CVE-2026-74665: net: fix skb length accounting after generic XDP frag adjustment

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

net: fix skb length accounting after generic XDP frag adjustment

Generic XDP exposes non-linear skb fragments through an xdpbuff. If an XDP program adjusts the fragment area, bpfprogrungenericxdp() copies xdpfragssize back to skb->datalen but leaves skb->len containing the old fragment contribution.

After a fragment shrink, this makes skbheadlen() larger than the actual linear area. In the reproduced UDP receive path, skbdatagramiter() copied 1024 bytes past the actual linear tail to userspace, starting at struct skbsharedinfo. The copied bytes included the affected skb's nrfrags, xdpfragssize and a kernel pointer from skbshinfo(skb)->frags[0]. Real packet data was displaced by the same amount and truncated at the end.

Subtract the old datalen before replacing it and add the new datalen afterwards, keeping skb->len and skb->datalen synchronized.

A 60000-byte UDP datagram on a veth pair with MTU 64000 was shortened by 1024 bytes from its fragment area. Before the fix, all 10 runs produced corrupted payloads. After the fix, all 10 runs matched the expected payload exactly.