From: Neal Cardwell ncardwell@google.com
[ Upstream commit 686dc2db2a0fdc1d34b424ec2c0a735becd8d62b ]
Fix a bug reported and analyzed by Nagaraj Arankal, where the handling of a spurious non-SACK RTO could cause a connection to fail to clear retrans_stamp, causing a later RTO to very prematurely time out the connection with ETIMEDOUT.
Here is the buggy scenario, expanding upon Nagaraj Arankal's excellent report:
(*1) Send one data packet on a non-SACK connection
(*2) Because no ACK packet is received, the packet is retransmitted and we enter CA_Loss; but this retransmission is spurious.
(*3) The ACK for the original data is received. The transmitted packet is acknowledged. The TCP timestamp is before the retrans_stamp, so tcp_may_undo() returns true, and tcp_try_undo_loss() returns true without changing state to Open (because tcp_is_sack() is false), and tcp_process_loss() returns without calling tcp_try_undo_recovery(). Normally after undoing a CA_Loss episode, tcp_fastretrans_alert() would see that the connection has returned to CA_Open and fall through and call tcp_try_to_open(), which would set retrans_stamp to 0. However, for non-SACK connections we hold the connection in CA_Loss, so do not fall through to call tcp_try_to_open() and do not set retrans_stamp to 0. So retrans_stamp is (erroneously) still non-zero.
At this point the first "retransmission event" has passed and been recovered from. Any future retransmission is a completely new "event". However, retrans_stamp is erroneously still set. (And we are still in CA_Loss, which is correct.)
(*4) After 16 minutes (to correspond with tcp_retries2=15), a new data packet is sent. Note: No data is transmitted between (*3) and (*4) and we disabled keep alives.
The socket's timeout SHOULD be calculated from this point in time, but instead it's calculated from the prior "event" 16 minutes ago (step (*2)).
(*5) Because no ACK packet is received, the packet is retransmitted.
(*6) At the time of the 2nd retransmission, the socket returns ETIMEDOUT, prematurely, because retrans_stamp is (erroneously) too far in the past (set at the time of (*2)).
This commit fixes this bug by ensuring that we reuse in tcp_try_undo_loss() the same careful logic for non-SACK connections that we have in tcp_try_undo_recovery(). To avoid duplicating logic, we factor out that logic into a new tcp_is_non_sack_preventing_reopen() helper and call that helper from both undo functions.
Fixes: da34ac7626b5 ("tcp: only undo on partial ACKs in CA_Loss") Reported-by: Nagaraj Arankal nagaraj.p.arankal@hpe.com Link: https://lore.kernel.org/all/SJ0PR84MB1847BE6C24D274C46A1B9B0EB27A9@SJ0PR84MB... Signed-off-by: Neal Cardwell ncardwell@google.com Signed-off-by: Yuchung Cheng ycheng@google.com Reviewed-by: Eric Dumazet edumazet@google.com Link: https://lore.kernel.org/r/20220903121023.866900-1-ncardwell.kernel@gmail.com Signed-off-by: Paolo Abeni pabeni@redhat.com Signed-off-by: Sasha Levin sashal@kernel.org --- net/ipv4/tcp_input.c | 25 ++++++++++++++++++------- 1 file changed, 18 insertions(+), 7 deletions(-)
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index e62500d6fe0d0..4ecd85b1e806c 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -2496,6 +2496,21 @@ static inline bool tcp_may_undo(const struct tcp_sock *tp) return tp->undo_marker && (!tp->undo_retrans || tcp_packet_delayed(tp)); }
+static bool tcp_is_non_sack_preventing_reopen(struct sock *sk) +{ + struct tcp_sock *tp = tcp_sk(sk); + + if (tp->snd_una == tp->high_seq && tcp_is_reno(tp)) { + /* Hold old state until something *above* high_seq + * is ACKed. For Reno it is MUST to prevent false + * fast retransmits (RFC2582). SACK TCP is safe. */ + if (!tcp_any_retrans_done(sk)) + tp->retrans_stamp = 0; + return true; + } + return false; +} + /* People celebrate: "We love our President!" */ static bool tcp_try_undo_recovery(struct sock *sk) { @@ -2518,14 +2533,8 @@ static bool tcp_try_undo_recovery(struct sock *sk) } else if (tp->rack.reo_wnd_persist) { tp->rack.reo_wnd_persist--; } - if (tp->snd_una == tp->high_seq && tcp_is_reno(tp)) { - /* Hold old state until something *above* high_seq - * is ACKed. For Reno it is MUST to prevent false - * fast retransmits (RFC2582). SACK TCP is safe. */ - if (!tcp_any_retrans_done(sk)) - tp->retrans_stamp = 0; + if (tcp_is_non_sack_preventing_reopen(sk)) return true; - } tcp_set_ca_state(sk, TCP_CA_Open); tp->is_sack_reneg = 0; return false; @@ -2561,6 +2570,8 @@ static bool tcp_try_undo_loss(struct sock *sk, bool frto_undo) NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSPURIOUSRTOS); inet_csk(sk)->icsk_retransmits = 0; + if (tcp_is_non_sack_preventing_reopen(sk)) + return true; if (frto_undo || tcp_is_sack(tp)) { tcp_set_ca_state(sk, TCP_CA_Open); tp->is_sack_reneg = 0;