netdev - Re: [PATCH bpf 2/2] net/flow_dissector: correctly cap nhoff and thoff in case of BPF

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Message-ID: <CAPhsuW5+jFS9vf3u+6DdDPVbttKCyaw6r3+Oh6e6pE4wUi8aPg@mail.gmail.com>
Date:   Thu, 6 Dec 2018 09:50:36 -0800
From:   Song Liu <liu.song.a23@...il.com>
To:     Stanislav Fomichev <sdf@...gle.com>
Cc:     Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        simon.horman@...ronome.com
Subject: Re: [PATCH bpf 2/2] net/flow_dissector: correctly cap nhoff and thoff
 in case of BPF

On Wed, Dec 5, 2018 at 8:41 PM Stanislav Fomichev <sdf@...gle.com> wrote:
>
> We want to make sure that the following condition holds:
> 0 <= nhoff <= thoff <= skb->len
>
> BPF program can set out-of-bounds nhoff and thoff, which is dangerous, see
> recent commit d0c081b49137 ("flow_dissector: properly cap thoff field")'.
>
> Signed-off-by: Stanislav Fomichev <sdf@...gle.com>

Acked-by: Song Liu <songliubraving@...com>

> ---
>  net/core/flow_dissector.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
> index ff5556d80570..af68207ee56c 100644
> --- a/net/core/flow_dissector.c
> +++ b/net/core/flow_dissector.c
> @@ -791,9 +791,12 @@ bool __skb_flow_dissect(const struct sk_buff *skb,
>                 /* Restore state */
>                 memcpy(cb, &cb_saved, sizeof(cb_saved));
>
> +               flow_keys.nhoff = clamp_t(u16, flow_keys.nhoff, 0, skb->len);
> +               flow_keys.thoff = clamp_t(u16, flow_keys.thoff,
> +                                         flow_keys.nhoff, skb->len);
> +
>                 __skb_flow_bpf_to_target(&flow_keys, flow_dissector,
>                                          target_container);
> -               key_control->thoff = min_t(u16, key_control->thoff, skb->len);
>                 rcu_read_unlock();
>                 return result == BPF_OK;
>         }
> --
> 2.20.0.rc1.387.gf8505762e3-goog
>