[PATCH] [ROSE] NULL ax25_cb kernel panic

When an internally generated frame is handled by rose_xmit(),
rose_route_frame() is called:

        if (!rose_route_frame(skb, NULL)) {
                dev_kfree_skb(skb);
                stats->tx_errors++;
                return NETDEV_TX_OK;
        }

We have the same code sequence in Net/Rom where an internally generated
frame is handled by nr_xmit() calling nr_route_frame(skb, NULL).
However, in this function NULL argument is tested while it is not in
rose_route_frame().
Then kernel panic occurs later on when calling ax25cmp() with a NULL
ax25_cb argument as reported many times and recently with syzbot.

We need to test if ax25 is NULL before using it.

Here is the patch:

diff --git a/net/rose/rose_route.c b/net/rose/rose_route.c
index 77e9f85a2c92..7f075255a372 100644
--- a/net/rose/rose_route.c
+++ b/net/rose/rose_route.c

@@ -850,6 +850,7 @@ void rose_link_device_down(struct net_device *dev)

 /*
  *     Route a frame to an appropriate AX.25 connection.
+ *     a NULL ax25_cb indicates an internally generated frame.
  */
 int rose_route_frame(struct sk_buff *skb, ax25_cb *ax25)
 {

@@ -867,6 +868,10 @@ int rose_route_frame(struct sk_buff *skb, ax25_cb

*ax25)

        if (skb->len < ROSE_MIN_LEN)
                return res;
+
+       if (!ax25)
+               return rose_loopback_queue(skb, NULL);
+
        frametype = skb->data[2];
        lci = ((skb->data[0] << 8) & 0xF00) + ((skb->data[1] << 0) &