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

[Eric Dumazet <eric.dumazet@gmail.com>]

On Tue, 2018-01-23 at 14:39 -0500, Neal Cardwell wrote:
> On Tue, Jan 23, 2018 at 2:20 PM, Lawrence Brakmo <brakmo@fb.com> wrote:
> > On 1/23/18, 9:30 AM, "Yuchung Cheng" <ycheng@google.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@fb.com> wrote:
> > On 1/23/18, 9:30 AM, "Yuchung Cheng" <ycheng@google.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.