[MPTCP] Re: [PATCH net] mptcp: fix NULL ptr dereference in MP_JOIN error path

[Christoph Paasch <cpaasch at apple.com>]

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On 05/28/20 - 15:27, Paolo Abeni wrote:
> When token lookup on MP_JOIN 3rd ack fails, the server
> socket closes with a reset the incoming child. Such socket
> has the 'is_mptcp' flag set, but no msk socket associated
> - due to the failed lookup.
> 
> While crafting the reset packet mptcp_established_options_mp()
> will try to dereference the child's master socket, causing
> a NULL ptr dereference.
> 
> This change addresses the issue with explicit fallback to
> TCP in such error path.
> 
> Fixes: f296234c98a8 ("mptcp: Add handling of incoming MP_JOIN requests")
> Signed-off-by: Paolo Abeni <pabeni(a)redhat.com>
> ---
>  net/mptcp/subflow.c | 17 +++++++++++++----
>  net/mptcp/token.c   | 10 +++++++---
>  2 files changed, 20 insertions(+), 7 deletions(-)
> 
> diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
> index ae7bcdb7f990..55fced4f136d 100644
> --- a/net/mptcp/subflow.c
> +++ b/net/mptcp/subflow.c
> @@ -408,6 +408,17 @@ static void subflow_ulp_fallback(struct sock *sk,
>  	tcp_sk(sk)->is_mptcp = 0;
>  }
>  
> +static void subflow_drop_ctx(struct sock *ssk)
> +{
> +	struct mptcp_subflow_context *ctx = mptcp_subflow_ctx(ssk);
> +
> +	if (!ctx)
> +		return;
> +
> +	subflow_ulp_fallback(ssk, ctx);
> +	kfree_rcu(ctx, rcu);
> +}
> +
>  static struct sock *subflow_syn_recv_sock(const struct sock *sk,
>  					  struct sk_buff *skb,
>  					  struct request_sock *req,
> @@ -480,10 +491,7 @@ static struct sock *subflow_syn_recv_sock(const struct sock *sk,
>  			if (fallback_is_fatal)
>  				goto dispose_child;
>  
> -			if (ctx) {
> -				subflow_ulp_fallback(child, ctx);
> -				kfree_rcu(ctx, rcu);
> -			}
> +			subflow_drop_ctx(child);
>  			goto out;
>  		}
>  
> @@ -532,6 +540,7 @@ static struct sock *subflow_syn_recv_sock(const struct sock *sk,
>  	return child;
>  
>  dispose_child:
> +	subflow_drop_ctx(child);
>  	tcp_rsk(req)->drop_req = true;
>  	tcp_send_active_reset(child, GFP_ATOMIC);
>  	inet_csk_prepare_for_destroy_sock(child);
> diff --git a/net/mptcp/token.c b/net/mptcp/token.c
> index c0327ff5d1b7..d1068b51a45a 100644
> --- a/net/mptcp/token.c
> +++ b/net/mptcp/token.c
> @@ -216,16 +216,20 @@ struct mptcp_sock *mptcp_token_get_sock(u32 token)
>  		msk = mptcp_sk(sk);
>  		if (READ_ONCE(msk->token) != token)
>  			continue;
> -		if (!refcount_inc_not_zero(&sk->sk_refcnt))
> +		if (!refcount_inc_not_zero(&sk->sk_refcnt)) {
> +			pr_warn("token match %x but 0 rfcnt\n", token);
>  			goto not_found;
> -		if (READ_ONCE(msk->token) != token) {
> +		} if (READ_ONCE(msk->token) != token) {
> +			pr_warn("token mis-match %x:%x after get\n", READ_ONCE(msk->token), token);
>  			sock_put(sk);
>  			goto again;
>  		}
>  		goto found;
>  	}
> -	if (get_nulls_value(pos) != (token & token_mask))
> +	if (get_nulls_value(pos) != (token & token_mask)) {
> +		pr_warn("moved nulls %ld id %d\n", get_nulls_value(pos), token & token_mask);
>  		goto again;
> +	}

Just wondering if these warns are necessary, as they could trigger often on
a busy server. Maybe they should be pr_debug ?


Christoph