CVE-2022-49340

In the Linux kernel, the following vulnerability has been resolved:

ipgre: test csumstart instead of transport header

GRE with TUNNELCSUM will apply local checksum offload on CHECKSUMPARTIAL packets.

ipgrexmit must validate csumstart after an optional skbpull, else lcocsum may trigger an overflow. The original check was

if (csum && skb_checksum_start(skb) < skb->data)
    return -EINVAL;

This had false positives when skbchecksumstart is undefined: when ipsummed is not CHECKSUMPARTIAL. 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 headerops and calls skbpull. - test skbtransportheader, which is set along with csumstart in skbpartialcsumset 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 csumstart directly, and only if CHECKSUMPARTIAL. Do leave the check in the updated location. Check field regardless of whether TUNNEL_CSUM is configured.