[PATCH net v2] tcp: reset tp->sacked_out when sack is enabled

[Lu Wei]

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@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..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)
 			err = tcp_repair_options_est(sk, optval, optlen);
 		else
 			err = -EPERM;
-- 
2.31.1