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: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: [e-mail]
Closes: https://lore.kernel.org/netdev/[e-mail]/T/#u
Signed-off-by: Eric Dumazet <[e-mail]>
Reviewed-by: Jamal Hadi Salim <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/net/pkt_cls.h

···
 		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

···
 			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

···
 	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

···
 			 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;