[Patch net] ax25: fix a use-after-free in ax25_fillin_cb()

[Cong Wang]

There are multiple issues here:

1. After freeing dev->ax25_ptr, we need to set it to NULL otherwise
   we may use a dangling pointer.

2. There is a race between ax25_setsockopt() and device notifier as
   reported by syzbot. Close it by holding RTNL lock.

3. We need to test if dev->ax25_ptr is NULL before using it.

Reported-and-tested-by: syzbot+ae6bb869cbed29b29...@syzkaller.appspotmail.com
Signed-off-by: Cong Wang <xiyou.wangc...@gmail.com>
---
 net/ax25/af_ax25.c  | 11 +++++++++--
 net/ax25/ax25_dev.c |  2 ++
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
index c603d33d5410..5d01edf8d819 100644
--- a/net/ax25/af_ax25.c
+++ b/net/ax25/af_ax25.c
@@ -653,15 +653,22 @@ static int ax25_setsockopt(struct socket *sock, int 
level, int optname,
                        break;
                }
 
-               dev = dev_get_by_name(&init_net, devname);
+               rtnl_lock();
+               dev = __dev_get_by_name(&init_net, devname);
                if (!dev) {
+                       rtnl_unlock();
                        res = -ENODEV;
                        break;
                }
 
                ax25->ax25_dev = ax25_dev_ax25dev(dev);
+               if (!ax25->ax25_dev) {
+                       rtnl_unlock();
+                       res = -ENODEV;
+                       break;
+               }
                ax25_fillin_cb(ax25, ax25->ax25_dev);
-               dev_put(dev);
+               rtnl_unlock();
                break;
 
        default:
diff --git a/net/ax25/ax25_dev.c b/net/ax25/ax25_dev.c
index 9a3a301e1e2f..d92195cd7834 100644
--- a/net/ax25/ax25_dev.c
+++ b/net/ax25/ax25_dev.c
@@ -116,6 +116,7 @@ void ax25_dev_device_down(struct net_device *dev)
        if ((s = ax25_dev_list) == ax25_dev) {
                ax25_dev_list = s->next;
                spin_unlock_bh(&ax25_dev_lock);
+               dev->ax25_ptr = NULL;
                dev_put(dev);
                kfree(ax25_dev);
                return;
@@ -125,6 +126,7 @@ void ax25_dev_device_down(struct net_device *dev)
                if (s->next == ax25_dev) {
                        s->next = ax25_dev->next;
                        spin_unlock_bh(&ax25_dev_lock);
+                       dev->ax25_ptr = NULL;
                        dev_put(dev);
                        kfree(ax25_dev);
                        return;
-- 
2.19.2