Subject: [PATCH net v2 2/2] net/sched: act_mirred: don't override retval if we already lost the skb

From: Jakub Kicinski <[email-masked]>
Date: Tue, 13 Feb 2024 19:38:48 -0800

If we're redirecting the skb, and haven't called tcf_mirred_forward(),
yet, we need to tell the core to drop the skb by setting the retcode
to SHOT. If we have called tcf_mirred_forward(), however, the skb
is out of our hands and returning SHOT will lead to UaF.

Move the retval override to the error path which actually need it.

Fixes: e5cf1baf92cb ("act_mirred: use TC_ACT_REINSERT when possible")
Signed-off-by: Jakub Kicinski <[email-masked]>
---
CC: [email-masked]
CC: [email-masked]
CC: [email-masked]
---
 net/sched/act_mirred.c | 22 ++++++++++------------
 1 file changed, 10 insertions(+), 12 deletions(-)

diff --git a/net/sched/act_mirred.c b/net/sched/act_mirred.c
index 291d47c9eb69..6faa7d00da09 100644
--- a/net/sched/act_mirred.c
+++ b/net/sched/act_mirred.c
@@ -266,8 +266,7 @@ static int tcf_mirred_to_dev(struct sk_buff *skb, struct tcf_mirred *m,
 	if (unlikely(!(dev->flags & IFF_UP)) || !netif_carrier_ok(dev)) {
 		net_notice_ratelimited("tc mirred to Houston: device %s is down\n",
 				       dev->name);
-		err = -ENODEV;
-		goto out;
+		goto err_cant_do;
 	}
 
 	/* we could easily avoid the clone only if called by ingress and clsact;
@@ -279,10 +278,8 @@ static int tcf_mirred_to_dev(struct sk_buff *skb, struct tcf_mirred *m,
 		tcf_mirred_can_reinsert(retval);
 	if (!dont_clone) {
 		skb_to_send = skb_clone(skb, GFP_ATOMIC);
-		if (!skb_to_send) {
-			err =  -ENOMEM;
-			goto out;
-		}
+		if (!skb_to_send)
+			goto err_cant_do;
 	}
 
 	want_ingress = tcf_mirred_act_wants_ingress(m_eaction);
@@ -319,15 +316,16 @@ static int tcf_mirred_to_dev(struct sk_buff *skb, struct tcf_mirred *m,
 	} else {
 		err = tcf_mirred_forward(at_ingress, want_ingress, skb_to_send);
 	}
-
-	if (err) {
-out:
+	if (err)
 		tcf_action_inc_overlimit_qstats(&m->common);
-		if (is_redirect)
-			retval = TC_ACT_SHOT;
-	}
 
 	return retval;
+
+err_cant_do:
+	if (is_redirect)
+		retval = TC_ACT_SHOT;
+	tcf_action_inc_overlimit_qstats(&m->common);
+	return retval;
 }
 
 static int tcf_blockcast_redir(struct sk_buff *skb, struct tcf_mirred *m,
-- 
2.43.0