Re: NULL dereference in econet AUN-over-UDP receive

[David Miller <davem@davemloft.net>]

From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Thu, 09 Dec 2010 02:37:47 +0100

> Le mercredi 08 décembre 2010 à 19:30 -0500, Nelson Elhage a écrit :
>> While testing one of my econet reproducers on a patched kernel, I triggered a
>> NULL pointer dereference in the econet AUN-over-UDP receive path. Upon further
>> investigation, I now suspect that this code path hasn't worked at all in years.
>> 
>> A copy of the oops is below for your reference, but here's my analysis:
>> 
>> When aun_data_available receives a data packet (ah->code == 2), it calls
>> aun_incoming to process the skb. The start of aun_incoming looks like:
>> 
>> static void aun_incoming(struct sk_buff *skb, struct aunhdr *ah, size_t len)
>> {
>> 	struct iphdr *ip = ip_hdr(skb);
>> 	unsigned char stn = ntohl(ip->saddr) & 0xff;
>> 	struct sock *sk = NULL;
>> 	struct sk_buff *newskb;
>> ---> 	struct ec_device *edev = skb->dev->ec_ptr;    
>> 
> 
> This can be changed to use skb_dst(skb)->dev instead
> 
> struct dst *dst = skb_dst(skb);
> 
> if (dst) {
> 	dev = dst->dev;
> 	...
> }

Nelson please test if this patch fixes your crash:

econet: Fix crash in aun_incoming().

Unconditional use of skb->dev won't work here,
try to fetch the econet device via skb_dst()->dev
instead.

Suggested by Eric Dumazet.

Reported-by: Nelson Elhage <nelhage@ksplice.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/econet/af_econet.c b/net/econet/af_econet.c
index f180371..15dcc1a 100644
--- a/net/econet/af_econet.c
+++ b/net/econet/af_econet.c
@@ -851,9 +851,13 @@ static void aun_incoming(struct sk_buff *skb, struct aunhdr *ah, size_t len)
 {
 	struct iphdr *ip = ip_hdr(skb);
 	unsigned char stn = ntohl(ip->saddr) & 0xff;
+	struct dst_entry *dst = skb_dst(skb);
+	struct ec_device *edev = NULL;
 	struct sock *sk = NULL;
 	struct sk_buff *newskb;
-	struct ec_device *edev = skb->dev->ec_ptr;
+
+	if (dst)
+		edev = dst->dev->ec_ptr;
 
 	if (! edev)
 		goto bad;