syzbot reported that tcf_get_base_ptr() can be called while transport header is not set [1]. Instead of returning a dangling pointer, return NULL. Fix tcf_get_base_ptr() callers to handle this NULL value. [1] WARNING: CPU: 1 PID: 6019 at ./include/linux/skbuff.h:3071 skb_transport_header include/linux/skbuff.h:3071 [inline] WARNING: CPU: 1 PID: 6019 at ./include/linux/skbuff.h:3071 tcf_get_base_ptr include/net/pkt_cls.h:539 [inline] WARNING: CPU: 1 PID: 6019 at ./include/linux/skbuff.h:3071 em_nbyte_match+0x2d8/0x3f0 net/sched/em_nbyte.c:43 Modules linked in: CPU: 1 UID: 0 PID: 6019 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full) Call Trace: <TASK> tcf_em_match net/sched/ematch.c:494 [inline] __tcf_em_tree_match+0x1ac/0x770 net/sched/ematch.c:520 tcf_em_tree_match include/net/pkt_cls.h:512 [inline] basic_classify+0x115/0x2d0 net/sched/cls_basic.c:50 tc_classify include/net/tc_wrapper.h:197 [inline] __tcf_classify net/sched/cls_api.c:1764 [inline] tcf_classify+0x4cf/0x1140 net/sched/cls_api.c:1860 multiq_classify net/sched/sch_multiq.c:39 [inline] multiq_enqueue+0xfd/0x4c0 net/sched/sch_multiq.c:66 dev_qdisc_enqueue+0x4e/0x260 net/core/dev.c:4118 __dev_xmit_skb net/core/dev.c:4214 [inline] __dev_queue_xmit+0xe83/0x3b50 net/core/dev.c:4729 packet_snd net/packet/af_packet.c:3076 [inline] packet_sendmsg+0x3e33/0x5080 net/packet/af_packet.c:3108 sock_sendmsg_nosec net/socket.c:727 [inline] __sock_sendmsg+0x21c/0x270 net/socket.c:742 ____sys_sendmsg+0x505/0x830 net/socket.c:2630 Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Reported-by: syzbot+f3a497f02c389d86ef16@syzkaller.appspotmail.com Closes: https://lore.kernel.org/netdev/6920855a.a70a0220.2ea503.0058.GAE@google.com/T/#u Signed-off-by: Eric Dumazet <edumazet@google.com> Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com> Link: https://patch.msgid.link/20251121154100.1616228-1-edumazet@google.com Signed-off-by: Jakub Kicinski <kuba@kernel.org>