CVE-2026-80839

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

batman-adv: reject unrepresentable multicast TVLV offsets

The network and transport header fields in struct sk buff are 16-bit offsets from skb->head, and U16 MAX is reserved as the unset transport header value. batadv tvlv call handler() sets both fields from a received multicast TVLV without checking whether the TVLV end is representable.

If the end offset exceeds the field's range, skb set transport header() truncates it so that the transport header precedes the network header. The negative difference is then returned by skb network header len() as a large u32. batadv mcast forw packet() consequently accepts an oversized multicast tracker and accesses memory beyond the skb data.

Add skb set transport header careful(), an offset-aware counterpart to skb reset transport header careful(), which validates the final head-relative offset before assigning it. Use the new helper in batadv tvlv call handler() and reject unrepresentable TVLVs before setting the network header.