netdev - Re: Possible read of uninitialized array values in reuseport_select

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Date:   Thu, 7 Jan 2021 16:07:39 +1100
From:   Baptiste Lepers <baptiste.lepers@...il.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: Possible read of uninitialized array values in reuseport_select_sock?

Thanks. I will submit a patch.

Baptiste.

On Thu, Jan 7, 2021 at 3:57 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> On Wed, Jan 6, 2021 at 10:54 PM Baptiste Lepers
> <baptiste.lepers@...il.com> wrote:
> >
> > Hello,
> >
> > While reading the code of net/core/sock_reuseport.c, I think I found a
> > possible race in reuseport_select_sock. The current code has the
> > following pattern:
> >
> >    socks = READ_ONCE(reuse->num_socks);
> >    smp_rmb(); // paired with reuseport_add_sock to make sure
> > reuse->socks[i < num_socks] are initialized
> >    while (reuse->socks[i]->sk_state == TCP_ESTABLISHED) {
> >         if (i >= reuse->num_socks) // should be "socks" and not
> > "reuse->num_socks"?
> >
> > The barrier seems to be here to make sure that all items of
> > reuse->socks are initialized before being read, but the barrier only
> > protects indexes < socks, not indexes < reuse->num_socks.
> >
> > I have a patch ready to fix this issue, but I wanted to confirm that
> > the current code is indeed incorrect (if the code is correct for a
> > non-obvious reason, I'd be happy to add some comments to document the
> > behavior).
> >
> >
> > Here is the diff of the patch I was planning to submit:
> >
> > diff --git a/net/core/sock_reuseport.c b/net/core/sock_reuseport.c
> > index bbdd3c7b6cb5..b065f0a103ed 100644
> > --- a/net/core/sock_reuseport.c
> > +++ b/net/core/sock_reuseport.c
> > @@ -293,7 +293,7 @@ struct sock *reuseport_select_sock(struct sock *sk,
> >              i = j = reciprocal_scale(hash, socks);
> >              while (reuse->socks[i]->sk_state == TCP_ESTABLISHED) {
> >                  i++;
> > -                if (i >= reuse->num_socks)
> > +                if (i >= socks)
> >                      i = 0;
> >                  if (i == j)
> >                      goto out;
> >
> >
> > Thanks,
> > Baptiste.
>
> Thanks for the clear description. Yes, I believe you're correct.