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From:   Jann Horn <jannh@google.com>
Date:   Wed, 6 Dec 2023 15:38:50 +0100
Message-ID: <CAG48ez07dJ_=KUzRONVhMmr2koW9PwiZ5KxMHfx8ERPA=j4cUw@mail.gmail.com>
Subject: Re: Is xt_owner's owner_mt() racy with sock_orphan()? [worse with new
 TYPESAFE_BY_RCU file lifetime?]
To:     Christian Brauner <brauner@kernel.org>
Cc:     Pablo Neira Ayuso <pablo@netfilter.org>,
        Jozsef Kadlecsik <kadlec@netfilter.org>,
        Florian Westphal <fw@strlen.de>,
        netfilter-devel <netfilter-devel@vger.kernel.org>,
        coreteam@netfilter.org, "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Network Development <netdev@vger.kernel.org>,
        kernel list <linux-kernel@vger.kernel.org>
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On Wed, Dec 6, 2023 at 2:58=E2=80=AFPM Christian Brauner <brauner@kernel.or=
g> wrote:
>
> On Tue, Dec 05, 2023 at 06:08:29PM +0100, Jann Horn wrote:
> > On Tue, Dec 5, 2023 at 5:40=E2=80=AFPM Jann Horn <jannh@google.com> wro=
te:
> > >
> > > 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.)