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

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Message-ID: <CAEivzxdfZaLD40cBKo7aqiwspwBeqeULR+RAv6jJ_wo-zV6UpQ@mail.gmail.com>
Date:   Wed, 10 May 2023 16:55:37 +0200
From:   Aleksandr Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
To:     Marcelo Ricardo Leitner <marcelo.leitner@...il.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] sctp: add bpf_bypass_getsockopt proto callback

On Wed, May 10, 2023 at 4:39 PM Marcelo Ricardo Leitner
<marcelo.leitner@...il.com> wrote:
>
> On Wed, May 10, 2023 at 03:15:27PM +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/
>
> I read some of the emails in there but I don't get why the fd leak is
> special here. I mean, I get that it leaks, but masking the error
> return like this can lead to several other problems in the application
> as well.
>
> For example, SCTP_SOCKOPT_CONNECTX3 will trigger a connect(). If it
> failed, and the hook returns success, the user app will at least log a
> wrong "connection successful".
>
> If the hook can't be responsible for cleaning up before returning a
> different value, then maybe we want to extend the list of sockopts in
> here. AFAICT these would be the 3 most critical sockopts.
>

Dear Marcelo,

Thanks for pointing this out. Initially this problem was discovered by
Christian Brauner and for SO_PEERPIDFD (a new SOL_SOCKET option that
we want to add),
after this I decided to check if we do fd_install in any other socket
options in the kernel and found that we have 2 cases in SCTP. It was
an accidental finding. :)

So, this patch isn't specific to fd_install things and probably we
should filter out bpf hook from being called for other socket options
as well.

So, I need to filter out SCTP_SOCKOPT_CONNECTX3 and
SCTP_SOCKOPT_PEELOFF* for SCTP, right?

Kind regards,
Alex