Re: [PATCH V2 net] net: fix tcp reset packet flowlabel for ipv6

[Shaohua Li]

On Mon, Jul 24, 2017 at 01:34:55PM -0700, David Miller wrote:
> From: Shaohua Li 
> Date: Tue, 18 Jul 2017 12:03:37 -0700
> 
> > +   /* Since this is being sent on the wire obfuscate hash a bit
> > +* to minimize possbility that any useful information to an
> > +* attacker is leaked. Only lower 20 bits are relevant.
> > +*/
> > +   rol32(hash, 16);
> 
> This doesn't help anything at all.
> 
> I don't like things that try to give a sense of security (however
> little) but actually don't give any at all.
> 
> I'd rather, therefore, that you remove this altogether.

ok, so you suggest removing the rol32() for ip6_make_flowlabel too, right? just
want to make sure.

Thanks,
Shaohua

Re: [PATCH V2 net] net: fix tcp reset packet flowlabel for ipv6

[Shaohua Li]

On Mon, Jul 24, 2017 at 04:12:53PM -0700, Cong Wang wrote:
> On Mon, Jul 24, 2017 at 1:34 PM, David Miller  wrote:
> > From: Shaohua Li 
> > Date: Tue, 18 Jul 2017 12:03:37 -0700
> >
> >> + /* Since this is being sent on the wire obfuscate hash a bit
> >> +  * to minimize possbility that any useful information to an
> >> +  * attacker is leaked. Only lower 20 bits are relevant.
> >> +  */
> >> + rol32(hash, 16);
> >
> > This doesn't help anything at all.
> 
> I believe the above code is copy-n-pasted from ip6_make_flowlabel()
> (with few adjustments). Don't know why we can't refactor that function
> for reuse.

There are just several lines of code, I really don't want to add adhoc if-else
for fast path.

Thanks,
Shaohua

Re: [PATCH V2 net] net: fix tcp reset packet flowlabel for ipv6

[Cong Wang]

On Mon, Jul 24, 2017 at 1:34 PM, David Miller  wrote:
> From: Shaohua Li 
> Date: Tue, 18 Jul 2017 12:03:37 -0700
>
>> + /* Since this is being sent on the wire obfuscate hash a bit
>> +  * to minimize possbility that any useful information to an
>> +  * attacker is leaked. Only lower 20 bits are relevant.
>> +  */
>> + rol32(hash, 16);
>
> This doesn't help anything at all.

I believe the above code is copy-n-pasted from ip6_make_flowlabel()
(with few adjustments). Don't know why we can't refactor that function
for reuse.

Re: [PATCH V2 net] net: fix tcp reset packet flowlabel for ipv6

[David Miller]

From: Shaohua Li 
Date: Tue, 18 Jul 2017 12:03:37 -0700

> + /* Since this is being sent on the wire obfuscate hash a bit
> +  * to minimize possbility that any useful information to an
> +  * attacker is leaked. Only lower 20 bits are relevant.
> +  */
> + rol32(hash, 16);

This doesn't help anything at all.

I don't like things that try to give a sense of security (however
little) but actually don't give any at all.

I'd rather, therefore, that you remove this altogether.

And note that maybe not using the same flow label prevents
attacks of that nature, ever consider that?

[PATCH V2 net] net: fix tcp reset packet flowlabel for ipv6

[Shaohua Li]

