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From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH net-next] kcm: do not attach sockets if sk_user_data is
 already used
Date: Tue, 16 Jan 2018 14:00:52 -0500 (EST)
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From: Tom Herbert <tom@herbertland.com>
Date: Tue, 16 Jan 2018 09:36:41 -0800

> sk_user_data is set with the sk_callback lock held in code below.
> Should be able to take the lock earlier can do this check under the
> lock.

csock, and this csk, is obtained from an arbitrary one of the
process's FDs.  It can be any socket type or family, and that socket's
family might set sk_user_data without the callback lock.

The only socket type check is making sure it is not another PF_KCM
socket.  So that doesn't help with this problem.