flow_dissector: fix ipv6 dst, hop-by-hop and routing ext hdrs

__skb_header_pointer() returns a pointer that must be checked.

Fixes infinite loop reported by Alexei, and add __must_check to
catch these errors earlier.

Fixes: 6a74fcf4 ("flow_dissector: add support for dst, hop-by-hop and routing ext hdrs")
Reported-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Tested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -2743,7 +2743,8 @@ __wsum __skb_checksum(const struct sk_buff *skb, int offset, int len,
 __wsum skb_checksum(const struct sk_buff *skb, int offset, int len,
 		    __wsum csum);
 
-static inline void *__skb_header_pointer(const struct sk_buff *skb, int offset,
+static inline void * __must_check
+__skb_header_pointer(const struct sk_buff *skb, int offset,
 		     int len, void *data, int hlen, void *buffer)
 {
 	if (hlen - offset >= len)
@@ -2756,8 +2757,8 @@ static inline void *__skb_header_pointer(const struct sk_buff *skb, int offset,
 	return buffer;
 }
 
-static inline void *skb_header_pointer(const struct sk_buff *skb, int offset,
-				       int len, void *buffer)
+static inline void * __must_check
+skb_header_pointer(const struct sk_buff *skb, int offset, int len, void *buffer)
 {
 	return __skb_header_pointer(skb, offset, len, skb->data,
 				    skb_headlen(skb), buffer);

net/core/flow_dissector.c

@@ -394,9 +394,11 @@ bool __skb_flow_dissect(const struct sk_buff *skb,
 
 		opthdr = __skb_header_pointer(skb, nhoff, sizeof(_opthdr),
 					      data, hlen, &_opthdr);
+		if (!opthdr)
+			return false;
 
-		ip_proto = _opthdr[0];
-		nhoff += (_opthdr[1] + 1) << 3;
+		ip_proto = opthdr[0];
+		nhoff += (opthdr[1] + 1) << 3;
 
 		goto ip_proto_again;
 	}