net: dsa: targeted MTU notifiers should only match on one port

dsa_slave_change_mtu() calls dsa_port_mtu_change() twice:
- it sends a cross-chip notifier with the MTU of the CPU port which is
  used to update the DSA links.
- it sends one targeted MTU notifier which is supposed to only match the
  user port on which we are changing the MTU. The "propagate_upstream"
  variable is used here to bypass the cross-chip notifier system from
  switch.c

But due to a mistake, the second, targeted notifier matches not only on
the user port, but also on the DSA link which is a member of the same
switch, if that exists.

And because the DSA links of the entire dst were programmed in a
previous round to the largest_mtu via a "propagate_upstream == true"
notification, then the dsa_port_mtu_change(propagate_upstream == false)
call that is immediately upcoming will break the MTU on the one DSA link
which is chip-wise local to the dp whose MTU is changing right now.

Example given this daisy chain topology:

   sw0p0     sw0p1     sw0p2     sw0p3     sw0p4
[  cpu  ] [  user ] [  user ] [  dsa  ] [  user ]
[   x   ] [       ] [       ] [   x   ] [       ]
                                  |
                                  +---------+
                                            |
   sw1p0     sw1p1     sw1p2     sw1p3     sw1p4
[  user ] [  user ] [  user ] [  dsa  ] [  dsa  ]
[       ] [       ] [       ] [       ] [   x   ]

ip link set sw0p1 mtu 9000
ip link set sw1p1 mtu 9000 # at this stage, sw0p1 and sw1p1 can talk
                           # to one another using jumbo frames
ip link set sw0p2 mtu 1500 # this programs the sw0p3 DSA link first to
                           # the largest_mtu of 9000, then reprograms it to
                           # 1500 with the "propagate_upstream == false"
                           # notifier, breaking communication between
                           # sw0p1 and sw1p1

To escape from this situation, make the targeted match really match on a
single port - the user port, and rename the "propagate_upstream"
variable to "targeted_match" to clarify the intention and avoid future
issues.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 3 deletions

diff --git a/net/dsa/switch.c b/net/dsa/switch.c
index 8b601ced6b45..75f567390a6b 100644
--- a/net/dsa/switch.c
+++ b/net/dsa/switch.c
@@ -52,10 +52,13 @@ static int dsa_switch_ageing_time(struct dsa_switch *ds,
 static bool dsa_switch_mtu_match(struct dsa_switch *ds, int port,
 				 struct dsa_notifier_mtu_info *info)
 {
-	if (ds->index == info->sw_index)
-		return (port == info->port) || dsa_is_dsa_port(ds, port);
+	if (ds->index == info->sw_index && port == info->port)
+		return true;
 
-	if (!info->propagate_upstream)
+	/* Do not propagate to other switches in the tree if the notifier was
+	 * targeted for a single switch.
+	 */
+	if (info->targeted_match)
 		return false;
 
 	if (dsa_is_dsa_port(ds, port) || dsa_is_cpu_port(ds, port))