From: Neal Cardwell ncardwell@google.com
[ Upstream commit 6de035fec045f8ae5ee5f3a02373a18b939e91fb ]
Currently if BBR congestion control is initialized after more than 2B packets have been delivered, depending on the phase of the tp->delivered counter the tracking of BBR round trips can get stuck.
The bug arises because if tp->delivered is between 2^31 and 2^32 at the time the BBR congestion control module is initialized, then the initialization of bbr->next_rtt_delivered to 0 will cause the logic to believe that the end of the round trip is still billions of packets in the future. More specifically, the following check will fail repeatedly:
!before(rs->prior_delivered, bbr->next_rtt_delivered)
and thus the connection will take up to 2B packets delivered before that check will pass and the connection will set:
bbr->round_start = 1;
This could cause many mechanisms in BBR to fail to trigger, for example bbr_check_full_bw_reached() would likely never exit STARTUP.
This bug is 5 years old and has not been observed, and as a practical matter this would likely rarely trigger, since it would require transferring at least 2B packets, or likely more than 3 terabytes of data, before switching congestion control algorithms to BBR.
This patch is a stable candidate for kernels as far back as v4.9, when tcp_bbr.c was added.
Fixes: 0f8782ea1497 ("tcp_bbr: add BBR congestion control") Signed-off-by: Neal Cardwell ncardwell@google.com Reviewed-by: Yuchung Cheng ycheng@google.com Reviewed-by: Kevin Yang yyd@google.com Reviewed-by: Eric Dumazet edumazet@google.com Link: https://lore.kernel.org/r/20210811024056.235161-1-ncardwell@google.com Signed-off-by: Jakub Kicinski kuba@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- net/ipv4/tcp_bbr.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/net/ipv4/tcp_bbr.c b/net/ipv4/tcp_bbr.c index bda10f7aea32..76a9652d90f2 100644 --- a/net/ipv4/tcp_bbr.c +++ b/net/ipv4/tcp_bbr.c @@ -840,7 +840,7 @@ static void bbr_init(struct sock *sk) bbr->prior_cwnd = 0; bbr->tso_segs_goal = 0; /* default segs per skb until first ACK */ bbr->rtt_cnt = 0; - bbr->next_rtt_delivered = 0; + bbr->next_rtt_delivered = tp->delivered; bbr->prev_ca_state = TCP_CA_Open; bbr->packet_conservation = 0;