From: Neal Cardwell <ncardwell@google.com>
Date: Wed, 11 Aug 2021 02:40:56 +0000 (-0400)
Subject: tcp_bbr: fix u32 wrap bug in round logic if bbr_init() called after 2B packets
X-Git-Url: https://git.stricted.de/?a=commitdiff_plain;h=347bebf79545e2f1c8dedd1ace02d15f3837eed8;p=GitHub%2FLineageOS%2FG12%2Fandroid_kernel_amlogic_linux-4.9.git

tcp_bbr: fix u32 wrap bug in round logic if bbr_init() called after 2B packets

[ 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>
---

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;