netdev - RE: [PATCH net-2.6] bridge: fix br_multicast_ipv6

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Date:	Thu, 30 Dec 2010 23:00:13 +0200
From:	"Winkler, Tomas" <tomas.winkler@...el.com>
To:	Stephen Hemminger <shemminger@...tta.com>,
	Johannes Berg <johannes@...solutions.net>
CC:	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>
Subject: RE: [PATCH net-2.6] bridge: fix br_multicast_ipv6_rcv for paged skbs



> -----Original Message-----
> From: Stephen Hemminger [mailto:shemminger@...tta.com]
> Sent: Thursday, December 30, 2010 9:06 PM
> To: Johannes Berg
> Cc: Winkler, Tomas; davem@...emloft.net; netdev@...r.kernel.org; linux-
> wireless@...r.kernel.org
> Subject: Re: [PATCH net-2.6] bridge: fix br_multicast_ipv6_rcv for paged
> skbs
> 
> On Thu, 30 Dec 2010 19:52:14 +0100
> Johannes Berg <johannes@...solutions.net> wrote:
> 
> > On Thu, 2010-12-30 at 10:46 -0800, Stephen Hemminger wrote:
> >
> > > This doesn't look correct. The calculation of the offset doesn't look
> correct.
> > > Just following the skb_clone(), the skb_pull value is "offset".
> > > Also, the other checks return -EINVAL for incorrectly formed packet.
> > >
> > > --- a/net/bridge/br_multicast.c	2010-12-30 10:29:58.579510488 -0800
> > > +++ b/net/bridge/br_multicast.c	2010-12-30 10:43:27.273386691 -0800
> > > @@ -1464,6 +1464,9 @@ static int br_multicast_ipv6_rcv(struct
> > >  	if (offset < 0 || nexthdr != IPPROTO_ICMPV6)
> > >  		return 0;
> > >
> > > +	if (!pskb_may_pull(skb, offset))
> > > +		return -EINVAL;
> > > +
> > >  	/* Okay, we found ICMPv6 header */
> > >  	skb2 = skb_clone(skb, GFP_ATOMIC);
> > >  	if (!skb2)
> >
> > Wouldn't that make more sense after the clone anyway? But if you look at
> > my email, you'll find that there's potentially, and conditionally, more
> > stuff that will be read from the skb's header, which hasn't necessarily
> > been pulled in, so I think this still won't fix all the issues.
> >
> > Seeing how this only affects some ICMPv6 packets, maybe we should just
> > use skb_copy() instead?
> 
> It comes out cleaner, and the check can be simplified.
> 
> --- a/net/bridge/br_multicast.c	2010-12-30 10:47:12.031733855 -0800
> +++ b/net/bridge/br_multicast.c	2010-12-30 11:00:12.135801266 -0800
> @@ -1465,19 +1465,19 @@ static int br_multicast_ipv6_rcv(struct
>  		return 0;
> 
>  	/* Okay, we found ICMPv6 header */
> -	skb2 = skb_clone(skb, GFP_ATOMIC);
> +	skb2 = skb_copy(skb, GFP_ATOMIC);
>  	if (!skb2)
>  		return -ENOMEM;
> 
> +	err = -EINVAL;
> +	if (skb2->len < offset + sizeof(*icmp6h))
> +		goto out;
> +
>  	len -= offset - skb_network_offset(skb2);
> 
>  	__skb_pull(skb2, offset);
>  	skb_reset_transport_header(skb2);
> 
> -	err = -EINVAL;
> -	if (!pskb_may_pull(skb2, sizeof(*icmp6h)))
> -		goto out;
> -
>  	icmp6h = icmp6_hdr(skb2);
> 
>  	switch (icmp6h->icmp6_type) {
> 
>
Sorry for dump question but isn't there performance penalty on using skb_copy vs. skb_clone?

Anyhow Below is a code snippet from ip6_input.c so you probably would want to fix it all over. 
BTW offset and the pointer arithmetic really gives the same number +1, I'm not surly why the original author would thought it be safer than just using offset.

					offset = ipv6_skip_exthdr(skb, sizeof(*hdr),
                                                          &nexthdr);
                                if (offset < 0)
                                        goto out;
 
                                if (nexthdr != IPPROTO_ICMPV6)
                                        goto out;
 
                                if (!pskb_may_pull(skb, (skb_network_header(skb) +
                                                   offset + 1 - skb->data)))
                                        goto out;
 
                                icmp6 = (struct icmp6hdr *)(skb_network_header(skb) + offset);



Thanks
Tomas


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