linux-kernel - Re: [PATCH net-next v2] sctp: add bpf_bypass

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Date:   Thu, 11 May 2023 10:12:09 -0300
From:   Marcelo Ricardo Leitner <marcelo.leitner@...il.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>,
        Stanislav Fomichev <sdf@...gle.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 v2] sctp: add bpf_bypass_getsockopt proto
 callback

Hi,

Two things:

On Thu, May 11, 2023 at 02:31:48PM +0200, Alexander Mikhalitsyn 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/

Cool, but the description is mentioning the CONNECTX3 sockopt.

> 
> 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
> Suggested-by: Stanislav Fomichev <sdf@...gle.com>
> Acked-by: Stanislav Fomichev <sdf@...gle.com>
> Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
> ---
>  net/sctp/socket.c | 31 +++++++++++++++++++++++++++++++
>  1 file changed, 31 insertions(+)
> 
> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> index cda8c2874691..a211a203003c 100644
> --- a/net/sctp/socket.c
> +++ b/net/sctp/socket.c
> @@ -8281,6 +8281,35 @@ static int sctp_getsockopt(struct sock *sk, int level, int optname,
>  	return retval;
>  }
>  
> +static bool sctp_bpf_bypass_getsockopt(int level, int optname)
> +{
> +	if (level == SOL_SCTP) {
> +		switch (optname) {
> +		/*
> +		 * 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.
> +		 */

This and..

> +		case SCTP_SOCKOPT_PEELOFF:
> +		case SCTP_SOCKOPT_PEELOFF_FLAGS:
> +		/*
> +		 * As pointed by Marcelo Ricardo Leitner it seems reasonable to skip
> +		 * bpf getsockopt hook for this sockopt too. Because internaly, it
> +		 * triggers connect() and if error will be masked userspace can be confused.
> +		 */

..this comments can be removed, as they are easily visible on the
description later on for who is interested on why such lines were
added.

Thanks,
Marcelo

> +		case SCTP_SOCKOPT_CONNECTX3:
> +			return true;
> +		default:
> +			return false;
> +		}
> +	}
> +
> +	return false;
> +}
> +
>  static int sctp_hash(struct sock *sk)
>  {
>  	/* STUB */
> @@ -9650,6 +9679,7 @@ 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 +9735,7 @@ 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
>