Re: PROBLEM: pppol2tp over pppoe NULL pointer dereference

[Eric Dumazet <eric.dumazet@gmail.com>]

Le vendredi 04 novembre 2011 à 22:28 -0400, David Miller a écrit :
> From: Eric Dumazet <eric.dumazet@gmail.com>
> Date: Wed, 02 Nov 2011 00:58:13 +0100
> 
> > Please try following patch, thanks !
> > 
> > [PATCH] l2tp: handle fragmented skbs in receive path
> > 
> > Modern drivers provide skb with fragments, and L2TP doesnt properly
> > handles them.
> > 
> > Some bad frames can also trigger panics because of insufficent checks.
> > 
> > Reported-by: Misha Labjuk <spiked.yar@gmail.com>
> > Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
> 
> I'm still waiting for testing results of this patch.

Of course.

If you prefer, I can submit a smaller patch for the obvious bug first,
and I can respin the thing when net-next reopens.

[PATCH] l2tp: fix l2tp_udp_recv_core()

pskb_may_pull() can change skb->data, so we have to load ptr/optr at the
right place.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
---
 net/l2tp/l2tp_core.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index bf8d50c..cf0f308 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -756,9 +756,6 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb,
 		goto error;
 	}
 
-	/* Point to L2TP header */
-	optr = ptr = skb->data;
-
 	/* Trace packet contents, if enabled */
 	if (tunnel->debug & L2TP_MSG_DATA) {
 		length = min(32u, skb->len);
@@ -769,12 +766,15 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb,
 
 		offset = 0;
 		do {
-			printk(" %02X", ptr[offset]);
+			printk(" %02X", skb->data[offset]);
 		} while (++offset < length);
 
 		printk("\n");
 	}
 
+	/* Point to L2TP header */
+	optr = ptr = skb->data;
+
 	/* Get L2TP header flags */
 	hdrflags = ntohs(*(__be16 *) ptr);