Re: Is xt_owner's owner_mt() racy with sock_orphan()? [worse with new TYPESAFE_BY_RCU file lifetime?]

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On Tue, Dec 5, 2023 at 10:40 PM Phil Sutter <phil@xxxxxx> 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@xxxxxxxxxx> 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

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