netdev - Re: [PATCH][next] mptcp: Avoid NULL dereference in mptcp_getsockopt_subflow

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Date:   Mon, 20 Sep 2021 17:05:19 -0700 (PDT)
From:   Mat Martineau <mathew.j.martineau@...ux.intel.com>
To:     Tim Gardner <tim.gardner@...onical.com>
cc:     mptcp@...ts.linux.dev,
        Matthieu Baerts <matthieu.baerts@...sares.net>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] mptcp: Avoid NULL dereference in
 mptcp_getsockopt_subflow_addrs()

On Mon, 20 Sep 2021, Tim Gardner wrote:

> Coverity complains of a possible NULL dereference in
> mptcp_getsockopt_subflow_addrs():
>
> 861        } else if (sk->sk_family == AF_INET6) {
>    	3. returned_null: inet6_sk returns NULL. [show details]
>    	4. var_assigned: Assigning: np = NULL return value from inet6_sk.
> 862                const struct ipv6_pinfo *np = inet6_sk(sk);
>
> Fix this by checking for NULL.
>
> Cc: Mat Martineau <mathew.j.martineau@...ux.intel.com>
> Cc: Matthieu Baerts <matthieu.baerts@...sares.net>
> Cc: "David S. Miller" <davem@...emloft.net>
> Cc: Jakub Kicinski <kuba@...nel.org>
> Cc: netdev@...r.kernel.org
> Cc: mptcp@...ts.linux.dev
> Cc: linux-kernel@...r.kernel.org
> Signed-off-by: Tim Gardner <tim.gardner@...onical.com>
>
> [ I'm not at all sure this is the right thing to do since the final result is to
> return garbage to user space in mptcp_getsockopt_subflow_addrs(). Is this one
> of those cases where inet6_sk() can't fail ?]

Hi Tim -

Thanks for noticing this and proposing a fix.

As you commented, this isn't the right change to merge since 
mptcp_getsockopt_subflow_addrs() would copy garbage.

This block of code already checks that CONFIG_IPV6 is enabled, so the 
question is whether sk_fullsock() would return false because the subflow 
is in TCP_TIME_WAIT or TCP_NEW_SYN_RECV. The caller is iterating over 
sockets in the MPTCP socket's conn_list, which does not contain 
request_socks (so there are no sockets in the TCP_NEW_SYN_RECV state).

TCP subflow sockets are normally removed from the conn_list before they 
are closed by their parent MPTCP socket, but I need to double-check for 
corner cases. I created a github issue to track this: 
https://github.com/multipath-tcp/mptcp_net-next/issues/231


Thanks,

Mat


> ---
> net/mptcp/sockopt.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/net/mptcp/sockopt.c b/net/mptcp/sockopt.c
> index 8137cc3a4296..c89f2bedce79 100644
> --- a/net/mptcp/sockopt.c
> +++ b/net/mptcp/sockopt.c
> @@ -861,6 +861,9 @@ static void mptcp_get_sub_addrs(const struct sock *sk, struct mptcp_subflow_addr
> 	} else if (sk->sk_family == AF_INET6) {
> 		const struct ipv6_pinfo *np = inet6_sk(sk);
>
> +		if (!np)
> +			return;
> +
> 		a->sin6_local.sin6_family = AF_INET6;
> 		a->sin6_local.sin6_port = inet->inet_sport;
>
> -- 
> 2.33.0
>
>

--
Mat Martineau
Intel