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

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Date: Wed, 6 Dec 2023 15:38:50 +0100
From: Jann Horn <jannh@...gle.com>
To: Christian Brauner <brauner@...nel.org>
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 Wed, Dec 6, 2023 at 2:58 PM Christian Brauner <brauner@...nel.org> wrote:
>
> 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.)

My understanding is that it could only happen due to a sock_orphan()
having happened first, and so just sk_callback_lock() should probably
be a sufficient fix. (I'm not an expert on net subsystem locking rules
though.)