In the Linux kernel, the following vulnerability has been resolved: tunnels: do not assume mac header is set in skb_tunnel_check_pmtu() Recently added debug in commit f9aefd6b2aa3 ("net: warn if mac header was not set") caught a bug in skb_tunnel_check_pmtu(), as shown in this syzbot report [1]. In ndo_start_xmit() paths, there is really no need to use skb->mac_header, because skb->data is supposed to point at it. [1] WARNING: CPU: 1 PID: 8604 at include/linux/skbuff.h:2784 skb_mac_header_len include/linux/skbuff.h:2784 [inline] WARNING: CPU: 1 PID: 8604 at include/linux/skbuff.h:2784 skb_tunnel_check_pmtu+0x5de/0x2f90 net/ipv4/ip_tunnel_core.c:413 Modules linked in: CPU: 1 PID: 8604 Comm: syz-executor.3 Not tainted 5.19.0-rc2-syzkaller-00443-g8720bd951b8e #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 RIP: 0010:skb_mac_header_len include/linux/skbuff.h:2784 [inline] RIP: 0010:skb_tunnel_check_pmtu+0x5de/0x2f90 net/ipv4/ip_tunnel_core.…