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authorNeal Cardwell <ncardwell@google.com>2021-08-10 22:40:56 -0400
committerJakub Kicinski <kuba@kernel.org>2021-08-11 15:00:15 -0700
commit6de035fec045f8ae5ee5f3a02373a18b939e91fb (patch)
tree8727e27769c4acbb768b7f3c0f1f913310c46907
parent2cad5d2ed1b47eded5a2f2372c2a94bb065a8f97 (diff)
tcp_bbr: fix u32 wrap bug in round logic if bbr_init() called after 2B packets
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>
-rw-r--r--net/ipv4/tcp_bbr.c2
1 files changed, 1 insertions, 1 deletions
diff --git a/net/ipv4/tcp_bbr.c b/net/ipv4/tcp_bbr.c
index 6ea3dc2e4219..6274462b86b4 100644
--- a/net/ipv4/tcp_bbr.c
+++ b/net/ipv4/tcp_bbr.c
@@ -1041,7 +1041,7 @@ static void bbr_init(struct sock *sk)
bbr->prior_cwnd = 0;
tp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
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;