[PATCH net 3/4] net/sched: act_api: fix skb sizing and action leak on reoffload delete

[Victor Nogueira <victor@mojatatu.com>]

tcf_reoffload_del_notify_msg() sizes the RTM_DELACTION skb with
tcf_action_fill_size(action) alone.  Unlike every other notification path
it never wraps that in tcf_action_full_attrs_size(), so the nlmsg_put()
header, struct tcamsg and the TCA_ACT_TAB nest that tca_get_fill() emits -
24 bytes on x86_64 - are not budgeted.  As long as the single action stays
well under NLMSG_GOODSIZE the floor in alloc_skb() hides this, but once its
fill size crosses NLMSG_GOODSIZE the allocation is exactly 24 bytes short
and tca_get_fill() runs out of tailroom.  That is now easy to reach for an
offloadable act_pedit with a large tcfp_nkeys, which commit 8e2efb3f45a5
("net/sched: add get_fill_size callbacks for actions missing them") started
accounting for properly.

When that happens tcf_reoffload_del_notify() returns early, before
tcf_idr_release_unsafe(), and tcf_action_reoffload_cb() discards the return
value:

	if (tc_act_skip_sw(p->tcfa_flags) && !tc_act_in_hw(p))
		tcf_reoffload_del_notify(net, p);

The action has just lost its last hardware instance and is skip_sw, so it
is left installed while processing no packets, and with no notification to
tell userspace about it.  An -ENOBUFS from alloc_skb() gets the same
treatment.

Fix this by budgeting the message header the way the add and delete paths
do, and release the action even when the notification cannot be built -
dropping the notification is strictly better than leaking a dead action,
and there is no caller left to report the error to.

Fixes: 13926d19a11e ("flow_offload: add reoffload process to update hw_count")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260810164357.1653956-1-victor%40mojatatu.com
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Victor Nogueira <victor@mojatatu.com>
---
 net/sched/act_api.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/net/sched/act_api.c b/net/sched/act_api.c
index 20b6501fd33b..37eced84dfa5 100644
--- a/net/sched/act_api.c
+++ b/net/sched/act_api.c
@@ -1867,11 +1867,13 @@ static int tcf_action_delete(struct net *net, struct tc_action *actions[])
 static struct sk_buff *tcf_reoffload_del_notify_msg(struct net *net,
 						    struct tc_action *action)
 {
-	size_t attr_size = tcf_action_fill_size(action);
 	struct tc_action *actions[TCA_ACT_MAX_PRIO] = {
 		[0] = action,
 	};
 	struct sk_buff *skb;
+	size_t attr_size;
+
+	attr_size = tcf_action_full_attrs_size(tcf_action_fill_size(action));
 
 	skb = alloc_skb(max(attr_size, NLMSG_GOODSIZE), GFP_KERNEL);
 	if (!skb)
@@ -1888,15 +1890,18 @@ static struct sk_buff *tcf_reoffload_del_notify_msg(struct net *net,
 static int tcf_reoffload_del_notify(struct net *net, struct tc_action *action)
 {
 	const struct tc_action_ops *ops = action->ops;
-	struct sk_buff *skb;
+	struct sk_buff *skb = NULL;
 	int ret;
 
-	if (!rtnl_notify_needed(net, 0, RTNLGRP_TC)) {
-		skb = NULL;
-	} else {
+	if (rtnl_notify_needed(net, 0, RTNLGRP_TC)) {
 		skb = tcf_reoffload_del_notify_msg(net, action);
+		/* The action has already lost its hardware instance and is
+		 * skip_sw, so it must be released whether or not the
+		 * notification can be built.  Drop the notification rather
+		 * than leave an action behind that processes no packets.
+		 */
 		if (IS_ERR(skb))
-			return PTR_ERR(skb);
+			skb = NULL;
 	}
 
 	ret = tcf_idr_release_unsafe(action);
-- 
2.55.0