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From:   Jann Horn <jannh@google.com>
Date:   Wed, 6 Dec 2023 22:02:04 +0100
Message-ID: <CAG48ez1ixOapt330sDoCfhnVhN0VmO=i9H8cSQontGkvi_NT7A@mail.gmail.com>
Subject: Re: Is xt_owner's owner_mt() racy with sock_orphan()? [worse with new
 TYPESAFE_BY_RCU file lifetime?]
To:     Phil Sutter <phil@nwl.cc>, Jann Horn <jannh@google.com>,
        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, Christian Brauner <brauner@kernel.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 9:42=E2=80=AFPM Phil Sutter <phil@nwl.cc> wrote:
>
> On Wed, Dec 06, 2023 at 05:28:44PM +0100, Jann Horn wrote:
> > On Tue, Dec 5, 2023 at 10:40=E2=80=AFPM Phil Sutter <phil@nwl.cc> 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>=
 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'r=
e 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 th=
e
> > > > 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 find the code pretty confusing since it combines three matches (socket
> UID, socket GID and socket existence) via binary ops. The second bail
> disregards socket existence bits, I assumed it was deliberate and thus
> decided to leave the first part as-is.
>
> > 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.
>
> Is this just annotation or do you see a practical effect of using
> READ_ONCE() there?

I mostly just meant that as an annotation. My understanding is that in
theory, racy reads can cause the compiler to do some terrible things
to your code (https://lore.kernel.org/all/CAG48ez2nFks+yN1Kp4TZisso+rjvv_4U=
W0FTo8iFUd4Qyq1qDw@mail.gmail.com/),
but that's almost certainly not going to happen here.

(Well, I guess doing a READ_ONCE() at one side without doing
WRITE_ONCE() on the other side is also unclean...)