015 18:34:38 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	vyasevic@...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.

On Mi, 2015-01-28 at 18:48 +0200, Michael S. Tsirkin wrote:
> On Wed, Jan 28, 2015 at 05:15:49PM +0100, Hannes Frederic Sowa wrote:
> > Hi,
> > 
> > On Mi, 2015-01-28 at 18:00 +0200, Michael S. Tsirkin wrote:
> > > On Wed, Jan 28, 2015 at 11:34:02AM +0100, 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.
> > > 
> > > In that case doing things like extending virtio
> > > is out of the question too, isn't it?
> > > It needs hypervisor changes.
> > 
> > Sure, but I would like to have the fragmentation id generator to reside
> > inside the end-host kernel. Hypervisor needs to carry the frag id along,
> > sure, and needs to be changed accordingly.
> > 
> > So in either case we need to change both kernels. ;)
> > 
> > > 
> > > > You would need to
> > > > patch both kernels in your case - non gso frames would still get the
> > > > fragmentation id generated in the host kernel.
> > > > 
> > > > > > 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?
> > > 
> > > No answer here?
> > 
> > Ups, sorry.
> > 
> > It is not re-randomized but only biased by a time delta (note the
> > prandom_u32_max). So even after such an increment happens you can still
> > guess the range of the current fragmentation ids for a longer time.
> > 
> > Otherwise it is 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.
> > > > 
> > > > Bye,
> > > > Hannes
> > > 
> > > OK then. Like this:
> > > 
> > > diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> > > index 679e6e9..1ee9a3a 100644
> > > --- a/include/linux/netdevice.h
> > > +++ b/include/linux/netdevice.h
> > > @@ -1508,6 +1508,9 @@ struct net_device {
> > >  	 *	part of the usual set specified in Space.c.
> > >  	 */
> > >  
> > > +	/* Extra hash to mix into IPv6 frag ID on packets received from here. */
> > > +	unsigned int		frag_id_hash;
> > > +
> > >  	unsigned long		state;
> > >  
> > >  	struct list_head	dev_list;
> > > diff --git a/net/core/dev.c b/net/core/dev.c
> > > index 683d493..56f1898 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->frag_id_hash;
> > >  
> > >  	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,
> > > 
> > > 
> > > Add to this a netlink/sysfs API to set the frag_id_hash for
> > > devices.
> > > 
> > > Now, user can set identical frag id hash for all devices
> > > for a given VM.
> > > 
> > > We can even expose this to guests: each guest would generate
> > > the ID on boot and send it to host, host would set it
> > > in sysfs.
> > 
> > jhash_1word shouldn't be a bijection, so we are randomizing here and are
> > increasing the probability of collisions.
> > Instead of jhash_1word you
> > would need to take a simple block cipher with the hash as key.
> > 
> > Bye,
> > Hannes
> 
> fhdr.identification is coming from jhash_3word itself, how is this
> different?
> 

Sorry, I currently cannot follow. Does it? We hash the ipv6 addresses
and the hash is used as an index into the ip_idents array.

Sorry, maybe I have overlooked something?

Bye,
Hannes


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