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SubjectRe: [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@fudan.edu.cn>
> Signed-off-by: Xin Xiong <xiongx18@fudan.edu.cn>
> Signed-off-by: Xin Tan <tanxin.ctf@gmail.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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