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    Subject[RFC PATCH v2 2/3] ipv6/icmp: l3mdev: Perform icmp error route lookup on source device routing table (v2)
    Date
    As per RFC4443, the destination address field for ICMPv6 error messages
    is copied from the source address field of the invoking packet.

    In configurations with Virtual Routing and Forwarding tables, looking up
    which routing table to use for sending ICMPv6 error messages is
    currently done by using the destination net_device.

    If the source and destination interfaces are within separate VRFs, or
    one in the global routing table and the other in a VRF, looking up the
    source address of the invoking packet in the destination interface's
    routing table will fail if the destination interface's routing table
    contains no route to the invoking packet's source address.

    One observable effect of this issue is that traceroute6 does not work in
    the following cases:

    - Route leaking between global routing table and VRF
    - Route leaking between VRFs

    Use the source device routing table when sending ICMPv6 error
    messages.

    [ In the context of ipv4, it has been pointed out that a similar issue
    may exist with ICMP errors triggered when forwarding between network
    namespaces. It would be worthwhile to investigate whether ipv6 has
    similar issues, but is outside of the scope of this investigation. ]

    [ Testing shows that similar issues exist with ipv6 unreachable /
    fragmentation needed messages. However, investigation of this
    additional failure mode is beyond this investigation's scope. ]

    Link: https://tools.ietf.org/html/rfc4443
    Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
    Cc: David Ahern <dsahern@kernel.org>
    Cc: David S. Miller <davem@davemloft.net>
    Cc: netdev@vger.kernel.org
    ---
    Changes since v1:
    - Introduce icmp6_get_route_lookup_dev.
    - Use skb->dev for routing table lookup, because it is guaranteed to be
    non-NULL.
    ---
    net/ipv6/icmp.c | 7 +++++--
    net/ipv6/ip6_output.c | 2 --
    2 files changed, 5 insertions(+), 4 deletions(-)

    diff --git a/net/ipv6/icmp.c b/net/ipv6/icmp.c
    index a4e4912ad607..91209a2760aa 100644
    --- a/net/ipv6/icmp.c
    +++ b/net/ipv6/icmp.c
    @@ -501,8 +501,11 @@ void icmp6_send(struct sk_buff *skb, u8 type, u8 code, __u32 info,
    if (__ipv6_addr_needs_scope_id(addr_type)) {
    iif = icmp6_iif(skb);
    } else {
    - dst = skb_dst(skb);
    - iif = l3mdev_master_ifindex(dst ? dst->dev : skb->dev);
    + /*
    + * The source device is used for looking up which routing table
    + * to use for sending an ICMP error.
    + */
    + iif = l3mdev_master_ifindex(skb->dev);
    }

    /*
    diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
    index c78e67d7747f..cd623068de53 100644
    --- a/net/ipv6/ip6_output.c
    +++ b/net/ipv6/ip6_output.c
    @@ -468,8 +468,6 @@ int ip6_forward(struct sk_buff *skb)
    * check and decrement ttl
    */
    if (hdr->hop_limit <= 1) {
    - /* Force OUTPUT device used as source address */
    - skb->dev = dst->dev;
    icmpv6_send(skb, ICMPV6_TIME_EXCEED, ICMPV6_EXC_HOPLIMIT, 0);
    __IP6_INC_STATS(net, idev, IPSTATS_MIB_INHDRERRORS);

    --
    2.17.1
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