Re: [PATCH bpf-next v3] bpf: check for BPF_F_ADJ_ROOM_FIXED_GSO when bpf_skb_change_proto
From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date: 2021-05-12 14:15:22
Also in:
bpf, lkml
On Wed, May 12, 2021 at 3:41 AM Dongseok Yi [off-list ref] wrote:
In the forwarding path GRO -> BPF 6 to 4 -> GSO for TCP traffic, the
coalesced packet payload can be > MSS, but < MSS + 20.
bpf_skb_proto_6_to_4 will upgrade the MSS and it can be > the payload
length. After then tcp_gso_segment checks for the payload length if it
is <= MSS. The condition is causing the packet to be dropped.
tcp_gso_segment():
[...]
mss = skb_shinfo(skb)->gso_size;
if (unlikely(skb->len <= mss))
goto out;
[...]
Allow to upgrade/downgrade MSS only when BPF_F_ADJ_ROOM_FIXED_GSO is
not set.
Signed-off-by: Dongseok Yi <redacted>Acked-by: Willem de Bruijn <willemb@google.com>