[SRU][Focal][PATCH v2 1/1] net/sched: cls_u32: Fix reference counter leak leading to overflow

[Yuxuan Luo]

From: Lee Jones <lee at kernel.org>

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 at google.com>
Signed-off-by: Lee Jones <lee at kernel.org>
Reviewed-by: Eric Dumazet <edumazet at google.com>
Acked-by: Jamal Hadi Salim <jhs at mojatatu.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
(cherry picked from commit 04c55383fa5689357bcdd2c8036725a55ed632bc)
CVE-2023-3609
Signed-off-by: Yuxuan Luo <yuxuan.luo at canonical.com>
---
 net/sched/cls_u32.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/net/sched/cls_u32.c b/net/sched/cls_u32.c
index ed8d26e6468ca..e5cc2b4d38d5a 100644
--- a/net/sched/cls_u32.c
+++ b/net/sched/cls_u32.c
@@ -716,12 +716,18 @@ static int u32_set_parms(struct net *net, struct tcf_proto *tp,
 			 struct nlattr *est, bool ovr,
 			 struct netlink_ext_ack *extack)
 {
-	int err;
+	int err, ifindex = -1;
 
 	err = tcf_exts_validate(net, tp, tb, est, &n->exts, ovr, true, extack);
 	if (err < 0)
 		return err;
 
+	if (tb[TCA_U32_INDEV]) {
+		ifindex = tcf_change_indev(net, tb[TCA_U32_INDEV], extack);
+		if (ifindex < 0)
+			return -EINVAL;
+	}
+
 	if (tb[TCA_U32_LINK]) {
 		u32 handle = nla_get_u32(tb[TCA_U32_LINK]);
 		struct tc_u_hnode *ht_down = NULL, *ht_old;
@@ -756,13 +762,9 @@ static int u32_set_parms(struct net *net, struct tcf_proto *tp,
 		tcf_bind_filter(tp, &n->res, base);
 	}
 
-	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;
+
 	return 0;
 }
 
-- 
2.34.1