From: Neal Cardwell <ncardwell@xxxxxxxxxx>
[ 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@xxxxxxxxxx>
Reviewed-by: Yuchung Cheng <ycheng@xxxxxxxxxx>
Reviewed-by: Kevin Yang <yyd@xxxxxxxxxx>
Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20210811024056.235161-1-ncardwell@xxxxxxxxxx
Signed-off-by: Jakub Kicinski <kuba@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
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 c22da42376fe..47f40e105044 100644
--- a/net/ipv4/tcp_bbr.c
+++ b/net/ipv4/tcp_bbr.c
@@ -811,7 +811,7 @@ 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;
--
2.30.2
