netdev - Re: [PATCH net] bpf: always re-init the congestion control after switching to it

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Message-ID: <20180123232527.7w77zfkub2elj4vs@ast-mbp>
Date:   Tue, 23 Jan 2018 15:25:29 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Lawrence Brakmo <brakmo@...com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "soheil@...gle.com" <soheil@...gle.com>
Subject: Re: [PATCH net] bpf: always re-init the congestion control after
 switching to it

On Tue, Jan 23, 2018 at 08:19:54PM +0000, Lawrence Brakmo wrote:
> On 1/23/18, 11:50 AM, "Eric Dumazet" <eric.dumazet@...il.com> wrote:
> 
>     On Tue, 2018-01-23 at 14:39 -0500, Neal Cardwell wrote:
>     > On Tue, Jan 23, 2018 at 2:20 PM, Lawrence Brakmo <brakmo@...com> wrote:
>     > > On 1/23/18, 9:30 AM, "Yuchung Cheng" <ycheng@...gle.com> wrote:
>     > > 
>     > >     The original patch that changes TCP's congestion control via eBPF only
>     > >     re-initializes the new congestion control, if the BPF op is set to an
>     > >     (invalid) value beyond BPF_SOCK_OPS_NEEDS_ECN. Consequently TCP will
>     > > 
>     > > What do you mean by “(invalid) value”?
>     > > 
>     > >     run the new congestion control from random states.
>     > > 
>     > > This has always been possible with setsockopt, no?
>     > > 
>     > >    This patch fixes
>     > >     the issue by always re-init the congestion control like other means
>     > >     such as setsockopt and sysctl changes.
>     > > 
>     > > The current code re-inits the congestion control when calling
>     > > tcp_set_congestion_control except when it is called early on (i.e. op <=
>     > > BPF_SOCK_OPS_NEEDS_ECN). In that case there is no need to re-initialize
>     > > since it will be initialized later by TCP when the connection is established.
>     > > 
>     > > Otherwise, if we always call tcp_reinit_congestion_control we would call
>     > > tcp_cleanup_congestion_control when the congestion control has not been
>     > > initialized yet.
>     > 
>     > On Tue, Jan 23, 2018 at 2:20 PM, Lawrence Brakmo <brakmo@...com> wrote:
>     > > On 1/23/18, 9:30 AM, "Yuchung Cheng" <ycheng@...gle.com> wrote:
>     > > 
>     > >     The original patch that changes TCP's congestion control via eBPF only
>     > >     re-initializes the new congestion control, if the BPF op is set to an
>     > >     (invalid) value beyond BPF_SOCK_OPS_NEEDS_ECN. Consequently TCP will
>     > > 
>     > > What do you mean by “(invalid) value”?
>     > > 
>     > >     run the new congestion control from random states.
>     > > 
>     > > This has always been possible with setsockopt, no?
>     > > 
>     > >    This patch fixes
>     > >     the issue by always re-init the congestion control like other means
>     > >     such as setsockopt and sysctl changes.
>     > > 
>     > > The current code re-inits the congestion control when calling
>     > > tcp_set_congestion_control except when it is called early on (i.e. op <=
>     > > BPF_SOCK_OPS_NEEDS_ECN). In that case there is no need to re-initialize
>     > > since it will be initialized later by TCP when the connection is established.
>     > > 
>     > > Otherwise, if we always call tcp_reinit_congestion_control we would call
>     > > tcp_cleanup_congestion_control when the congestion control has not been
>     > > initialized yet.
>     > 
>     > Interesting. So I wonder if the symptoms we were seeing were due to
>     > kernels that did not yet have this fix:
>     > 
>     >   27204aaa9dc6 ("tcp: uniform the set up of sockets after successful
>     > connection):
>     >   https://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next.git/commit/?id=27204aaa9dc67b833b77179fdac556288ec3a4bf
>     > 
>     > Before that fix, there could be TFO passive connections that at SYN time called:
>     >   tcp_init_congestion_control(child);
>     > and then:
>     >   tcp_call_bpf(child, BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB);
>     > 
>     > So that if the CC was switched in the
>     > BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB handler then there would be no
>     > init for the new module?
>     
>     
>     Note that bpf_sock->op can be written by a malicious BPF filter.
>     
>     So, a malicious filter can switch from Cubic to BBR without re-init,
>     and bad things can happen.
>     
>     I do not believe we should trust BPF here.
>     
> Very good point Eric. One solution would be to make bpf_sock->op not writeable by
> the BPF program. 
> 
> Neal, you are correct that would have been a problem. I leave it up to you guys whether
> making bpf_sock->op not writeable by BPF program is enough or if it is safer to always
> re-init (as long as there is no problem calling tcp_cleanup_congestion_control when it
> has not been initialized.

I think allowing write into 'op' and 'replylong' was a mistake.
Only 'reply' field is used by tcp_call_bpf().
No reason to let programs write into other fields.
I think we have to fix it now before programs start to rely
on this undefined behavior.