For gso packets, virtio_net_hdr_set_proto sets the protocol (if it isn't set) based on the type in the virtio net hdr, but the skb could contain anything since it could come from packet_snd through a raw socket. If there is a mismatch between what virtio_net_hdr_set_proto sets and the actual protocol, then the skb could be handled incorrectly later on by gso.
The network header of gso packets starts at 14 bytes, but a specially crafted packet could fool the call to skb_flow_dissect_flow_keys_basic as the network header offset in the skb could be incorrect. Consequently, EINVAL is not returned. There are even packets that can cause an infinite loop. For example, a packet with ethernet type ETH_P_MPLS_UC (which is unnoticed by virtio_net_hdr_to_skb) that is sent to a geneve interface will be handled by geneve_build_skb. In turn, it calls udp_tunnel_handle_offloads which then calls skb_reset_inner_headers. After that, the packet gets passed to mpls_gso_segment. That function calculates the mpls header length by taking the difference between network_header and inner_network_header. Since the two are equal (due to the earlier call to skb_reset_inner_headers), it will calculate a header of length 0, and it will not pull any headers. Then, it will call skb_mac_gso_segment which will again call mpls_gso_segment, etc... This leads to the infinite loop. For that reason, address the root cause of the issue: don't blindly trust the information provided by the virtio net header. Instead, check if the protocol in the packet actually matches the protocol set by virtio_net_hdr_set_proto. Fixes: 9274124f023b ("net: stricter validation of untrusted gso packets") Signed-off-by: Balazs Nemeth <bnem...@redhat.com> --- include/linux/virtio_net.h | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h index e8a924eeea3d..cf2c53563f22 100644 --- a/include/linux/virtio_net.h +++ b/include/linux/virtio_net.h @@ -79,8 +79,13 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb, if (gso_type && skb->network_header) { struct flow_keys_basic keys; - if (!skb->protocol) + if (!skb->protocol) { + const struct ethhdr *eth = skb_eth_hdr(skb); + virtio_net_hdr_set_proto(skb, hdr); + if (skb->protocol != eth->h_proto) + return -EINVAL; + } retry: if (!skb_flow_dissect_flow_keys_basic(NULL, skb, &keys, NULL, 0, 0, 0, -- 2.29.2