Re: [PATCH net] net: ipv6: tcp: fix potential use after free in tcp_v6_do_rcv

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

On Tue, 2013-09-03 at 19:29 +0200, Daniel Borkmann wrote:
> In tcp_v6_do_rcv() code, when processing pkt options, we soley work
> on our skb clone opt_skb that we've created earlier before entering
> tcp_rcv_established() on our way. However, only in condition ...
> 
>   if (np->rxopt.bits.rxtclass)
>     np->rcv_tclass = ipv6_get_dsfield(ipv6_hdr(skb));
> 
> ... we work on skb itself. As we extract every other information out
> of opt_skb in ipv6_pktoptions path, this seems wrong, since skb can
> already be released by tcp_rcv_established() earlier on. When we try
> to access it in ipv6_hdr(), we will dereference freed skb.
> 
> Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
> Cc: Eric Dumazet <eric.dumazet@gmail.com>
> ---
>  net/ipv6/tcp_ipv6.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
> index 6e1649d..eeb4cb0 100644
> --- a/net/ipv6/tcp_ipv6.c
> +++ b/net/ipv6/tcp_ipv6.c
> @@ -1427,7 +1427,7 @@ ipv6_pktoptions:
>  		if (np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim)
>  			np->mcast_hops = ipv6_hdr(opt_skb)->hop_limit;
>  		if (np->rxopt.bits.rxtclass)
> -			np->rcv_tclass = ipv6_get_dsfield(ipv6_hdr(skb));
> +			np->rcv_tclass = ipv6_get_dsfield(ipv6_hdr(opt_skb));
>  		if (ipv6_opt_accepted(sk, opt_skb)) {
>  			skb_set_owner_r(opt_skb, sk);
>  			opt_skb = xchg(&np->pktoptions, opt_skb);

Bug added in commit 4c507d2897bd9b
("net: implement IP_RECVTOS for IP_PKTOPTIONS")

CC Jiri

Acked-by: Eric Dumazet <edumazet@google.com>