netdev - [PATCH net] rtnetlink: make sure to refresh master_dev/m_ops in __rtnl

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Date:   Mon, 31 Jan 2022 17:21:06 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Jiri Pirko <jiri@...dia.com>
Subject: [PATCH net] rtnetlink: make sure to refresh master_dev/m_ops in __rtnl_newlink()

From: Eric Dumazet <edumazet@...gle.com>

While looking at one unrelated syzbot bug, I found the replay logic
in __rtnl_newlink() to potentially trigger use-after-free.

It is better to clear master_dev and m_ops inside the loop,
in case we have to replay it.

Fixes: ba7d49b1f0f8 ("rtnetlink: provide api for getting and setting slave info")
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Jiri Pirko <jiri@...dia.com>
---
 net/core/rtnetlink.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index e476403231f00053e1a261f31a8760325c75c941..710da8a36729d6a9293b610b029bd33aa38d6514 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -3275,8 +3275,8 @@ static int __rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 	struct nlattr *slave_attr[RTNL_SLAVE_MAX_TYPE + 1];
 	unsigned char name_assign_type = NET_NAME_USER;
 	struct nlattr *linkinfo[IFLA_INFO_MAX + 1];
-	const struct rtnl_link_ops *m_ops = NULL;
-	struct net_device *master_dev = NULL;
+	const struct rtnl_link_ops *m_ops;
+	struct net_device *master_dev;
 	struct net *net = sock_net(skb->sk);
 	const struct rtnl_link_ops *ops;
 	struct nlattr *tb[IFLA_MAX + 1];
@@ -3314,6 +3314,8 @@ static int __rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 	else
 		dev = NULL;
 
+	master_dev = NULL;
+	m_ops = NULL;
 	if (dev) {
 		master_dev = netdev_master_upper_dev_get(dev);
 		if (master_dev)
-- 
2.35.0.rc2.247.g8bbb082509-goog