From: Xin Long <lucien.xin@gmail.com>
Date: Mon, 12 Feb 2018 17:15:40 +0800
Subject: bridge: check brport attr show in brport_show
Patch-mainline: v4.16-rc3
Git-commit: 1b12580af1d0677c3c3a19e35bfe5d59b03f737f
References: bsc#1076830

Now br_sysfs_if file flush doesn't have attr show. To read it will
cause kernel panic after users chmod u+r this file.

Xiong found this issue when running the commands:

  ip link add br0 type bridge
  ip link add type veth
  ip link set veth0 master br0
  chmod u+r /sys/devices/virtual/net/veth0/brport/flush
  timeout 3 cat /sys/devices/virtual/net/veth0/brport/flush

kernel crashed with NULL a pointer dereference call trace.

This patch is to fix it by return -EINVAL when brport_attr->show
is null, just the same as the check for brport_attr->store in
brport_store().

Fixes: 9cf637473c85 ("bridge: add sysfs hook to flush forwarding table")
Reported-by: Xiong Zhou <xzhou@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Michal Kubecek <mkubecek@suse.cz>

---
 net/bridge/br_sysfs_if.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/bridge/br_sysfs_if.c b/net/bridge/br_sysfs_if.c
index 5d5d413a6cf8..a097a8613a02 100644
--- a/net/bridge/br_sysfs_if.c
+++ b/net/bridge/br_sysfs_if.c
@@ -235,6 +235,9 @@ static ssize_t brport_show(struct kobject *kobj,
 	struct brport_attribute *brport_attr = to_brport_attr(attr);
 	struct net_bridge_port *p = to_brport(kobj);
 
+	if (!brport_attr->show)
+		return -EINVAL;
+
 	return brport_attr->show(p, buf);
 }
 
-- 
2.16.2