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

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Date:   Thu, 21 Feb 2019 09:53:44 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Kees Cook <keescook@...omium.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     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 02/21/2019 06:31 AM, Kees Cook wrote:
> 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.

Yeah, I think it's okay to add once actually technically needed. Last
time I looked, if I recall correctly, at least Chrome installs some
heavy duty seccomp programs that go close to prog limit.

Thanks,
Daniel