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 17:49:51 +0100
From: Christian Brauner <brauner@...nel.org>
To: Jann Horn <jannh@...gle.com>
Cc: Phil Sutter <phil@....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 06, 2023 at 05:28:44PM +0100, Jann Horn wrote:
> On Tue, Dec 5, 2023 at 10:40 PM Phil Sutter <phil@....cc> 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?
> >
> > Sounds reasonable, although I wonder how likely a socket is to
> > orphan while netfilter is processing a packet it just sent.
> >
> > How about the attached patch? Not sure what hash to put into a Fixes:
> > tag given this is a day 1 bug and ipt_owner/ip6t_owner predate git.
> 
> Looks mostly reasonable to me; though I guess it's a bit weird to have
> two separate bailout paths for checking whether sk->sk_socket is NULL,
> where the first check can race, and the second check uses different
> logic for determining the return value; I don't know whether that
> actually matters semantically. But I'm not sure how to make it look
> nicer either.
> I guess you could add a READ_ONCE() around the first read to signal
> that that's a potentially racy read, but I don't feel strongly about
> that.

It should be possible to split it into two static inlin helpers:

owner_mt_fast()
owner_mt_slow()

And then abstract the lockless and locked fetches into the two helpers.