From: Shaohua Li 

Please see below tcpdump output:
21:00:48.109122 IP6 (flowlabel 0x43304, hlim 64, next-header TCP (6) payload 
length: 40) fec0::5054:ff:fe12:3456.55804 > fec0::5054:ff:fe12:3456.: Flags 
[S], cksum 0x0529 (incorrect -> 0xf56c), seq 3282214508, win 43690, options 
[mss 65476,sackOK,TS val 2500903437 ecr 0,nop,wscale 7], length 0
21:00:48.109381 IP6 (flowlabel 0xd827f, hlim 64, next-header TCP (6) payload 
length: 40) fec0::5054:ff:fe12:3456. > fec0::5054:ff:fe12:3456.55804: Flags 
[S.], cksum 0x0529 (incorrect -> 0x49ad), seq 1923801573, ack 3282214509, win 
43690, options [mss 65476,sackOK,TS val 2500903437 ecr 2500903437,nop,wscale 
7], length 0
21:00:48.109548 IP6 (flowlabel 0x43304, hlim 64, next-header TCP (6) payload 
length: 32) fec0::5054:ff:fe12:3456.55804 > fec0::5054:ff:fe12:3456.: Flags 
[.], cksum 0x0521 (incorrect -> 0x1bdf), seq 1, ack 1, win 342, options 
[nop,nop,TS val 2500903437 ecr 2500903437], length 0
21:00:48.109823 IP6 (flowlabel 0x43304, hlim 64, next-header TCP (6) payload 
length: 62) fec0::5054:ff:fe12:3456.55804 > fec0::5054:ff:fe12:3456.: Flags 
[P.], cksum 0x053f (incorrect -> 0xb8b1), seq 1:31, ack 1, win 342, options 
[nop,nop,TS val 2500903437 ecr 2500903437], length 30
21:00:48.109910 IP6 (flowlabel 0xd827f, hlim 64, next-header TCP (6) payload 
length: 32) fec0::5054:ff:fe12:3456. > fec0::5054:ff:fe12:3456.55804: Flags 
[.], cksum 0x0521 (incorrect -> 0x1bc1), seq 1, ack 31, win 342, options 
[nop,nop,TS val 2500903437 ecr 2500903437], length 0
21:00:48.110043 IP6 (flowlabel 0xd827f, hlim 64, next-header TCP (6) payload 
length: 56) fec0::5054:ff:fe12:3456. > fec0::5054:ff:fe12:3456.55804: Flags 
[P.], cksum 0x0539 (incorrect -> 0xb726), seq 1:25, ack 31, win 342, options 
[nop,nop,TS val 2500903438 ecr 2500903437], length 24
21:00:48.110173 IP6 (flowlabel 0x43304, hlim 64, next-header TCP (6) payload 
length: 32) fec0::5054:ff:fe12:3456.55804 > fec0::5054:ff:fe12:3456.: Flags 
[.], cksum 0x0521 (incorrect -> 0x1ba7), seq 31, ack 25, win 342, options 
[nop,nop,TS val 2500903438 ecr 2500903438], length 0
21:00:48.110211 IP6 (flowlabel 0xd827f, hlim 64, next-header TCP (6) payload 
length: 32) fec0::5054:ff:fe12:3456. > fec0::5054:ff:fe12:3456.55804: Flags 
[F.], cksum 0x0521 (incorrect -> 0x1ba7), seq 25, ack 31, win 342, options 
[nop,nop,TS val 2500903438 ecr 2500903437], length 0
21:00:48.151099 IP6 (flowlabel 0x43304, hlim 64, next-header TCP (6) payload 
length: 32) fec0::5054:ff:fe12:3456.55804 > fec0::5054:ff:fe12:3456.: Flags 
[.], cksum 0x0521 (incorrect -> 0x1ba6), seq 31, ack 26, win 342, options 
[nop,nop,TS val 2500903438 ecr 2500903438], length 0
21:00:49.110524 IP6 (flowlabel 0x43304, hlim 64, next-header TCP (6) payload 
length: 56) fec0::5054:ff:fe12:3456.55804 > fec0::5054:ff:fe12:3456.: Flags 
[P.], cksum 0x0539 (incorrect -> 0xb324), seq 31:55, ack 26, win 342, options 
[nop,nop,TS val 2500904438 ecr 2500903438], length 24
21:00:49.110637 IP6 (flowlabel 0xb34d5, hlim 64, next-header TCP (6) payload 
length: 20) fec0::5054:ff:fe12:3456. > fec0::5054:ff:fe12:3456.55804: Flags 
[R], cksum 0x0515 (incorrect -> 0x668c), seq 1923801599, win 0, length 0

The tcp reset packet has a different flowlabel, which causes our router
doesn't correctly close tcp connection. The reason is the normal packet
gets the skb->hash from sk->sk_txhash, which is generated randomly.
ip6_make_flowlabel then uses the hash to create a flowlabel. The reset
packet doesn't get assigned a hash, so the flowlabel is calculated with
flowi6.

Since user can't change timewait sock flowlabel, we create a flowlabel
for timewait socket with the random generated hash (sk->sk_txhash), then
use it in reset packet. In this way, the reset packet will have the same
flowlabel as normal packets.

This also fixes the flowlabel issue for reset packet if user configures
flowlabel, which is ignored previously.

Cc: Eric Dumazet 
Cc: Florent Fourcot 
Signed-off-by: Shaohua Li 
---
 include/net/ipv6.h   | 25 +
 net/ipv4/tcp_minisocks.c |  8 +++-
 net/ipv6/tcp_ipv6.c  | 18 +-
 3 files changed, 49 insertions(+), 2 deletions(-)

diff --git a/include/net/ipv6.h b/include/net/ipv6.h
index f2a1ddb..0a21c17 100644
--- a/include/net/ipv6.h
+++ b/include/net/ipv6.h
@@ -804,6 +804,31 @@ static inline __be32 ip6_make_flowlabel(struct net *net, 
struct sk_buff *skb,
return flowlabel;
 }
 
+/* Like ip6_make_flowlabel, but already has hash */
+static inline __be32 ip6_make_flowlabel_from_hash(struct net *net,
+ bool autolabel, u32 hash)
+{
+   __be32 flowlabel;
+
+   if (net->ipv6.sysctl.auto_flowlabels == IP6_AUTO_FLOW_LABEL_OFF ||
+   (!autolabel &&
+net->ipv6.sysctl.auto_flowlabels != IP6_AUTO_FLOW_LABEL_FORCED))
+   return 0;
+
+   /* Since