4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Bernard Pidoux <f6...@free.fr>

[ Upstream commit b0cf029234f9b18e10703ba5147f0389c382bccc ]

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.

Testing:
Built kernel with CONFIG_ROSE=y.

Signed-off-by: Bernard Pidoux <f6...@free.fr>
Acked-by: Dmitry Vyukov <dvyu...@google.com>
Reported-by: syzbot+1a2c456a1ea08fa5b...@syzkaller.appspotmail.com
Cc: "David S. Miller" <da...@davemloft.net>
Cc: Ralf Baechle <r...@linux-mips.org>
Cc: Bernard Pidoux <f6...@free.fr>
Cc: linux-hams@vger.kernel.org
Cc: net...@vger.kernel.org
Cc: linux-ker...@vger.kernel.org
Signed-off-by: David S. Miller <da...@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
---
 net/rose/rose_route.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/net/rose/rose_route.c
+++ b/net/rose/rose_route.c
@@ -848,6 +848,7 @@ void rose_link_device_down(struct net_de
 
 /*
  *     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)
 {
@@ -865,6 +866,10 @@ int rose_route_frame(struct sk_buff *skb
 
        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) & 0x0FF);
        if (frametype == ROSE_CALL_REQUEST &&


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