[PATCH net] tunnels: do not assume transport header in iptunnel_pmtud_check_icmp()

[Eric Dumazet <edumazet@google.com>]

In some cases, iptunnel_pmtud_check_icmp() can be called while
skb transport header is not set.

This triggers an out-of-bound access, because
(typeof(skb->transport_header))~0U is 65535.

Access the icmp header based on IPv4 network header,
after making sure icmp->type is present in skb linear part.

Note that iptunnel_pmtud_check_icmpv6()) is fine.

Fixes: 4cb47a8644cc ("tunnels: PMTU discovery support for directly bridged IP packets")
Reported-by: Damiano Melotti <melotti@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/ipv4/ip_tunnel_core.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/ip_tunnel_core.c b/net/ipv4/ip_tunnel_core.c
index 2667f53482bdaf1e693907fc527063d0ddd45580..5f82d84f0f1d0bf863716d8e777f1007f871b9b3 100644
--- a/net/ipv4/ip_tunnel_core.c
+++ b/net/ipv4/ip_tunnel_core.c
@@ -281,7 +281,6 @@ static int iptunnel_pmtud_build_icmp(struct sk_buff *skb, int mtu)
  */
 static int iptunnel_pmtud_check_icmp(struct sk_buff *skb, int mtu)
 {
-	const struct icmphdr *icmph = icmp_hdr(skb);
 	const struct iphdr *iph = ip_hdr(skb);
 
 	if (mtu < 576 || iph->frag_off != htons(IP_DF))
@@ -292,9 +291,17 @@ static int iptunnel_pmtud_check_icmp(struct sk_buff *skb, int mtu)
 	    ipv4_is_lbcast(iph->saddr)  || ipv4_is_multicast(iph->saddr))
 		return 0;
 
-	if (iph->protocol == IPPROTO_ICMP && icmp_is_err(icmph->type))
-		return 0;
+	if (iph->protocol == IPPROTO_ICMP) {
+		const struct icmphdr *icmph;
 
+		if (!pskb_network_may_pull(skb, iph->ihl * 4 +
+						offsetofend(struct icmphdr, type)))
+			return 0;
+		iph = ip_hdr(skb);
+		icmph = (void *)iph + iph->ihl * 4;
+		if (icmp_is_err(icmph->type))
+			return 0;
+	}
 	return iptunnel_pmtud_build_icmp(skb, mtu);
 }
 
-- 
2.54.0.746.g67dd491aae-goog