linux-kernel - Re: Is xt_owner's owner_mt() racy with sock_orphan()? [worse with new TYPESAFE_BY

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Message-ID: <20231206-refinanzieren-werkhalle-22db5334f256@brauner>
Date:   Wed, 6 Dec 2023 14:58:36 +0100
From:   Christian Brauner <brauner@...nel.org>
To:     Jann Horn <jannh@...gle.com>
Cc:     Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...filter.org>,
        Florian Westphal <fw@...len.de>,
        netfilter-devel <netfilter-devel@...r.kernel.org>,
        coreteam@...filter.org, "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Network Development <netdev@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: Is xt_owner's owner_mt() racy with sock_orphan()? [worse with
 new TYPESAFE_BY_RCU file lifetime?]

On Tue, Dec 05, 2023 at 06:08:29PM +0100, Jann Horn wrote:
> On Tue, Dec 5, 2023 at 5:40 PM Jann Horn <jannh@...gle.com> wrote:
> >
> > Hi!
> >
> > I think this code is racy, but testing that seems like a pain...
> >
> > owner_mt() in xt_owner runs in context of a NF_INET_LOCAL_OUT or
> > NF_INET_POST_ROUTING hook. It first checks that sk->sk_socket is
> > non-NULL, then checks that sk->sk_socket->file is non-NULL, then
> > accesses the ->f_cred of that file.
> >
> > I don't see anything that protects this against a concurrent
> > sock_orphan(), which NULLs out the sk->sk_socket pointer, if we're in
> 
> Ah, and all the other users of ->sk_socket in net/netfilter/ do it
> under the sk_callback_lock... so I guess the fix would be to add the
> same in owner_mt?

In your other mail you wrote:

> I also think we have no guarantee here that the socket's ->file won't
> go away due to a concurrent __sock_release(), which could cause us to
> continue reading file credentials out of a file whose refcount has
> already dropped to zero?

Is this an independent worry or can the concurrent __sock_release()
issue only happen due to a sock_orphan() having happened first? I think
that it requires a sock_orphan() having happend, presumably because the
socket gets marked SOCK_DEAD and can thus be released via
__sock_release() asynchronously?

If so then taking sk_callback_lock() in owner_mt() should fix this.
(Otherwise we might need an additional get_active_file() on
sk->sk_socker->file in owner_mt() in addition to the other fix.)