[PATCH 1/1] ax25: Fix segfault when receiving an iframe with net2kiss loaded

AX.25 uses sock_queue_rcv_skb() to queue an iframe received packet.
This routine writes NULL to the socket buffer device structure
pointer. The socket buffer is subsequently serviced by
__netif_receiv_skb_core() which dereferences the device structure
pointer & segfaults.

The fix puts the ax25 device structure pointer back in the socket
buffer struct after sock_queue_rcv_skb() is called.

To trigger the segfault setup an ax.25 device (ax0) then run net2kiss
(net2kiss -v -i ax0 /dev/ptmx). In another console make an ax.25
connection (call udr0 jnbbs). Within 2 received packets a segfault
will occur.

Please submit to -stable.

Signed-off-by: Basil Gunn <redacted>
---
 net/ax25/ax25_in.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/ax25/ax25_in.c b/net/ax25/ax25_in.c
index bb5a0e4..417f21a 100644
--- a/net/ax25/ax25_in.c
+++ b/net/ax25/ax25_in.c

@@ -144,10 +144,15 @@ int ax25_rx_iframe(ax25_cb *ax25, struct sk_buff *skb)
        if (ax25->sk != NULL && ax25->ax25_dev->values[AX25_VALUES_CONMODE] == 2) {
                if ((!ax25->pidincl && ax25->sk->sk_protocol == pid) ||
                    ax25->pidincl) {
+                       /* Will set socket buffer device struct pointer,
+                        *  skb->dev to NULL
+                        */
                        if (sock_queue_rcv_skb(ax25->sk, skb) == 0)
                                queued = 1;
                        else
                                ax25->condition |= AX25_COND_OWN_RX_BUSY;
+
+                       skb->dev = ax25->ax25_dev->dev;
                }
        }

--

2.1.4

From: Basil Gunn <redacted>
Date: Wed, 30 Nov 2016 11:15:25 -0800

AX.25 uses sock_queue_rcv_skb() to queue an iframe received packet.
This routine writes NULL to the socket buffer device structure
pointer. The socket buffer is subsequently serviced by
__netif_receiv_skb_core() which dereferences the device structure
pointer & segfaults.

The fix puts the ax25 device structure pointer back in the socket
buffer struct after sock_queue_rcv_skb() is called.

You cannot do this.

We have no reference count held on the device object, therefore once
access to this SKB leaves the receive packet processing path and thus
the RCU protected region, we must set skb->dev to NULL.

Queueing the SKB to the socket causes the SKB to leave the receive
processing path.

You will have to find another way to propagate this device pointer
to the code that needs it.