net: dsa: Fix use-after-free in probing of DSA switch tree

DSA sets up a switch tree little by little. Every switch of the N
members of the tree calls dsa_register_switch, and (N - 1) will just
touch the dst->ports list with their ports and quickly exit. Only the
last switch that calls dsa_register_switch will find all DSA links
complete in dsa_tree_setup_routing_table, and not return zero as a
result but instead go ahead and set up the entire DSA switch tree
(practically on behalf of the other switches too).

The trouble is that the (N - 1) switches don't clean up after themselves
after they get an error such as EPROBE_DEFER. Their footprint left in
dst->ports by dsa_switch_touch_ports is still there. And switch N, the
one responsible with actually setting up the tree, is going to work with
those stale dp, dp->ds and dp->ds->dev pointers. In particular ds and
ds->dev might get freed by the device driver.

Be there a 2-switch tree and the following calling order:
- Switch 1 calls dsa_register_switch
  - Calls dsa_switch_touch_ports, populates dst->ports
  - Calls dsa_port_parse_cpu, gets -EPROBE_DEFER, exits.
- Switch 2 calls dsa_register_switch
  - Calls dsa_switch_touch_ports, populates dst->ports
  - Probe doesn't get deferred, so it goes ahead.
  - Calls dsa_tree_setup_routing_table, which returns "complete == true"
    due to Switch 1 having called dsa_switch_touch_ports before.
  - Because the DSA links are complete, it calls dsa_tree_setup_switches
    now.
  - dsa_tree_setup_switches iterates through dst->ports, initializing
    the Switch 1 ds structure (invalid) and the Switch 2 ds structure
    (valid).
  - Undefined behavior (use after free, sometimes NULL pointers, etc).

Real example below (debugging prints added by me, as well as guards
against NULL pointers):

[    5.477947] dsa_tree_setup_switches: Setting up port 0 of switch ffffff803df0b980 (dev ffffff803f775c00)
[    6.313002] dsa_tree_setup_switches: Setting up port 1 of switch ffffff803df0b980 (dev ffffff803f775c00)
[    6.319932] dsa_tree_setup_switches: Setting up port 2 of switch ffffff803df0b980 (dev ffffff803f775c00)
[    6.329693] dsa_tree_setup_switches: Setting up port 3 of switch ffffff803df0b980 (dev ffffff803f775c00)
[    6.339458] dsa_tree_setup_switches: Setting up port 4 of switch ffffff803df0b980 (dev ffffff803f775c00)
[    6.349226] dsa_tree_setup_switches: Setting up port 5 of switch ffffff803df0b980 (dev ffffff803f775c00)
[    6.358991] dsa_tree_setup_switches: Setting up port 6 of switch ffffff803df0b980 (dev ffffff803f775c00)
[    6.368758] dsa_tree_setup_switches: Setting up port 7 of switch ffffff803df0b980 (dev ffffff803f775c00)
[    6.378524] dsa_tree_setup_switches: Setting up port 8 of switch ffffff803df0b980 (dev ffffff803f775c00)
[    6.388291] dsa_tree_setup_switches: Setting up port 9 of switch ffffff803df0b980 (dev ffffff803f775c00)
[    6.398057] dsa_tree_setup_switches: Setting up port 10 of switch ffffff803df0b980 (dev ffffff803f775c00)
[    6.407912] dsa_tree_setup_switches: Setting up port 0 of switch ffffff803da02f80 (dev 0000000000000000)
[    6.417682] dsa_tree_setup_switches: Setting up port 1 of switch ffffff803da02f80 (dev 0000000000000000)
[    6.427446] dsa_tree_setup_switches: Setting up port 2 of switch ffffff803da02f80 (dev 0000000000000000)
[    6.437212] dsa_tree_setup_switches: Setting up port 3 of switch ffffff803da02f80 (dev 0000000000000000)
[    6.446979] dsa_tree_setup_switches: Setting up port 4 of switch ffffff803da02f80 (dev 0000000000000000)
[    6.456744] dsa_tree_setup_switches: Setting up port 5 of switch ffffff803da02f80 (dev 0000000000000000)
[    6.466512] dsa_tree_setup_switches: Setting up port 6 of switch ffffff803da02f80 (dev 0000000000000000)
[    6.476277] dsa_tree_setup_switches: Setting up port 7 of switch ffffff803da02f80 (dev 0000000000000000)
[    6.486043] dsa_tree_setup_switches: Setting up port 8 of switch ffffff803da02f80 (dev 0000000000000000)
[    6.495810] dsa_tree_setup_switches: Setting up port 9 of switch ffffff803da02f80 (dev 0000000000000000)
[    6.505577] dsa_tree_setup_switches: Setting up port 10 of switch ffffff803da02f80 (dev 0000000000000000)
[    6.515433] dsa_tree_setup_switches: Setting up port 0 of switch ffffff803db15b80 (dev ffffff803d8e4800)
[    7.354120] dsa_tree_setup_switches: Setting up port 1 of switch ffffff803db15b80 (dev ffffff803d8e4800)
[    7.361045] dsa_tree_setup_switches: Setting up port 2 of switch ffffff803db15b80 (dev ffffff803d8e4800)
[    7.370805] dsa_tree_setup_switches: Setting up port 3 of switch ffffff803db15b80 (dev ffffff803d8e4800)
[    7.380571] dsa_tree_setup_switches: Setting up port 4 of switch ffffff803db15b80 (dev ffffff803d8e4800)
[    7.390337] dsa_tree_setup_switches: Setting up port 5 of switch ffffff803db15b80 (dev ffffff803d8e4800)
[    7.400104] dsa_tree_setup_switches: Setting up port 6 of switch ffffff803db15b80 (dev ffffff803d8e4800)
[    7.409872] dsa_tree_setup_switches: Setting up port 7 of switch ffffff803db15b80 (dev ffffff803d8e4800)
[    7.419637] dsa_tree_setup_switches: Setting up port 8 of switch ffffff803db15b80 (dev ffffff803d8e4800)
[    7.429403] dsa_tree_setup_switches: Setting up port 9 of switch ffffff803db15b80 (dev ffffff803d8e4800)
[    7.439169] dsa_tree_setup_switches: Setting up port 10 of switch ffffff803db15b80 (dev ffffff803d8e4800)

