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

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Date:   Wed, 6 Jan 2021 23:56:59 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Baptiste Lepers <baptiste.lepers@...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?

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.