netdev - Re: [PATCH bpf-next v2] bpf, seccomp: fix false positive preemption splat for cbpf->ebpf progs

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Message-ID: <CAGXu5jLHV78-d+_yTKVsgawDAVn7FXZVCcvhtDesXcxhZf_NKg@mail.gmail.com>
Date:   Wed, 20 Feb 2019 21:31:16 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Jann Horn <jannh@...gle.com>,
        Andy Lutomirski <luto@...capital.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2] bpf, seccomp: fix false positive preemption
 splat for cbpf->ebpf progs

On Wed, Feb 20, 2019 at 8:03 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Wed, Feb 20, 2019 at 3:59 PM Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
> >
> > On Thu, Feb 21, 2019 at 12:01:35AM +0100, Daniel Borkmann wrote:
> > > In 568f196756ad ("bpf: check that BPF programs run with preemption disabled")
> > > a check was added for BPF_PROG_RUN() that for every invocation preemption is
> > > disabled to not break eBPF assumptions (e.g. per-cpu map). Of course this does
> > > not count for seccomp because only cBPF -> eBPF is loaded here and it does
> > > not make use of any functionality that would require this assertion. Fix this
> > > false positive by adding and using SECCOMP_RUN() variant that does not have
> > > the cant_sleep(); check.
> > >
> > > Fixes: 568f196756ad ("bpf: check that BPF programs run with preemption disabled")
> > > Reported-by: syzbot+8bf19ee2aa580de7a2a7@...kaller.appspotmail.com
> > > Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> > > Acked-by: Kees Cook <keescook@...omium.org>
> >
> > Applied, Thanks
>
> Actually I think it's a wrong approach to go long term.
> I'm thinking to revert it.
> I think it's better to disable preemption for duration of
> seccomp cbpf prog.
> It's short and there is really no reason for it to be preemptible.
> When seccomp switches to ebpf we'll have this weird inconsistency.
> Let's just disable preemption for seccomp as well.

A lot of changes will be needed for seccomp ebpf -- not the least of
which is convincing me there is a use-case. ;)

But the main issue is that I'm not a huge fan of dropping two
barriers() across syscall entry. That seems pretty heavy-duty for
something that is literally not needed right now.

-- 
Kees Cook