net: fix rtnetlink IFF_PROMISC and IFF_ALLMULTI handling

When device flags are set using rtnetlink, IFF_PROMISC and IFF_ALLMULTI
flags are handled specially. Function dev_change_flags sets IFF_PROMISC and
IFF_ALLMULTI bits in dev->gflags according to the passed value but
do_setlink passes a result of rtnl_dev_combine_flags which takes those bits
from dev->flags.

This can be easily trigerred by doing:

tcpdump -i eth0 &
ip l s up eth0

ip sets IFF_UP flag in ifi_flags and ifi_change, which is combined with
IFF_PROMISC by rtnl_dev_combine_flags, causing __dev_change_flags to set
IFF_PROMISC in gflags.
Reported-by: Max Matveev <makc@redhat.com>
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -659,6 +659,12 @@ static void set_operstate(struct net_device *dev, unsigned char transition)
 	}
 }
 
+static unsigned int rtnl_dev_get_flags(const struct net_device *dev)
+{
+	return (dev->flags & ~(IFF_PROMISC | IFF_ALLMULTI)) |
+	       (dev->gflags & (IFF_PROMISC | IFF_ALLMULTI));
+}
+
 static unsigned int rtnl_dev_combine_flags(const struct net_device *dev,
 					   const struct ifinfomsg *ifm)
 {
@@ -667,7 +673,7 @@ static unsigned int rtnl_dev_combine_flags(const struct net_device *dev,
 	/* bugwards compatibility: ifi_change == 0 is treated as ~0 */
 	if (ifm->ifi_change)
 		flags = (flags & ifm->ifi_change) |
-			(dev->flags & ~ifm->ifi_change);
+			(rtnl_dev_get_flags(dev) & ~ifm->ifi_change);
 
 	return flags;
 }