ip_gre: test csum_start instead of transport header

Vulnerability Description

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.

References

• https://git.kernel.org/stable/c/7596bd7920985f7fc8579a92e48bc53ce4475b21
• https://git.kernel.org/stable/c/3d08bc3a5d9b2106f5c8bcf1adb73147824aa006
• https://git.kernel.org/stable/c/fbeb8dfa8b87ef259eef0c89e39b53962a3cf604
• https://git.kernel.org/stable/c/e6b6f98fc7605c06c0a3baa70f62c534d7b4ce58
• https://git.kernel.org/stable/c/0c92d813c7c9ca2212ecd879232e7d87362fce98
• https://git.kernel.org/stable/c/0ffa268724656633af5f37a38c212326d98ebe8c
• https://git.kernel.org/stable/c/8d21e9963bec1aad2280cdd034c8993033ef2948