Re: [PATCH net] tcp: fix race in tcp_v6_syn_recv_sock()

[Simon Horman <horms@kernel.org>]

+ David Ahern

On Thu, Jun 06, 2024 at 03:46:51PM +0000, Eric Dumazet wrote:
> tcp_v6_syn_recv_sock() calls ip6_dst_store() before
> inet_sk(newsk)->pinet6 has been set up.
> 
> This means ip6_dst_store() writes over the parent (listener)
> np->dst_cookie.
> 
> This is racy because multiple threads could share the same
> parent and their final np->dst_cookie could be wrong.
> 
> Move ip6_dst_store() call after inet_sk(newsk)->pinet6
> has been changed and after the copy of parent ipv6_pinfo.
> 
> Fixes: e994b2f0fb92 ("tcp: do not lock listener to process SYN packets")
> Signed-off-by: Eric Dumazet <edumazet@google.com>

Reviewed-by: Simon Horman <horms@kernel.org>

> ---
>  net/ipv6/tcp_ipv6.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
> index 8c577b651bfcd2f94b45e339ed4a2b47e93ff17a..729faf8bd366ad25d093a4ae931fb46ebd45b79c 100644
> --- a/net/ipv6/tcp_ipv6.c
> +++ b/net/ipv6/tcp_ipv6.c
> @@ -1439,7 +1439,6 @@ static struct sock *tcp_v6_syn_recv_sock(const struct sock *sk, struct sk_buff *
>  	 */
>  
>  	newsk->sk_gso_type = SKB_GSO_TCPV6;
> -	ip6_dst_store(newsk, dst, NULL, NULL);
>  	inet6_sk_rx_dst_set(newsk, skb);
>  
>  	inet_sk(newsk)->pinet6 = tcp_inet6_sk(newsk);
> @@ -1450,6 +1449,8 @@ static struct sock *tcp_v6_syn_recv_sock(const struct sock *sk, struct sk_buff *
>  
>  	memcpy(newnp, np, sizeof(struct ipv6_pinfo));
>  
> +	ip6_dst_store(newsk, dst, NULL, NULL);
> +
>  	newsk->sk_v6_daddr = ireq->ir_v6_rmt_addr;
>  	newnp->saddr = ireq->ir_v6_loc_addr;
>  	newsk->sk_v6_rcv_saddr = ireq->ir_v6_loc_addr;
> -- 
> 2.45.1.467.gbab1589fc0-goog
> 
>