netdev - Re: [PATCH bpf] flow_dissector: Fix potential use-after-free on BPF_PROG

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Date:   Wed, 21 Aug 2019 23:00:18 -0700
From:   Petar Penkov <ppenkov.kernel@...il.com>
To:     Jakub Sitnicki <jakub@...udflare.com>
Cc:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        kernel-team@...udflare.com, Petar Penkov <ppenkov@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Lorenz Bauer <lmb@...udflare.com>
Subject: Re: [PATCH bpf] flow_dissector: Fix potential use-after-free on BPF_PROG_DETACH

This makes sense, thanks!

Acked-by: Petar Penkov <ppenkov@...gle.com>

On Wed, Aug 21, 2019 at 5:19 AM Jakub Sitnicki <jakub@...udflare.com> wrote:
>
> Call to bpf_prog_put(), with help of call_rcu(), queues an RCU-callback to
> free the program once a grace period has elapsed. The callback can run
> together with new RCU readers that started after the last grace period.
> New RCU readers can potentially see the "old" to-be-freed or already-freed
> pointer to the program object before the RCU update-side NULLs it.
>
> Reorder the operations so that the RCU update-side resets the protected
> pointer before the end of the grace period after which the program will be
> freed.
>
> Fixes: d58e468b1112 ("flow_dissector: implements flow dissector BPF hook")
> Reported-by: Lorenz Bauer <lmb@...udflare.com>
> Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
> ---
>  net/core/flow_dissector.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
> index 3e6fedb57bc1..2470b4b404e6 100644
> --- a/net/core/flow_dissector.c
> +++ b/net/core/flow_dissector.c
> @@ -142,8 +142,8 @@ int skb_flow_dissector_bpf_prog_detach(const union bpf_attr *attr)
>                 mutex_unlock(&flow_dissector_mutex);
>                 return -ENOENT;
>         }
> -       bpf_prog_put(attached);
>         RCU_INIT_POINTER(net->flow_dissector_prog, NULL);
> +       bpf_prog_put(attached);
>         mutex_unlock(&flow_dissector_mutex);
>         return 0;
>  }
> --
> 2.20.1
>