netdev - Re: [PATCH net v2] tcp: reset tp->sacked

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Date:   Thu, 27 Oct 2022 04:18:30 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Pavel Emelyanov <ovzxemul@...il.com>
Cc:     Lu Wei <luwei32@...wei.com>, davem@...emloft.net,
        yoshfuji@...ux-ipv6.org, dsahern@...nel.org, kuba@...nel.org,
        pabeni@...hat.com, Pavel Emelyanov <xemul@...allels.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Pavel Tikhomirov <ptikhomirov@...tuozzo.com>, den@...tuozzo.com
Subject: Re: [PATCH net v2] tcp: reset tp->sacked_out when sack is enabled

On Thu, Oct 27, 2022 at 4:14 AM Pavel Emelyanov <ovzxemul@...il.com> wrote:
>
>
>
> чт, 27 окт. 2022 г., 14:08 Eric Dumazet <edumazet@...gle.com>:
>>
>> On Thu, Oct 27, 2022 at 3:45 AM Lu Wei <luwei32@...wei.com> 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, 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
>> >
>> > This warning occurs in several steps:
>> > Step1. If sack is not enabled, when server receives dup-ack,
>> >        it calls tcp_add_reno_sack() to increase tp->sacked_out.
>> >
>> > Step2. Setsockopt() is called to enable sack
>> >
>> > Step3. The retransmit timer expires, it calls tcp_timeout_mark_lost()
>> >        to increase tp->lost_out but not clear tp->sacked_out because
>> >        sack is enabled and tcp_is_reno() is false.
>> >
>> > So tp->left_out is increased repeatly in Step1 and Step3 and it is
>> > greater than tp->packets_out and trigger the warning. In function
>> > tcp_timeout_mark_lost(), tp->sacked_out will be cleared if Step2 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.
>> >
>> > 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@...wei.com>
>> > ---
>> >  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..ef876e70f7fe 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->packets_out)
>>
>> You keep focusing on packets_out  :/
>>
>> What I said was : TCP_REPAIR_OPTIONS must be denied if any packets
>> have been sent (and possibly already ACK)
>
>
>
> If repair mode is ON, why does socket send any packets? This shouldn't happen from my perspective.

Exactly.

TCP_REPAIR is easily abused by fuzzers.

We need to enforce sanity rules in the kernel, not assuming user space
is following the CRIU way of doing things.

>
> -- Pavel
>
>>
>> Looking at tp->packets_out alone is not sufficient.
>>
>> >                         err = tcp_repair_options_est(sk, optval, optlen);
>> >                 else
>> >                         err = -EPERM;
>> > --
>> > 2.31.1
>> >