[MPTCP] Re: [PATCH] mptcp: Protect subflow socket options before connection completes

[Florian Westphal <fw at strlen.de>]

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Mat Martineau <mathew.j.martineau(a)linux.intel.com> wrote:
> Userspace should not be able to directly manipulate subflow socket
> options before a connection is established since it is not yet known if
> it will be an MPTCP subflow or a TCP fallback subflow. TCP fallback
> subflows can be more directly controlled by userspace because they are
> regular TCP connections, while MPTCP subflow sockets need to be
> configured for the specific needs of MPTCP. Use the same logic as
> sendmsg/recvmsg to ensure that socket option calls are only passed
> through to known TCP fallback subflows.
> 
> Signed-off-by: Mat Martineau <mathew.j.martineau(a)linux.intel.com>
> ---
> 
> One other question: is it racy to have __mptcp_tcp_fallback() return an
> unlocked msk, then do the sock_hold(ssk) from an unlocked state?

Yes.

> It seems like if holding the ssk reference count is necessary at all it
> should be done while the msk is still locked. I'm considering a change
> to __mptcp_tcp_fallback() to have it not release the msk lock and
> instead have all the callers do that.

Yes, that seems like the right thing to do.