[PATCH 4.19] net/sched: cls_u32: Fix reference counter leak leading to overflow

[Rishabh Bhatnagar <risbhat@amazon.com>]

From: Lee Jones <lee@kernel.org>

Upstream commit 04c55383fa5689357bcdd2c8036725a55ed632bc.

In the event of a failure in tcf_change_indev(), u32_set_parms() will
immediately return without decrementing the recently incremented
reference counter.  If this happens enough times, the counter will
rollover and the reference freed, leading to a double free which can be
used to do 'bad things'.

In order to prevent this, move the point of possible failure above the
point where the reference counter is incremented.  Also save any
meaningful return values to be applied to the return data at the
appropriate point in time.

This issue was caught with KASAN.

Fixes: 705c7091262d ("net: sched: cls_u32: no need to call tcf_exts_change for newly allocated struct")
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Rishabh Bhatnagar <risbhat@amazon.com>
---
 net/sched/cls_u32.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/net/sched/cls_u32.c b/net/sched/cls_u32.c
index d30256ac3537..ee8ef606a8e9 100644
--- a/net/sched/cls_u32.c
+++ b/net/sched/cls_u32.c
@@ -778,11 +778,22 @@ static int u32_set_parms(struct net *net, struct tcf_proto *tp,
 			 struct netlink_ext_ack *extack)
 {
 	int err;
+#ifdef CONFIG_NET_CLS_IND
+	int ifindex = -1;
+#endif
 
 	err = tcf_exts_validate(net, tp, tb, est, &n->exts, ovr, extack);
 	if (err < 0)
 		return err;
 
+#ifdef CONFIG_NET_CLS_IND
+	if (tb[TCA_U32_INDEV]) {
+		ifindex = tcf_change_indev(net, tb[TCA_U32_INDEV], extack);
+		if (ifindex < 0)
+			return -EINVAL;
+	}
+#endif
+
 	if (tb[TCA_U32_LINK]) {
 		u32 handle = nla_get_u32(tb[TCA_U32_LINK]);
 		struct tc_u_hnode *ht_down = NULL, *ht_old;
@@ -814,13 +825,8 @@ static int u32_set_parms(struct net *net, struct tcf_proto *tp,
 	}
 
 #ifdef CONFIG_NET_CLS_IND
-	if (tb[TCA_U32_INDEV]) {
-		int ret;
-		ret = tcf_change_indev(net, tb[TCA_U32_INDEV], extack);
-		if (ret < 0)
-			return -EINVAL;
-		n->ifindex = ret;
-	}
+	if (ifindex >= 0)
+		n->ifindex = ifindex;
 #endif
 	return 0;
 }
-- 
2.40.1