net: mscc: ocelot: mark traps with a bool instead of keeping them in a list

Since the blamed commit, VCAP filters can appear on more than one list.
If their action is "trap", they are chained on ocelot->traps via
filter->trap_list. This is in addition to their normal placement on the
VCAP block->rules list head.

Therefore, when we free a VCAP filter, we must remove it from all lists
it is a member of, including ocelot->traps.

There are at least 2 bugs which are direct consequences of this design
decision.

First is the incorrect usage of list_empty(), meant to denote whether
"filter" is chained into ocelot->traps via filter->trap_list.
This does not do the correct thing, because list_empty() checks whether
"head->next == head", but in our case, head->next == head->prev == NULL.
So we dereference NULL pointers and die when we call list_del().

Second is the fact that not all places that should remove the filter
from ocelot->traps do so. One example is ocelot_vcap_block_remove_filter(),
which is where we have the main kfree(filter). By keeping freed filters
in ocelot->traps we end up in a use-after-free in
felix_update_trapping_destinations().

Attempting to fix all the buggy patterns is a whack-a-mole game which
makes the driver unmaintainable. Actually this is what the previous
patch version attempted to do:
https://patchwork.kernel.org/project/netdevbpf/patch/20220503115728.834457-3-vladimir.oltean@nxp.com/

but it introduced another set of bugs, because there are other places in
which create VCAP filters, not just ocelot_vcap_filter_create():

- ocelot_trap_add()
- felix_tag_8021q_vlan_add_rx()
- felix_tag_8021q_vlan_add_tx()

Relying on the convention that all those code paths must call
INIT_LIST_HEAD(&filter->trap_list) is not going to scale.

So let's do what should have been done in the first place and keep a
bool in struct ocelot_vcap_filter which denotes whether we are looking
at a trapping rule or not. Iterating now happens over the main VCAP IS2
block->rules. The advantage is that we no longer risk having stale
references to a freed filter, since it is only present in that list.

Fixes: e42bd4ed09 ("net: mscc: ocelot: keep traps in a list")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/dsa/ocelot/felix.c

@@ -403,6 +403,7 @@ static int felix_update_trapping_destinations(struct dsa_switch *ds,
 {
 	struct ocelot *ocelot = ds->priv;
 	struct felix *felix = ocelot_to_felix(ocelot);
+	struct ocelot_vcap_block *block_vcap_is2;
 	struct ocelot_vcap_filter *trap;
 	enum ocelot_mask_mode mask_mode;
 	unsigned long port_mask;
@@ -422,9 +423,13 @@ static int felix_update_trapping_destinations(struct dsa_switch *ds,
 	/* We are sure that "cpu" was found, otherwise
 	 * dsa_tree_setup_default_cpu() would have failed earlier.
 	 */
+	block_vcap_is2 = &ocelot->block[VCAP_IS2];
 
 	/* Make sure all traps are set up for that destination */
-	list_for_each_entry(trap, &ocelot->traps, trap_list) {
+	list_for_each_entry(trap, &block_vcap_is2->rules, list) {
+		if (!trap->is_trap)
+			continue;
+
 		/* Figure out the current trapping destination */
 		if (using_tag_8021q) {
 			/* Redirect to the tag_8021q CPU port. If timestamps

drivers/net/ethernet/mscc/ocelot.c

@@ -1622,7 +1622,7 @@ int ocelot_trap_add(struct ocelot *ocelot, int port,
 		trap->action.mask_mode = OCELOT_MASK_MODE_PERMIT_DENY;
 		trap->action.port_mask = 0;
 		trap->take_ts = take_ts;
-		list_add_tail(&trap->trap_list, &ocelot->traps);
+		trap->is_trap = true;
 		new = true;
 	}
 
@@ -1634,10 +1634,8 @@ int ocelot_trap_add(struct ocelot *ocelot, int port,
 		err = ocelot_vcap_filter_replace(ocelot, trap);
 	if (err) {
 		trap->ingress_port_mask &= ~BIT(port);
-		if (!trap->ingress_port_mask) {
-			list_del(&trap->trap_list);
+		if (!trap->ingress_port_mask)
 			kfree(trap);
-		}
 		return err;
 	}
 
@@ -1657,11 +1655,8 @@ int ocelot_trap_del(struct ocelot *ocelot, int port, unsigned long cookie)
 		return 0;
 
 	trap->ingress_port_mask &= ~BIT(port);
-	if (!trap->ingress_port_mask) {
-		list_del(&trap->trap_list);
-
+	if (!trap->ingress_port_mask)
 		return ocelot_vcap_filter_del(ocelot, trap);
-	}
 
 	return ocelot_vcap_filter_replace(ocelot, trap);
 }

drivers/net/ethernet/mscc/ocelot_flower.c

@@ -295,7 +295,7 @@ static int ocelot_flower_parse_action(struct ocelot *ocelot, int port,
 			filter->action.cpu_copy_ena = true;
 			filter->action.cpu_qu_num = 0;
 			filter->type = OCELOT_VCAP_FILTER_OFFLOAD;
-			list_add_tail(&filter->trap_list, &ocelot->traps);
+			filter->is_trap = true;
 			break;
 		case FLOW_ACTION_POLICE:
 			if (filter->block_id == PSFP_BLOCK_ID) {
@@ -878,8 +878,6 @@ int ocelot_cls_flower_replace(struct ocelot *ocelot, int port,
 
 	ret = ocelot_flower_parse(ocelot, port, ingress, f, filter);
 	if (ret) {
-		if (!list_empty(&filter->trap_list))
-			list_del(&filter->trap_list);
 		kfree(filter);
 		return ret;
 	}

include/soc/mscc/ocelot_vcap.h

@@ -681,7 +681,6 @@ struct ocelot_vcap_id {
 
 struct ocelot_vcap_filter {
 	struct list_head list;
-	struct list_head trap_list;
 
 	enum ocelot_vcap_filter_type type;
 	int block_id;
@@ -695,6 +694,7 @@ struct ocelot_vcap_filter {
 	struct ocelot_vcap_stats stats;
 	/* For VCAP IS1 and IS2 */
 	bool take_ts;
+	bool is_trap;
 	unsigned long ingress_port_mask;
 	/* For VCAP ES0 */
 	struct ocelot_vcap_port ingress_port;