netdev - Re: [PATCH v2 bpf-next 4/5] bpf: tcp: Stop bpf_setsockopt(TCP

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Date:   Wed, 28 Sep 2022 19:04:51 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Martin KaFai Lau <kafai@...com>
Cc:     bpf <bpf@...r.kernel.org>, netdev <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        kernel-team <kernel-team@...com>,
        Paolo Abeni <pabeni@...hat.com>,
        Martin KaFai Lau <martin.lau@...nel.org>
Subject: Re: [PATCH v2 bpf-next 4/5] bpf: tcp: Stop bpf_setsockopt(TCP_CONGESTION)
 in init ops to recur itself

On Fri, Sep 23, 2022 at 3:48 PM Martin KaFai Lau <kafai@...com> wrote:
>
> From: Martin KaFai Lau <martin.lau@...nel.org>
>
> When a bad bpf prog '.init' calls
> bpf_setsockopt(TCP_CONGESTION, "itself"), it will trigger this loop:
>
> .init => bpf_setsockopt(tcp_cc) => .init => bpf_setsockopt(tcp_cc) ...
> ... => .init => bpf_setsockopt(tcp_cc).
>
> It was prevented by the prog->active counter before but the prog->active
> detection cannot be used in struct_ops as explained in the earlier
> patch of the set.
>
> In this patch, the second bpf_setsockopt(tcp_cc) is not allowed
> in order to break the loop.  This is done by using a bit of
> an existing 1 byte hole in tcp_sock to check if there is
> on-going bpf_setsockopt(TCP_CONGESTION) in this tcp_sock.
>
> Note that this essentially limits only the first '.init' can
> call bpf_setsockopt(TCP_CONGESTION) to pick a fallback cc (eg. peer
> does not support ECN) and the second '.init' cannot fallback to
> another cc.  This applies even the second
> bpf_setsockopt(TCP_CONGESTION) will not cause a loop.
>
> Signed-off-by: Martin KaFai Lau <martin.lau@...nel.org>
> ---
>  include/linux/tcp.h |  6 ++++++
>  net/core/filter.c   | 28 +++++++++++++++++++++++++++-
>  2 files changed, 33 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/tcp.h b/include/linux/tcp.h
> index a9fbe22732c3..3bdf687e2fb3 100644
> --- a/include/linux/tcp.h
> +++ b/include/linux/tcp.h
> @@ -388,6 +388,12 @@ struct tcp_sock {
>         u8      bpf_sock_ops_cb_flags;  /* Control calling BPF programs
>                                          * values defined in uapi/linux/tcp.h
>                                          */
> +       u8      bpf_chg_cc_inprogress:1; /* In the middle of
> +                                         * bpf_setsockopt(TCP_CONGESTION),
> +                                         * it is to avoid the bpf_tcp_cc->init()
> +                                         * to recur itself by calling
> +                                         * bpf_setsockopt(TCP_CONGESTION, "itself").
> +                                         */
>  #define BPF_SOCK_OPS_TEST_FLAG(TP, ARG) (TP->bpf_sock_ops_cb_flags & ARG)
>  #else
>  #define BPF_SOCK_OPS_TEST_FLAG(TP, ARG) 0
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 96f2f7a65e65..ac4c45c02da5 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -5105,6 +5105,9 @@ static int bpf_sol_tcp_setsockopt(struct sock *sk, int optname,
>  static int sol_tcp_sockopt_congestion(struct sock *sk, char *optval,
>                                       int *optlen, bool getopt)
>  {
> +       struct tcp_sock *tp;
> +       int ret;
> +
>         if (*optlen < 2)
>                 return -EINVAL;
>
> @@ -5125,8 +5128,31 @@ static int sol_tcp_sockopt_congestion(struct sock *sk, char *optval,
>         if (*optlen >= sizeof("cdg") - 1 && !strncmp("cdg", optval, *optlen))
>                 return -ENOTSUPP;
>
> -       return do_tcp_setsockopt(sk, SOL_TCP, TCP_CONGESTION,
> +       /* It stops this looping
> +        *
> +        * .init => bpf_setsockopt(tcp_cc) => .init =>
> +        * bpf_setsockopt(tcp_cc)" => .init => ....
> +        *
> +        * The second bpf_setsockopt(tcp_cc) is not allowed
> +        * in order to break the loop when both .init
> +        * are the same bpf prog.
> +        *
> +        * This applies even the second bpf_setsockopt(tcp_cc)
> +        * does not cause a loop.  This limits only the first
> +        * '.init' can call bpf_setsockopt(TCP_CONGESTION) to
> +        * pick a fallback cc (eg. peer does not support ECN)
> +        * and the second '.init' cannot fallback to
> +        * another.
> +        */
> +       tp = tcp_sk(sk);
> +       if (tp->bpf_chg_cc_inprogress)
> +               return -EBUSY;
> +

Is the socket locked (and owned by current thread) at this point ?
If not, changing bpf_chg_cc_inprogress would be racy.


> +       tp->bpf_chg_cc_inprogress = 1;
> +       ret = do_tcp_setsockopt(sk, SOL_TCP, TCP_CONGESTION,
>                                 KERNEL_SOCKPTR(optval), *optlen);
> +       tp->bpf_chg_cc_inprogress = 0;
> +       return ret;
>  }
>
>  static int sol_tcp_sockopt(struct sock *sk, int optname,
> --
> 2.30.2
>