Re: [PATCH net] net/dccp: fix use-after-free in dccp_invalid_packet

[Eric Dumazet]

On Mon, 2016-11-28 at 11:40 -0300, Arnaldo Carvalho de Melo wrote:
> Em Mon, Nov 28, 2016 at 06:26:49AM -0800, Eric Dumazet escreveu:
> > From: Eric Dumazet <edumazet@xxxxxxxxxx>
> > 
> > pskb_may_pull() can reallocate skb->head, we need to reload dh pointer
> > in dccp_invalid_packet() or risk use after free.
> > 
> > Bug found by Andrey Konovalov using syzkaller.
> > 
> > Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
> > Reported-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> 
> Acked-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> 
> I was about to send exactly this patch, and while looking at it I think
> the patch below needs to go in as well, no? To follow the advice of that
> Warning line there :-)
> 
> From: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> 
> pskb_may_pull() can reallocate skb->head, so we can't access
> iph->frag_off or risk use after free, save it to a variable and us that
> later.
> 
> Cc: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> 
> diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
> index 5ddf5cda07f4..9462070561a3 100644
> --- a/net/ipv4/af_inet.c
> +++ b/net/ipv4/af_inet.c
> @@ -1198,6 +1198,7 @@ struct sk_buff *inet_gso_segment(struct sk_buff *skb,
>  	struct iphdr *iph;
>  	int proto, tot_len;
>  	int nhoff;
> +	u16 frag_off;
>  	int ihl;
>  	int id;
>  
> @@ -1213,6 +1214,7 @@ struct sk_buff *inet_gso_segment(struct sk_buff *skb,
>  
>  	id = ntohs(iph->id);
>  	proto = iph->protocol;
> +	frag_off = iph->frag_off;
>  
>  	/* Warning: after this point, iph might be no longer valid */
>  	if (unlikely(!pskb_may_pull(skb, ihl)))
> @@ -1233,7 +1235,7 @@ struct sk_buff *inet_gso_segment(struct sk_buff *skb,
>  		fixedid = !!(skb_shinfo(skb)->gso_type & SKB_GSO_TCP_FIXEDID);
>  
>  		/* fixed ID is invalid if DF bit is not set */
> -		if (fixedid && !(iph->frag_off & htons(IP_DF)))
> +		if (fixedid && !(frag_off & htons(IP_DF)))
>  			goto out;
>  	}
>  


I do not see why this patch would be needed ?