• Hannes Frederic Sowa's avatar
    ipv4: introduce ip_dst_mtu_maybe_forward and protect forwarding path against pmtu spoofing · f87c10a8
    Hannes Frederic Sowa authored
    While forwarding we should not use the protocol path mtu to calculate
    the mtu for a forwarded packet but instead use the interface mtu.
    
    We mark forwarded skbs in ip_forward with IPSKB_FORWARDED, which was
    introduced for multicast forwarding. But as it does not conflict with
    our usage in unicast code path it is perfect for reuse.
    
    I moved the functions ip_sk_accept_pmtu, ip_sk_use_pmtu and ip_skb_dst_mtu
    along with the new ip_dst_mtu_maybe_forward to net/ip.h to fix circular
    dependencies because of IPSKB_FORWARDED.
    
    Because someone might have written a software which does probe
    destinations manually and expects the kernel to honour those path mtus
    I introduced a new per-namespace "ip_forward_use_pmtu" knob so someone
    can disable this new behaviour. We also still use mtus which are locked on a
    route for forwarding.
    
    The reason for this change is, that path mtus information can be injected
    into the kernel via e.g. icmp_err protocol handler without verification
    of local sockets. As such, this could cause the IPv4 forwarding path to
    wrongfully emit fragmentation needed notifications or start to fragment
    packets along a path.
    
    Tunnel and ipsec output paths clear IPCB again, thus IPSKB_FORWARDED
    won't be set and further fragmentation logic will use the path mtu to
    determine the fragmentation size. They also recheck packet size with
    help of path mtu discovery and report appropriate errors.
    
    Cc: Eric Dumazet <eric.dumazet@gmail.com>
    Cc: David Miller <davem@davemloft.net>
    Cc: John Heffner <johnwheffner@gmail.com>
    Cc: Steffen Klassert <steffen.klassert@secunet.com>
    Signed-off-by: default avatarHannes Frederic Sowa <hannes@stressinduktion.org>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    f87c10a8
ip_forward.c 3.43 KB