Subject: Re: [PATCH] ipv4: ip_check_defrag must not modify skb before unsharing

From: Eric Leblond <[email-masked]>
Date: Mon, 10 Dec 2012 12:02:58 +0100

Hello,

On Mon, 2012-12-10 at 10:41 +0100, Johannes Berg wrote:
> From: Johannes Berg <[email-masked]>
> 
> ip_check_defrag() might be called from af_packet within the
> RX path where shared SKBs are used, so it must not modify
> the input SKB before it has unshared it for defragmentation.
> Use skb_copy_bits() to get the IP header and only pull in
> everything later.
> 
> The same is true for the other caller in macvlan as it is
> called from dev->rx_handler which can also get a shared SKB.

I've applied the patch and built a new kernel. I did not manage to get
it crashed when using the two techniques (suspend to ram and down/up
interface) that were working well to crash kernel without the patch.

BR,

> Reported-by: Eric Leblond <[email-masked]>
> Cc: [email-masked]
> Signed-off-by: Johannes Berg <[email-masked]>
> ---
> For some versions of the kernel, this code goes into af_packet.c
> 
>  net/ipv4/ip_fragment.c | 19 +++++++++----------
>  1 file changed, 9 insertions(+), 10 deletions(-)
> 
> diff --git a/net/ipv4/ip_fragment.c b/net/ipv4/ip_fragment.c
> index 448e685..8d5cc75 100644
> --- a/net/ipv4/ip_fragment.c
> +++ b/net/ipv4/ip_fragment.c
> @@ -707,28 +707,27 @@ EXPORT_SYMBOL(ip_defrag);
>  
>  struct sk_buff *ip_check_defrag(struct sk_buff *skb, u32 user)
>  {
> -	const struct iphdr *iph;
> +	struct iphdr iph;
>  	u32 len;
>  
>  	if (skb->protocol != htons(ETH_P_IP))
>  		return skb;
>  
> -	if (!pskb_may_pull(skb, sizeof(struct iphdr)))
> +	if (!skb_copy_bits(skb, 0, &iph, sizeof(iph)))
>  		return skb;
>  
> -	iph = ip_hdr(skb);
> -	if (iph->ihl < 5 || iph->version != 4)
> +	if (iph.ihl < 5 || iph.version != 4)
>  		return skb;
> -	if (!pskb_may_pull(skb, iph->ihl*4))
> -		return skb;
> -	iph = ip_hdr(skb);
> -	len = ntohs(iph->tot_len);
> -	if (skb->len < len || len < (iph->ihl * 4))
> +
> +	len = ntohs(iph.tot_len);
> +	if (skb->len < len || len < (iph.ihl * 4))
>  		return skb;
>  
> -	if (ip_is_fragment(ip_hdr(skb))) {
> +	if (ip_is_fragment(&iph)) {
>  		skb = skb_share_check(skb, GFP_ATOMIC);
>  		if (skb) {
> +			if (!pskb_may_pull(skb, iph.ihl*4))
> +				return skb;
>  			if (pskb_trim_rcsum(skb, len))
>  				return skb;
>  			memset(IPCB(skb), 0, sizeof(struct inet_skb_parm));

-- 
Eric Leblond <[email-masked]>
Blog: https://home.regit.org/