linux-kernel - Re: [PATCH net-next] sctp: add bpf_bypass

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Date:   Wed, 10 May 2023 07:31:55 -0700
From:   Stanislav Fomichev <sdf@...gle.com>
To:     Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
Cc:     nhorman@...driver.com, davem@...emloft.net,
        Daniel Borkmann <daniel@...earbox.net>,
        Christian Brauner <brauner@...nel.org>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Xin Long <lucien.xin@...il.com>, linux-sctp@...r.kernel.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] sctp: add bpf_bypass_getsockopt proto callback

On Wed, May 10, 2023 at 6:15 AM Alexander Mikhalitsyn
<aleksandr.mikhalitsyn@...onical.com> wrote:
>
> Add bpf_bypass_getsockopt proto callback and filter out
> SCTP_SOCKOPT_PEELOFF and SCTP_SOCKOPT_PEELOFF_FLAGS socket options
> from running eBPF hook on them.
>
> These options do fd_install(), and if BPF_CGROUP_RUN_PROG_GETSOCKOPT
> hook returns an error after success of the original handler
> sctp_getsockopt(...), userspace will receive an error from getsockopt
> syscall and will be not aware that fd was successfully installed into fdtable.
>
> This patch was born as a result of discussion around a new SCM_PIDFD interface:
> https://lore.kernel.org/all/20230413133355.350571-3-aleksandr.mikhalitsyn@canonical.com/
>
> Fixes: 0d01da6afc54 ("bpf: implement getsockopt and setsockopt hooks")
> Cc: Daniel Borkmann <daniel@...earbox.net>
> Cc: Christian Brauner <brauner@...nel.org>
> Cc: Stanislav Fomichev <sdf@...gle.com>
> Cc: Neil Horman <nhorman@...driver.com>
> Cc: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
> Cc: Xin Long <lucien.xin@...il.com>
> Cc: linux-sctp@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Cc: netdev@...r.kernel.org
> Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>

Acked-by: Stanislav Fomichev <sdf@...gle.com>

with a small nit below

> ---
>  net/sctp/socket.c | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
>
> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> index cda8c2874691..a9a0ababea90 100644
> --- a/net/sctp/socket.c
> +++ b/net/sctp/socket.c
> @@ -8281,6 +8281,29 @@ static int sctp_getsockopt(struct sock *sk, int level, int optname,
>         return retval;
>  }
>

[...]

> +bool sctp_bpf_bypass_getsockopt(int level, int optname)

static bool ... ? You're not making it indirect-callable, so seems
fine to keep private to this compilation unit?

> +{
> +       /*
> +        * These options do fd_install(), and if BPF_CGROUP_RUN_PROG_GETSOCKOPT
> +        * hook returns an error after success of the original handler
> +        * sctp_getsockopt(...), userspace will receive an error from getsockopt
> +        * syscall and will be not aware that fd was successfully installed into fdtable.
> +        *
> +        * Let's prevent bpf cgroup hook from running on them.
> +        */
> +       if (level == SOL_SCTP) {
> +               switch (optname) {
> +               case SCTP_SOCKOPT_PEELOFF:
> +               case SCTP_SOCKOPT_PEELOFF_FLAGS:
> +                       return true;
> +               default:
> +                       return false;
> +               }
> +       }
> +
> +       return false;
> +}
> +
>  static int sctp_hash(struct sock *sk)
>  {
>         /* STUB */
> @@ -9650,6 +9673,7 @@ struct proto sctp_prot = {
>         .shutdown    =  sctp_shutdown,
>         .setsockopt  =  sctp_setsockopt,
>         .getsockopt  =  sctp_getsockopt,
> +       .bpf_bypass_getsockopt  = sctp_bpf_bypass_getsockopt,
>         .sendmsg     =  sctp_sendmsg,
>         .recvmsg     =  sctp_recvmsg,
>         .bind        =  sctp_bind,
> @@ -9705,6 +9729,7 @@ struct proto sctpv6_prot = {
>         .shutdown       = sctp_shutdown,
>         .setsockopt     = sctp_setsockopt,
>         .getsockopt     = sctp_getsockopt,
> +       .bpf_bypass_getsockopt  = sctp_bpf_bypass_getsockopt,
>         .sendmsg        = sctp_sendmsg,
>         .recvmsg        = sctp_recvmsg,
>         .bind           = sctp_bind,
> --
> 2.34.1
>