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Date:   Fri, 9 Nov 2018 16:33:32 +0100
From:   Guillaume Nault <g.nault@alphalink.fr>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        "David S. Miller" <davem@davemloft.net>,
        Sasha Levin <sashal@kernel.org>
Subject: Re: [PATCH 4.4 063/114] l2tp: hold socket before dropping lock in
 l2tp_ip{, 6}_recv()
Message-ID: <20181109153332.GD1415@alphalink.fr>
References: <20181108215059.051093652@linuxfoundation.org>
 <20181108215106.791041209@linuxfoundation.org>
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On Thu, Nov 08, 2018 at 01:51:18PM -0800, Greg Kroah-Hartman wrote:
> 4.4-stable review patch.  If anyone has any objections, please let me know.
> 
This patch was partly wrong. Here is the followup fix:
94d7ee0baa8b ("l2tp: hold tunnel socket when handling control frames in l2tp_ip and l2tp_ip6")

Both a3c18422a4 ("l2tp: hold socket before dropping lock in l2tp_ip{, 6}_recv()")
and 94d7ee0baa8b ("l2tp: hold tunnel socket when handling control frames in l2tp_ip and l2tp_ip6")
should be applied/backported together.

> ------------------
> 
> [ Upstream commit a3c18422a4b4e108bcf6a2328f48867e1003fd95 ]
> 
> Socket must be held while under the protection of the l2tp lock; there
> is no guarantee that sk remains valid after the read_unlock_bh() call.
> 
> Same issue for l2tp_ip and l2tp_ip6.
> 
> Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
> Signed-off-by: David S. Miller <davem@davemloft.net>
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---
>  net/l2tp/l2tp_ip.c  | 11 ++++++-----
>  net/l2tp/l2tp_ip6.c | 11 ++++++-----
>  2 files changed, 12 insertions(+), 10 deletions(-)
> 
> diff --git a/net/l2tp/l2tp_ip.c b/net/l2tp/l2tp_ip.c
> index 48ab93842322..c7e6098c924e 100644
> --- a/net/l2tp/l2tp_ip.c
> +++ b/net/l2tp/l2tp_ip.c
> @@ -184,14 +184,15 @@ pass_up:
>  
>  		read_lock_bh(&l2tp_ip_lock);
>  		sk = __l2tp_ip_bind_lookup(net, iph->daddr, 0, tunnel_id);
> +		if (!sk) {
> +			read_unlock_bh(&l2tp_ip_lock);
> +			goto discard;
> +		}
> +
> +		sock_hold(sk);
sock_hold(sk) is missing in the 'if' branch of this condition.

>  		read_unlock_bh(&l2tp_ip_lock);
>  	}
>  
> -	if (sk == NULL)
> -		goto discard;
> -
> -	sock_hold(sk);
> -
Original sock_hold(sk) covered both parts of the condition.

>  	if (!xfrm4_policy_check(sk, XFRM_POLICY_IN, skb))
>  		goto discard_put;
>  
> diff --git a/net/l2tp/l2tp_ip6.c b/net/l2tp/l2tp_ip6.c
> index bcdab1cba773..5fe0a6f6af3d 100644
> --- a/net/l2tp/l2tp_ip6.c
> +++ b/net/l2tp/l2tp_ip6.c
> @@ -196,14 +196,15 @@ pass_up:
>  		read_lock_bh(&l2tp_ip6_lock);
>  		sk = __l2tp_ip6_bind_lookup(&init_net, &iph->daddr,
>  					    0, tunnel_id);
> +		if (!sk) {
> +			read_unlock_bh(&l2tp_ip6_lock);
> +			goto discard;
> +		}
> +
> +		sock_hold(sk);
Same here.

>  		read_unlock_bh(&l2tp_ip6_lock);
>  	}
>  
> -	if (sk == NULL)
> -		goto discard;
> -
> -	sock_hold(sk);
> -
>  	if (!xfrm6_policy_check(sk, XFRM_POLICY_IN, skb))
>  		goto discard_put;
>  
> -- 
> 2.17.1
>