netfilter: ipv6: fix crash caused by ipv6_find_hdr()

When calling:
    ipv6_find_hdr(skb, &thoff, -1, NULL)
on a fragmented packet, thoff would be left with a random
value causing callers to read random memory offsets with:
    skb_header_pointer(skb, thoff, ...)

Now we force ipv6_find_hdr() to return a failure in this case.
Calling:
  ipv6_find_hdr(skb, &thoff, -1, &fragoff)
will set fragoff as expected, and not return a failure.

Change-Id: Ib474e8a4267dd2b300feca325811330329684a88
Signed-off-by: JP Abgrall <jpa@google.com>

diff --git a/net/ipv6/netfilter/ip6_tables.c b/net/ipv6/netfilter/ip6_tables.c
index 94874b0..14cb310 100644 (file)
--- a/net/ipv6/netfilter/ip6_tables.c
+++ b/net/ipv6/netfilter/ip6_tables.c
@@ -2292,16 +2292,15 @@ static void __exit ip6_tables_fini(void)
  * "No next header".
  *
  * If target header is found, its offset is set in *offset and return protocol
- * number. Otherwise, return -1.
+ * number. Otherwise, return -ENOENT or -EBADMSG.
  *
  * If the first fragment doesn't contain the final protocol header or
  * NEXTHDR_NONE it is considered invalid.
  *
  * Note that non-1st fragment is special case that "the protocol number
  * of last header" is "next header" field in Fragment header. In this case,
- * *offset is meaningless and fragment offset is stored in *fragoff if fragoff
- * isn't NULL.
- *
+ * *offset is meaningless. If fragoff is not NULL, the fragment offset is
+ * stored in *fragoff; if it is NULL, return -EINVAL.
  */
 int ipv6_find_hdr(const struct sk_buff *skb, unsigned int *offset,
                  int target, unsigned short *fragoff)
@@ -2342,9 +2341,12 @@ int ipv6_find_hdr(const struct sk_buff *skb, unsigned int *offset,
                                if (target < 0 &&
                                    ((!ipv6_ext_hdr(hp->nexthdr)) ||
                                     hp->nexthdr == NEXTHDR_NONE)) {
-                                       if (fragoff)
+                                       if (fragoff) {
                                                *fragoff = _frag_off;
-                                       return hp->nexthdr;
+                                               return hp->nexthdr;
+                                       } else {
+                                               return -EINVAL;
+                                       }
                                }
                                return -ENOENT;
                        }