Patch "ipv6: fix flow labels when the traffic class is non-0" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

ipv6: fix flow labels when the traffic class is non-0

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
ipv6-fix-flow-labels-when-the-traffic-class-is-non-0.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Dimitris Michailidis <[email protected]>
Date: Tue, 23 May 2017 21:54:07 -0400
Subject: ipv6: fix flow labels when the traffic class is non-0

From: Dimitris Michailidis <[email protected]>

[ Upstream commit 90427ef5d2a4b9a24079889bf16afdcdaebc4240 ]

ip6_make_flowlabel() determines the flow label for IPv6 packets. It's supposed to be passed a flow label, which it returns as is if non-0 and in some other cases, otherwise it calculates a new value.

The problem is callers often pass a flowi6.flowlabel, which may also contain traffic class bits. If the traffic class is non-0 ip6_make_flowlabel() mistakes the non-0 it gets as a flow label and returns the whole thing. Thus it can return a 'flow label' longer than 20b and the low 20b of that is typically 0 resulting in packets with 0 label. Moreover, different packets of a flow may be labeled differently. For a TCP flow with ECN non-payload and payload packets get different labels as exemplified by this pair of consecutive packets:

(pure ACK)
Internet Protocol Version 6, Src: 2002:af5:11a3::, Dst: 2002:af5:11a2::

    0110 .... = Version: 6
    .... 0000 0000 .... .... .... .... .... = Traffic Class: 0x00 (DSCP: CS0, ECN: Not-ECT)
        .... 0000 00.. .... .... .... .... .... = Differentiated Services Codepoint: Default (0)
        .... .... ..00 .... .... .... .... .... = Explicit Congestion Notification: Not ECN-Capable Transport (0)
    .... .... .... 0001 1100 1110 0100 1001 = Flow Label: 0x1ce49
    Payload Length: 32
    Next Header: TCP (6)

(payload)
Internet Protocol Version 6, Src: 2002:af5:11a3::, Dst: 2002:af5:11a2::

    0110 .... = Version: 6
    .... 0000 0010 .... .... .... .... .... = Traffic Class: 0x02 (DSCP: CS0, ECN: ECT(0))
        .... 0000 00.. .... .... .... .... .... = Differentiated Services Codepoint: Default (0)
        .... .... ..10 .... .... .... .... .... = Explicit Congestion Notification: ECN-Capable Transport codepoint '10' (2)
    .... .... .... 0000 0000 0000 0000 0000 = Flow Label: 0x00000
    Payload Length: 688
    Next Header: TCP (6)

This patch allows ip6_make_flowlabel() to be passed more than just a flow label and has it extract the part it really wants. This was simpler than modifying the callers. With this patch packets like the above become

Internet Protocol Version 6, Src: 2002:af5:11a3::, Dst: 2002:af5:11a2::

    0110 .... = Version: 6
    .... 0000 0000 .... .... .... .... .... = Traffic Class: 0x00 (DSCP: CS0, ECN: Not-ECT)
        .... 0000 00.. .... .... .... .... .... = Differentiated Services Codepoint: Default (0)
        .... .... ..00 .... .... .... .... .... = Explicit Congestion Notification: Not ECN-Capable Transport (0)
    .... .... .... 1010 1111 1010 0101 1110 = Flow Label: 0xafa5e
    Payload Length: 32
    Next Header: TCP (6)

Internet Protocol Version 6, Src: 2002:af5:11a3::, Dst: 2002:af5:11a2::

    0110 .... = Version: 6
    .... 0000 0010 .... .... .... .... .... = Traffic Class: 0x02 (DSCP: CS0, ECN: ECT(0))
        .... 0000 00.. .... .... .... .... .... = Differentiated Services Codepoint: Default (0)
        .... .... ..10 .... .... .... .... .... = Explicit Congestion Notification: ECN-Capable Transport codepoint '10' (2)
    .... .... .... 1010 1111 1010 0101 1110 = Flow Label: 0xafa5e
    Payload Length: 688
    Next Header: TCP (6)

---
 include/net/ipv6.h |    5 +++++
 1 file changed, 5 insertions(+)

@@ -744,6 +744,11 @@ static inline __be32 ip6_make_flowlabel(
 {
 	u32 hash;
 
+	/* @flowlabel may include more than a flow label, eg, the traffic class.
+	 * Here we want only the flow label value.
+	 */
+	flowlabel &= IPV6_FLOWLABEL_MASK;
+
 	if (flowlabel ||
 	    net->ipv6.sysctl.auto_flowlabels == IP6_AUTO_FLOW_LABEL_OFF ||
 	    (!autolabel &&

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/ipv6-fix-flow-labels-when-the-traffic-class-is-non-0.patch