In the Linux kernel, the following vulnerability has been resolved:
ip_gre: test csum_start instead of transport header
GRE with TUNNEL_CSUM will apply local checksum offload on CHECKSUM_PARTIAL packets.
ipgre_xmit must validate csum_start after an optional skb_pull, else lco_csum may trigger an overflow. The original check was
if (csum && skb_checksum_start(skb) < skb->data) return -EINVAL;
This had false positives when skb_checksum_start is undefined: when ip_summed is not CHECKSUM_PARTIAL. A discussed refinement was straightforward
if (csum && skb->ip_summed == CHECKSUM_PARTIAL && skb_checksum_start(skb) < skb->data) return -EINVAL;
But was eventually revised more thoroughly:
- restrict the check to the only branch where needed, in an
- test skb_transport_header, which is set along with csum_start
Turns out skbs can arrive in this branch without the transport header set, e.g., through BPF redirection.
Revise the check back to check csum_start directly, and only if CHECKSUM_PARTIAL. Do leave the check in the updated location. Check field regardless of whether TUNNEL_CSUM is configured.