CVE-2026-52942: netfilter: nf_log: validate MAC header was set before dumping it

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

netfilter: nflog: validate MAC header was set before dumping it

The fallback path of dumpmacheader() guards the MAC header access only with "skb->macheader != skb->networkheader", without checking skbmacheaderwasset(). When the MAC header is unset, macheader is 0xffff, so the test passes and skbmacheader(skb) returns skb->head + 0xffff, ~64 KiB past the buffer; the loop then reads dev->hardheaderlen bytes out of bounds into the kernel log.

This is reachable via the netdev logger: nflogunknownpacket() calls dumpmacheader() unconditionally, and an skb sent through AFPACKET with PACKETQDISCBYPASS reaches the egress hook with macheader still unset (devqueuexmit(), which would reset it, is bypassed).

Add the skbmacheaderwasset() check the ARPHRDETHER path already uses, and replace the open-coded MAC header length test with skbmacheaderlen(). Only skbs with an unset MAC header are affected; valid ones are dumped as before.

BUG: KASAN: slab-out-of-bounds in dumpmacheader (net/netfilter/nflogsyslog.c:831) Read of size 1 at addr ffff88800ea49d3f by task exploit/148 Call Trace: kasanreport (mm/kasan/report.c:595) dumpmacheader (net/netfilter/nflogsyslog.c:831) nflognetdevpacket (net/netfilter/nflogsyslog.c:938 net/netfilter/nflogsyslog.c:963) nflogpacket (net/netfilter/nflog.c:260) nftlogeval (net/netfilter/nftlog.c:60) nftdochain (net/netfilter/nftablescore.c:285) nftdochainnetdev (net/netfilter/nftchainfilter.c:307) nfhookslow (net/netfilter/core.c:619) nfhookdirectegress (net/packet/afpacket.c:257) packetxmit (net/packet/afpacket.c:280) packetsendmsg (net/packet/afpacket.c:3114) syssendto (net/socket.c:2265)