6.1-stable review patch. If anyone has any objections, please let me know.
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From: Neal Cardwell ncardwell@google.com
[ Upstream commit 0ec986ed7bab6801faed1440e8839dcc710331ff ]
Loss recovery undo_retrans bookkeeping had a long-standing bug where a DSACK from a spurious TLP retransmit packet could cause an erroneous undo of a fast recovery or RTO recovery that repaired a single really-lost packet (in a sequence range outside that of the TLP retransmit). Basically, because the loss recovery state machine didn't account for the fact that it sent a TLP retransmit, the DSACK for the TLP retransmit could erroneously be implicitly be interpreted as corresponding to the normal fast recovery or RTO recovery retransmit that plugged a real hole, thus resulting in an improper undo.
For example, consider the following buggy scenario where there is a real packet loss but the congestion control response is improperly undone because of this bug:
+ send packets P1, P2, P3, P4 + P1 is really lost + send TLP retransmit of P4 + receive SACK for original P2, P3, P4 + enter fast recovery, fast-retransmit P1, increment undo_retrans to 1 + receive DSACK for TLP P4, decrement undo_retrans to 0, undo (bug!) + receive cumulative ACK for P1-P4 (fast retransmit plugged real hole)
The fix: when we initialize undo machinery in tcp_init_undo(), if there is a TLP retransmit in flight, then increment tp->undo_retrans so that we make sure that we receive a DSACK corresponding to the TLP retransmit, as well as DSACKs for all later normal retransmits, before triggering a loss recovery undo. Note that we also have to move the line that clears tp->tlp_high_seq for RTO recovery, so that upon RTO we remember the tp->tlp_high_seq value until tcp_init_undo() and clear it only afterward.
Also note that the bug dates back to the original 2013 TLP implementation, commit 6ba8a3b19e76 ("tcp: Tail loss probe (TLP)").
However, this patch will only compile and work correctly with kernels that have tp->tlp_retrans, which was added only in v5.8 in 2020 in commit 76be93fc0702 ("tcp: allow at most one TLP probe per flight"). So we associate this fix with that later commit.
Fixes: 76be93fc0702 ("tcp: allow at most one TLP probe per flight") Signed-off-by: Neal Cardwell ncardwell@google.com Reviewed-by: Eric Dumazet edumazet@google.com Cc: Yuchung Cheng ycheng@google.com Cc: Kevin Yang yyd@google.com Link: https://patch.msgid.link/20240703171246.1739561-1-ncardwell.sw@gmail.com Signed-off-by: Jakub Kicinski kuba@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- net/ipv4/tcp_input.c | 11 ++++++++++- net/ipv4/tcp_timer.c | 2 -- 2 files changed, 10 insertions(+), 3 deletions(-)
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 317cb90d77102..359ffda9b736b 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -2101,8 +2101,16 @@ void tcp_clear_retrans(struct tcp_sock *tp) static inline void tcp_init_undo(struct tcp_sock *tp) { tp->undo_marker = tp->snd_una; + /* Retransmission still in flight may cause DSACKs later. */ - tp->undo_retrans = tp->retrans_out ? : -1; + /* First, account for regular retransmits in flight: */ + tp->undo_retrans = tp->retrans_out; + /* Next, account for TLP retransmits in flight: */ + if (tp->tlp_high_seq && tp->tlp_retrans) + tp->undo_retrans++; + /* Finally, avoid 0, because undo_retrans==0 means "can undo now": */ + if (!tp->undo_retrans) + tp->undo_retrans = -1; }
static bool tcp_is_rack(const struct sock *sk) @@ -2181,6 +2189,7 @@ void tcp_enter_loss(struct sock *sk)
tcp_set_ca_state(sk, TCP_CA_Loss); tp->high_seq = tp->snd_nxt; + tp->tlp_high_seq = 0; tcp_ecn_queue_cwr(tp);
/* F-RTO RFC5682 sec 3.1 step 1: retransmit SND.UNA if no previous diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c index 44b49f7d1a9e6..f36492331ef0b 100644 --- a/net/ipv4/tcp_timer.c +++ b/net/ipv4/tcp_timer.c @@ -496,8 +496,6 @@ void tcp_retransmit_timer(struct sock *sk) if (WARN_ON_ONCE(!skb)) return;
- tp->tlp_high_seq = 0; - if (!tp->snd_wnd && !sock_flag(sk, SOCK_DEAD) && !((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV))) { /* Receiver dastardly shrinks window. Our retransmits