ip_gre: test csum_start instead of transport header

Summary

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 uncommon GRE path that uses header_ops and calls skb_pull. • test skb_transport_header, which is set along with csum_start in skb_partial_csum_set in the normal header_ops datapath. 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.

Timeline

1. Feb 26, 2025, 2:10 AM UTCCVE published
2. Aug 5, 2026, 8:54 AM UTCRecord last updated

Affected versions

• Linux Linux

  From 4.19.207 · fixed in 4.19.247; From 5.4.148 · fixed in 5.4.198; From 5.10.68 · fixed in 5.10.122; From 5.14.7 · fixed in 5.15

• Linux Linux

  5.15

References 7