[PATCH] net/dsa: fix oob in dsa_user_prechangeupper

[Edward Adam Davis <eadavis@qq.com>]

If the private data is not allocated memory when generating an instance of 
struct net_device, i.e. priv_size is too small, then its corresponding private
data should not be accessed.

Reported-and-tested-by: syzbot+7ec955e36bb239bd720f@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
---
 net/dsa/user.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/dsa/user.h b/net/dsa/user.h
index 996069130bea..9a40918ee7fc 100644
--- a/net/dsa/user.h
+++ b/net/dsa/user.h
@@ -53,7 +53,11 @@ int dsa_user_manage_vlan_filtering(struct net_device *dev,
 
 static inline struct dsa_port *dsa_user_to_port(const struct net_device *dev)
 {
-	struct dsa_user_priv *p = netdev_priv(dev);
+	const struct rtnl_link_ops *ops = dev->rtnl_link_ops;
+	struct dsa_user_priv *p = ops->priv_size >= sizeof(*p) ? 
+		netdev_priv(dev) : NULL;
+	if (!p)
+		return NULL;
 
 	return p->dp;
 }
-- 
2.43.0