The solution is to recognize that the functions that call
dsa_switch_touch_ports (dsa_switch_parse_of, dsa_switch_parse) have side
effects, and therefore one should clean up their side effects on error
path. The cleanup of dst->ports was taken from dsa_switch_remove and
moved into a dedicated dsa_switch_release_ports function, which should
really be per-switch (free only the members of dst->ports that are also
members of ds, instead of all switch ports).
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/dsa2.c

@@ -849,6 +849,19 @@ static int dsa_switch_parse(struct dsa_switch *ds, struct dsa_chip_data *cd)
 	return dsa_switch_parse_ports(ds, cd);
 }
 
+static void dsa_switch_release_ports(struct dsa_switch *ds)
+{
+	struct dsa_switch_tree *dst = ds->dst;
+	struct dsa_port *dp, *next;
+
+	list_for_each_entry_safe(dp, next, &dst->ports, list) {
+		if (dp->ds != ds)
+			continue;
+		list_del(&dp->list);
+		kfree(dp);
+	}
+}
+
 static int dsa_switch_probe(struct dsa_switch *ds)
 {
 	struct dsa_switch_tree *dst;
@@ -865,12 +878,17 @@ static int dsa_switch_probe(struct dsa_switch *ds)
 	if (!ds->num_ports)
 		return -EINVAL;
 
-	if (np)
+	if (np) {
 		err = dsa_switch_parse_of(ds, np);
-	else if (pdata)
+		if (err)
+			dsa_switch_release_ports(ds);
+	} else if (pdata) {
 		err = dsa_switch_parse(ds, pdata);
-	else
+		if (err)
+			dsa_switch_release_ports(ds);
+	} else {
 		err = -ENODEV;
+	}
 
 	if (err)
 		return err;
@@ -878,8 +896,10 @@ static int dsa_switch_probe(struct dsa_switch *ds)
 	dst = ds->dst;
 	dsa_tree_get(dst);
 	err = dsa_tree_setup(dst);
-	if (err)
+	if (err) {
+		dsa_switch_release_ports(ds);
 		dsa_tree_put(dst);
+	}
 
 	return err;
 }
@@ -900,15 +920,9 @@ EXPORT_SYMBOL_GPL(dsa_register_switch);
 static void dsa_switch_remove(struct dsa_switch *ds)
 {
 	struct dsa_switch_tree *dst = ds->dst;
-	struct dsa_port *dp, *next;
 
 	dsa_tree_teardown(dst);
-
-	list_for_each_entry_safe(dp, next, &dst->ports, list) {
-		list_del(&dp->list);
-		kfree(dp);
-	}
-
+	dsa_switch_release_ports(ds);
 	dsa_tree_put(dst);
 }