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bpf: Fix bpf_fib_lookup helper MTU check for SKB ctx 

[ 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: 4c79579b44 ("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>
This commit is contained in:
Jesper Dangaard Brouer 2021-02-09 14:38:14 +01:00 committed by Greg Kroah-Hartman
parent 5af224ab94
commit 88f8f40c90
1 changed files with 10 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

13
net/core/filter.c
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@ -4880,6 +4880,7 @@ 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;
@ -4887,22 +4888,28 @@ 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;