[PATCH net] net: icmp: avoid invalid transport header access in icmp_send tracepoint

syzbot reported a WARNING triggered by DEBUG_NET_WARN_ON_ONCE():

 WARNING: at skb_transport_header include/linux/skbuff.h:3087 [inline]
 WARNING: at udp_hdr include/linux/udp.h:23 [inline]
 WARNING: at do_trace_event_raw_event_icmp_send include/trace/events/icmp.h:30 [inline]
 WARNING: at trace_event_raw_event_icmp_send+0x48c/0x6ec include/trace/events/icmp.h:11
 Call trace:
  skb_transport_header include/linux/skbuff.h:3087 [inline]
  udp_hdr include/linux/udp.h:23 [inline]
  do_trace_event_raw_event_icmp_send include/trace/events/icmp.h:30 [inline]
  trace_event_raw_event_icmp_send+0x48c/0x6ec include/trace/events/icmp.h:11
  __traceiter_icmp_send include/trace/events/icmp.h:11 [inline]
  __do_trace_icmp_send include/trace/events/icmp.h:11 [inline]
  trace_icmp_send+0x320/0x49c include/trace/events/icmp.h:11
  __icmp_send+0xcfc/0x11d8 net/ipv4/icmp.c:1013
  ipv4_send_dest_unreach net/ipv4/route.c:1280 [inline]
  ipv4_link_failure+0x57c/0x8dc net/ipv4/route.c:1287
  dst_link_failure include/net/dst.h:438 [inline]
  vti_tunnel_xmit+0xe40/0x17a4 net/ipv4/ip_vti.c:307

TP_fast_assign() unconditionally calls udp_hdr(skb) before checking
whether the packet is UDP. Furthermore, __icmp_send() can be invoked
from paths (e.g., link failures, ARP errors, forwarding, AF_PACKET)
where skb->transport_header was never initialized (~0U).

Under CONFIG_DEBUG_NET=y, calling skb_transport_header(skb) triggers
DEBUG_NET_WARN_ON_ONCE(!skb_transport_header_was_set(skb)).

Fix this by:
1. Only parsing transport info when iph->protocol == IPPROTO_UDP.
2. Using skb_header_pointer() at skb_network_offset(skb) + (iph->ihl << 2)
   to safely fetch the UDP header without assuming transport_header is set.

Fixes: db3efdcf70c7 ("net/ipv4: add tracepoint for icmp_send")
Reported-by: syzbot+6d2762674103618994b0@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/6a8d5538.91706f20.ef82.0009.GAE@google.com/T/#u (local)
Signed-off-by: Eric Dumazet <redacted>
Cc: Peilin He <redacted>
Cc: xu xin <redacted>
Cc: Steven Rostedt <rostedt@goodmis.org>
---
 include/trace/events/icmp.h | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/include/trace/events/icmp.h b/include/trace/events/icmp.h
index 09ae115099dfe25616b6c74d5b92be1af4acff55..6937b778ae54dbf4442b127957a017da3551aaff 100644
--- a/include/trace/events/icmp.h
+++ b/include/trace/events/icmp.h

@@ -27,17 +27,20 @@ TRACE_EVENT(icmp_send,
 
 		TP_fast_assign(
 			struct iphdr *iph = ip_hdr(skb);
-			struct udphdr *uh = udp_hdr(skb);
-			int proto_4 = iph->protocol;
+			struct udphdr _uh, *uh = NULL;
 			__be32 *p32;
 
 			__entry->skbaddr = skb;
 			__entry->type = type;
 			__entry->code = code;
 
-			if (proto_4 != IPPROTO_UDP || (u8 *)uh < skb->head ||
-				(u8 *)uh + sizeof(struct udphdr)
-				> skb_tail_pointer(skb)) {
+			if (iph->protocol == IPPROTO_UDP)
+				uh = skb_header_pointer(skb,
+							skb_network_offset(skb) +
+							(iph->ihl << 2),
+							sizeof(_uh), &_uh);
+
+			if (!uh) {
 				__entry->sport = 0;
 				__entry->dport = 0;
 				__entry->ulen = 0;

-- 
2.55.0.860.g4b6b3295ed-goog