netdev - Re: [PATCH] net/l2tp: Fix reference count leak in l2tp_udp_recv

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Date:   Thu, 9 Sep 2021 10:01:56 +0100
From:   Tom Parkin <tparkin@...alix.com>
To:     Xiyu Yang <xiyuyang19@...an.edu.cn>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Cong Wang <cong.wang@...edance.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Xin Xiong <xiongx18@...an.edu.cn>,
        "Gong, Sishuai" <sishuai@...due.edu>,
        Matthias Schiffer <mschiffer@...verse-factory.net>,
        Bhaskar Chowdhury <unixbhaskar@...il.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        yuanxzhang@...an.edu.cn, Xin Tan <tanxin.ctf@...il.com>
Subject: Re: [PATCH] net/l2tp: Fix reference count leak in l2tp_udp_recv_core

On  Thu, Sep 09, 2021 at 12:32:00 +0800, Xiyu Yang wrote:
> The reference count leak issue may take place in an error handling
> path. If both conditions of tunnel->version == L2TP_HDR_VER_3 and the
> return value of l2tp_v3_ensure_opt_in_linear is nonzero, the function
> would directly jump to label invalid, without decrementing the reference
> count of the l2tp_session object session increased earlier by
> l2tp_tunnel_get_session(). This may result in refcount leaks.

I agree with your analysis.  Thanks for catching this!

> 
> Fix this issue by decrease the reference count before jumping to the
> label invalid.
> 
> Signed-off-by: Xiyu Yang <xiyuyang19@...an.edu.cn>
> Signed-off-by: Xin Xiong <xiongx18@...an.edu.cn>
> Signed-off-by: Xin Tan <tanxin.ctf@...il.com>
> ---
>  net/l2tp/l2tp_core.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
> index 53486b162f01..93271a2632b8 100644
> --- a/net/l2tp/l2tp_core.c
> +++ b/net/l2tp/l2tp_core.c
> @@ -869,8 +869,10 @@ static int l2tp_udp_recv_core(struct l2tp_tunnel *tunnel, struct sk_buff *skb)
>  	}
>  
>  	if (tunnel->version == L2TP_HDR_VER_3 &&
> -	    l2tp_v3_ensure_opt_in_linear(session, skb, &ptr, &optr))
> +	    l2tp_v3_ensure_opt_in_linear(session, skb, &ptr, &optr)) {
> +		l2tp_session_dec_refcount(session);
>  		goto invalid;
> +	}

The error paths in l2tp_udp_recv_core are a bit convoluted because of
the check (!session || !session->recv_skb) which may or may not need
to drop a session reference if triggered.

I think it could be simplified since session->recv_skb is always set
for all the current session types in the tree, but doing that is probably
a little patch series on its own.

>  
>  	l2tp_recv_common(session, skb, ptr, optr, hdrflags, length);
>  	l2tp_session_dec_refcount(session);
> -- 
> 2.7.4
> 

-- 
Tom Parkin
Katalix Systems Ltd
https://katalix.com
Catalysts for your Embedded Linux software development

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