bridge: fix smatch warning / potential null pointer dereference

"New smatch warnings:
  net/bridge/br_multicast.c:1368 br_ip6_multicast_query() error:
    we previously assumed 'group' could be null (see line 1349)"

In the rare (sort of broken) case of a query having a Maximum
Response Delay of zero, we could create a potential null pointer
dereference.

Fixing this by skipping the multicast specific MLD Query parsing again
if no multicast group address is available.

Introduced by dc4eb53a996a78bfb8ea07b47423ff5a3aadc362
("bridge: adhere to querier election mechanism specified by RFCs")

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Linus Lüssing <linus.luessing@web.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 0 deletions

diff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c
index cd3cf394c477..876e5fb2a786 100644
--- a/net/bridge/br_multicast.c
+++ b/net/bridge/br_multicast.c
@@ -1373,6 +1373,8 @@ static int br_ip6_multicast_query(struct net_bridge *br,
                 br_multicast_query_received(br, port, &br->ip6_other_query,
                                             &saddr, max_delay);
                 goto out;
+        } else if (!group) {
+                goto out;
         }
 
         mp = br_mdb_ip6_get(mlock_dereference(br->mdb, br), group, vid);