Re: [patch net v3] tcp: prohibit TCP_REPAIR_OPTIONS if data was already sent

[luwei (O)]

在 2022/11/2 10:46 PM, Neal Cardwell 写道:
> On Wed, Nov 2, 2022 at 8:23 AM Lu Wei <luwei32@xxxxxxxxxx> wrote:
> > If setsockopt with option name of TCP_REPAIR_OPTIONS and opt_code
> > of TCPOPT_SACK_PERM is called to enable sack after data is sent
> > and before data is acked, ...
> This "before data is acked" phrase does not quite seem to match the
> sequence below, AFAICT?
>
> How about something like:
>
>   If setsockopt TCP_REPAIR_OPTIONS with opt_code TCPOPT_SACK_PERM
>   is called to enable SACK after data is sent and the data sender receives a
>   dupack, ...
     yes, thanks for suggestion
> > ... it will trigger a warning in function
> > tcp_verify_left_out() as follows:
> >
> > ============================================
> > WARNING: CPU: 8 PID: 0 at net/ipv4/tcp_input.c:2132
> > tcp_timeout_mark_lost+0x154/0x160
> > tcp_enter_loss+0x2b/0x290
> > tcp_retransmit_timer+0x50b/0x640
> > tcp_write_timer_handler+0x1c8/0x340
> > tcp_write_timer+0xe5/0x140
> > call_timer_fn+0x3a/0x1b0
> > __run_timers.part.0+0x1bf/0x2d0
> > run_timer_softirq+0x43/0xb0
> > __do_softirq+0xfd/0x373
> > __irq_exit_rcu+0xf6/0x140
> >
> > The warning is caused in the following steps:
> > 1. a socket named socketA is created
> > 2. socketA enters repair mode without build a connection
> > 3. socketA calls connect() and its state is changed to TCP_ESTABLISHED
> >     directly
> > 4. socketA leaves repair mode
> > 5. socketA calls sendmsg() to send data, packets_out and sack_outs(dup
> >     ack receives) increase
> > 6. socketA enters repair mode again
> > 7. socketA calls setsockopt with TCPOPT_SACK_PERM to enable sack
> > 8. retransmit timer expires, it calls tcp_timeout_mark_lost(), lost_out
> >     increases
> > 9. sack_outs + lost_out > packets_out triggers since lost_out and
> >     sack_outs increase repeatly
> >
> > In function tcp_timeout_mark_lost(), tp->sacked_out will be cleared if
> > Step7 not happen and the warning will not be triggered. As suggested by
> > Denis and Eric, TCP_REPAIR_OPTIONS should be prohibited if data was
> > already sent. So this patch checks tp->segs_out, only TCP_REPAIR_OPTIONS
> > can be set only if tp->segs_out is 0.
> >
> > socket-tcp tests in CRIU has been tested as follows:
> > $ sudo ./test/zdtm.py run -t zdtm/static/socket-tcp*  --keep-going \
> >         --ignore-taint
> >
> > socket-tcp* represent all socket-tcp tests in test/zdtm/static/.
> >
> > Fixes: b139ba4e90dc ("tcp: Repair connection-time negotiated parameters")
> > Signed-off-by: Lu Wei <luwei32@xxxxxxxxxx>
> > ---
> >   net/ipv4/tcp.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> > index ef14efa1fb70..1f5cc32cf0cc 100644
> > --- a/net/ipv4/tcp.c
> > +++ b/net/ipv4/tcp.c
> > @@ -3647,7 +3647,7 @@ int do_tcp_setsockopt(struct sock *sk, int level, int optname,
> >          case TCP_REPAIR_OPTIONS:
> >                  if (!tp->repair)
> >                          err = -EINVAL;
> > -               else if (sk->sk_state == TCP_ESTABLISHED)
> > +               else if (sk->sk_state == TCP_ESTABLISHED && !tp->segs_out)
> The tp->segs_out field is only 32 bits wide. By my math, at 200
> Gbit/sec with 1500 byte MTU it can wrap roughly every 260 secs. So a
> caller could get unlucky or carefully sequence its call to
> TCP_REPAIR_OPTIONS (based on packets sent so far) to mess up the
> accounting and trigger the kernel warning.
>
> How about using some other method to determine if this is safe?
> Perhaps using tp->bytes_sent, which is a 64-bit field, which by my
> math would take 23 years to wrap at 200 Gbit/sec?
>
> If we're more paranoid about wrapping we could also check
> tp->packets_out, and refuse to allow TCP_REPAIR_OPTIONS if either
> tp->bytes_sent or tp->packets_out are non-zero. (Or if we're even more
> paranoid I suppose we could have a special new bit to track whether
> we've ever sent something, but that probably seems like overkill?)
>
> neal
> .

I didn't notice that u32 will be easily wrapped in huge network throughput,
thank you neal.

But tcp->packets_out shoud not be used because tp->packets_out can decrease
when expected ack is received, so it can decrease to 0 and this is the common
condition.

--
Best Regards,
Lu Wei