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    Subject[PATCH 5.11 121/775] bpf: Fix bpf_fib_lookup helper MTU check for SKB ctx
    Date
    From: Jesper Dangaard Brouer <brouer@redhat.com>

    [ Upstream commit 2c0a10af688c02adcf127aad29e923e0056c6b69 ]

    BPF end-user on Cilium slack-channel (Carlo Carraro) wants to use
    bpf_fib_lookup for doing MTU-check, but *prior* to extending packet size,
    by adjusting fib_params 'tot_len' with the packet length plus the expected
    encap size. (Just like the bpf_check_mtu helper supports). He discovered
    that for SKB ctx the param->tot_len was not used, instead skb->len was used
    (via MTU check in is_skb_forwardable() that checks against netdev MTU).

    Fix this by using fib_params 'tot_len' for MTU check. If not provided (e.g.
    zero) then keep existing TC behaviour intact. Notice that 'tot_len' for MTU
    check is done like XDP code-path, which checks against FIB-dst MTU.

    V16:
    - Revert V13 optimization, 2nd lookup is against egress/resulting netdev

    V13:
    - Only do ifindex lookup one time, calling dev_get_by_index_rcu().

    V10:
    - Use same method as XDP for 'tot_len' MTU check

    Fixes: 4c79579b44b1 ("bpf: Change bpf_fib_lookup to return lookup status")
    Reported-by: Carlo Carraro <colrack@gmail.com>
    Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
    Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
    Link: https://lore.kernel.org/bpf/161287789444.790810.15247494756551413508.stgit@firesoul
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    net/core/filter.c | 13 ++++++++++---
    1 file changed, 10 insertions(+), 3 deletions(-)

    diff --git a/net/core/filter.c b/net/core/filter.c
    index 255aeee724026..ee665720a41a0 100644
    --- a/net/core/filter.c
    +++ b/net/core/filter.c
    @@ -5552,6 +5552,7 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,
    {
    struct net *net = dev_net(skb->dev);
    int rc = -EAFNOSUPPORT;
    + bool check_mtu = false;

    if (plen < sizeof(*params))
    return -EINVAL;
    @@ -5559,22 +5560,28 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,
    if (flags & ~(BPF_FIB_LOOKUP_DIRECT | BPF_FIB_LOOKUP_OUTPUT))
    return -EINVAL;

    + if (params->tot_len)
    + check_mtu = true;
    +
    switch (params->family) {
    #if IS_ENABLED(CONFIG_INET)
    case AF_INET:
    - rc = bpf_ipv4_fib_lookup(net, params, flags, false);
    + rc = bpf_ipv4_fib_lookup(net, params, flags, check_mtu);
    break;
    #endif
    #if IS_ENABLED(CONFIG_IPV6)
    case AF_INET6:
    - rc = bpf_ipv6_fib_lookup(net, params, flags, false);
    + rc = bpf_ipv6_fib_lookup(net, params, flags, check_mtu);
    break;
    #endif
    }

    - if (!rc) {
    + if (rc == BPF_FIB_LKUP_RET_SUCCESS && !check_mtu) {
    struct net_device *dev;

    + /* When tot_len isn't provided by user, check skb
    + * against MTU of FIB lookup resulting net_device
    + */
    dev = dev_get_by_index_rcu(net, params->ifindex);
    if (!is_skb_forwardable(dev, skb))
    rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
    --
    2.27.0


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