net: sched: fix TCF_LAYER_TRANSPORT handling in tcf_get_base_ptr()

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: 1da177e4c3 ("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>

include/net/pkt_cls.h

@@ -536,6 +536,8 @@ static inline unsigned char * tcf_get_base_ptr(struct sk_buff *skb, int layer)
 		case TCF_LAYER_NETWORK:
 			return skb_network_header(skb);
 		case TCF_LAYER_TRANSPORT:
+			if (!skb_transport_header_was_set(skb))
+				break;
 			return skb_transport_header(skb);
 	}
 

net/sched/em_cmp.c

@@ -22,9 +22,12 @@ static int em_cmp_match(struct sk_buff *skb, struct tcf_ematch *em,
 			struct tcf_pkt_info *info)
 {
 	struct tcf_em_cmp *cmp = (struct tcf_em_cmp *) em->data;
-	unsigned char *ptr = tcf_get_base_ptr(skb, cmp->layer) + cmp->off;
+	unsigned char *ptr = tcf_get_base_ptr(skb, cmp->layer);
 	u32 val = 0;
 
+	if (!ptr)
+		return 0;
+	ptr += cmp->off;
 	if (!tcf_valid_offset(skb, ptr, cmp->align))
 		return 0;
 

net/sched/em_nbyte.c

@@ -42,6 +42,8 @@ static int em_nbyte_match(struct sk_buff *skb, struct tcf_ematch *em,
 	struct nbyte_data *nbyte = (struct nbyte_data *) em->data;
 	unsigned char *ptr = tcf_get_base_ptr(skb, nbyte->hdr.layer);
 
+	if (!ptr)
+		return 0;
 	ptr += nbyte->hdr.off;
 
 	if (!tcf_valid_offset(skb, ptr, nbyte->hdr.len))

net/sched/em_text.c

@@ -29,12 +29,19 @@ static int em_text_match(struct sk_buff *skb, struct tcf_ematch *m,
 			 struct tcf_pkt_info *info)
 {
 	struct text_match *tm = EM_TEXT_PRIV(m);
+	unsigned char *ptr;
 	int from, to;
 
-	from = tcf_get_base_ptr(skb, tm->from_layer) - skb->data;
+	ptr = tcf_get_base_ptr(skb, tm->from_layer);
+	if (!ptr)
+		return 0;
+	from = ptr - skb->data;
 	from += tm->from_offset;
 
-	to = tcf_get_base_ptr(skb, tm->to_layer) - skb->data;
+	ptr = tcf_get_base_ptr(skb, tm->to_layer);
+	if (!ptr)
+		return 0;
+	to = ptr - skb->data;
 	to += tm->to_offset;
 
 	return skb_find_text(skb, from, to, tm->config) != UINT_MAX;