CVE-2026-82962: Out-of-bounds write in Zephyr Ethernet L2 when adding the link-layer header to a forwarded packet

ethernetfillheader() in subsys/net/l2/ethernet/ethernet.c can prepend the Ethernet header in place, by pushing it into the headroom of the packet's first fragment, instead of allocating a separate header fragment. Before this fix the in-place path was selected purely from the build-time value of CONFIGNETL2ETHERNETRESERVEHEADER, with no check that the packet actually has that headroom, and then called netbufpush(pkt->buffer, hdrlen). Only Ethernet TX allocations reserve that space: netpktallocbuffer() invokes the L2 alloc hook (ethernetl2alloc()) only when ispkttx() is true and the packet's interface is the Ethernet interface.

A packet that was allocated by a different L2 and then handed to Ethernet for transmission therefore has too little (usually zero) headroom. Such packets are produced by ordinary routing and tunnelling paths: netroutepacketif() re-targets a received packet to another interface when CONFIGNETIPV4FORWARDING / CONFIGNETIPV6FORWARDING are enabled (for example an 802.15.4/OpenThread or PPP interface forwarding to Wi-Fi/Ethernet), netrouteipv6mcastforwardpacket() forwards an RX-slab clone that carries no reserve, and the IPIP tunnel interfacesend() prepends an outer IP header allocated on the virtual interface. In these configurations a remote or adjacent attacker triggers the condition simply by sending traffic that the device forwards.

Because netbufsimplepush() only asserts on the headroom bound, a build without CONFIGASSERT (and without the newer CONFIGNETBUFHARDENING) moves the fragment data pointer below the start of its data block and writes 14 bytes — 18 with VLAN — of Ethernet header out of bounds. With the default fixed-size buffer pools this overwrites the tail of the neighbouring buffer's payload or memory preceding the pool array; with variable-size pools it overwrites the buffer reference count and the heap chunk header. The attacker controls neither the written bytes nor the offset, so the practical impact is memory corruption leading to a crash, packet corruption, or heap inconsistency rather than controlled code execution. The fix takes the in-place path only when netbufheadroom(pkt->buffer) is large enough and otherwise falls back to a separate header fragment.