CVE-2026-43501

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

ipv6: rpl: reserve mac_len headroom when recompressed SRH grows

ipv6rplsrhrcv() decompresses an RFC 6554 Source Routing Header, swaps the next segment into ipv6hdr->daddr, recompresses, then pulls the old header and pushes the new one plus the IPv6 header back. The recompressed header can be larger than the received one when the swap reduces the common-prefix length the segments share with daddr (CmprI=0, CmprE>0, seg[0][0] != daddr[0] gives the maximum +8 bytes).

pskbexpandhead() was gated on segmentsleft == 0, so on earlier segments the push consumed unchecked headroom. Once skbpush() leaves fewer than skb->maclen bytes in front of data, skbmacheaderrebuild()'s call to:

skb_set_mac_header(skb, -skb->mac_len);

will store (data - head) - maclen into the u16 macheader field, which wraps to ~65530, and the following memmove() writes mac_len bytes ~64KiB past skb->head.

A single AFINET6/SOCKRAW/IPV6HDRINCL packet over lo with a two segment type-3 SRH (CmprI=0, CmprE=15) reaches headroom 8 after one pass; KASAN reports a 14-byte OOB write in ipv6rthdr_rcv.

Fix this by expanding the head whenever the remaining room is less than the push size plus mac_len, and request that much extra so the rebuilt MAC header fits afterwards.