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Neal Cardwell 5dfe9d2739 tcp: fix tcp_rcv_fastopen_synack() to enter TCP_CA_Loss for failed TFO 
Testing determined that the recent commit 9e046bb111 ("tcp: clear
tp->retrans_stamp in tcp_rcv_fastopen_synack()") has a race, and does
not always ensure retrans_stamp is 0 after a TFO payload retransmit.

If transmit completion for the SYN+data skb happens after the client
TCP stack receives the SYNACK (which sometimes happens), then
retrans_stamp can erroneously remain non-zero for the lifetime of the
connection, causing a premature ETIMEDOUT later.

Testing and tracing showed that the buggy scenario is the following
somewhat tricky sequence:

+ Client attempts a TFO handshake. tcp_send_syn_data() sends SYN + TFO
  cookie + data in a single packet in the syn_data skb. It hands the
  syn_data skb to tcp_transmit_skb(), which makes a clone. Crucially,
  it then reuses the same original (non-clone) syn_data skb,
  transforming it by advancing the seq by one byte and removing the
  FIN bit, and enques the resulting payload-only skb in the
  sk->tcp_rtx_queue.

+ Client sets retrans_stamp to the start time of the three-way
  handshake.

+ Cookie mismatches or server has TFO disabled, and server only ACKs
  SYN.

+ tcp_ack() sees SYN is acked, tcp_clean_rtx_queue() clears
  retrans_stamp.

+ Since the client SYN was acked but not the payload, the TFO failure
  code path in tcp_rcv_fastopen_synack() tries to retransmit the
  payload skb.  However, in some cases the transmit completion for the
  clone of the syn_data (which had SYN + TFO cookie + data) hasn't
  happened.  In those cases, skb_still_in_host_queue() returns true
  for the retransmitted TFO payload, because the clone of the syn_data
  skb has not had its tx completetion.

+ Because skb_still_in_host_queue() finds skb_fclone_busy() is true,
  it sets the TSQ_THROTTLED bit and the retransmit does not happen in
  the tcp_rcv_fastopen_synack() call chain.

+ The tcp_rcv_fastopen_synack() code next implicitly assumes the
  retransmit process is finished, and sets retrans_stamp to 0 to clear
  it, but this is later overwritten (see below).

+ Later, upon tx completion, tcp_tsq_write() calls
  tcp_xmit_retransmit_queue(), which puts the retransmit in flight and
  sets retrans_stamp to a non-zero value.

+ The client receives an ACK for the retransmitted TFO payload data.

+ Since we're in CA_Open and there are no dupacks/SACKs/DSACKs/ECN to
  make tcp_ack_is_dubious() true and make us call
  tcp_fastretrans_alert() and reach a code path that clears
  retrans_stamp, retrans_stamp stays nonzero.

+ Later, if there is a TLP, RTO, RTO sequence, then the connection
  will suffer an early ETIMEDOUT due to the erroneously ancient
  retrans_stamp.

The fix: this commit refactors the code to have
tcp_rcv_fastopen_synack() retransmit by reusing the relevant parts of
tcp_simple_retransmit() that enter CA_Loss (without changing cwnd) and
call tcp_xmit_retransmit_queue(). We have tcp_simple_retransmit() and
tcp_rcv_fastopen_synack() share code in this way because in both cases
we get a packet indicating non-congestion loss (MTU reduction or TFO
failure) and thus in both cases we want to retransmit as many packets
as cwnd allows, without reducing cwnd. And given that retransmits will
set retrans_stamp to a non-zero value (and may do so in a later
calling context due to TSQ), we also want to enter CA_Loss so that we
track when all retransmitted packets are ACked and clear retrans_stamp
when that happens (to ensure later recurring RTOs are using the
correct retrans_stamp and don't declare ETIMEDOUT prematurely).

Fixes: 9e046bb111 ("tcp: clear tp->retrans_stamp in tcp_rcv_fastopen_synack()")
Fixes: a7abf3cd76 ("tcp: consider using standard rtx logic in tcp_rcv_fastopen_synack()")
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Link: https://patch.msgid.link/20240624144323.2371403-1-ncardwell.sw@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
2024-06-25 17:22:49 -07:00
arch - fix for BCM6538 boards 2024-06-19 10:19:41 -07:00
block block: unmap and free user mapped integrity via submitter 2024-06-12 11:00:50 -06:00
certs kbuild: use $(src) instead of $(srctree)/$(src) for source directory 2024-05-10 04:34:52 +09:00
crypto This push fixes a bug in the new ecc P521 code as well as a buggy 2024-05-20 08:47:54 -07:00
Documentation dt-bindings: net: fman: remove ptp-timer from required list 2024-06-23 12:51:59 +01:00
drivers ionic: use dev_consume_skb_any outside of napi 2024-06-25 16:44:08 -07:00
fs 19 hotfixes, 8 of which are cc:stable. 2024-06-17 12:30:07 -07:00
include Fix race for duplicate reqsk on identical SYN 2024-06-25 11:37:45 +02:00
init gcc: disable '-Warray-bounds' for gcc-9 2024-06-15 10:43:04 -07:00
io_uring io_uring: fix cancellation overwriting req->flags 2024-06-13 19:25:28 -06:00
ipc Mainly singleton patches, documented in their respective changelogs. 2024-05-19 14:02:03 -07:00
kernel bpf-for-netdev 2024-06-24 18:15:22 -07:00
lib cpumask: limit FORCE_NR_CPUS to just the UP case 2024-06-18 09:00:04 -07:00
LICENSES LICENSES: Add the copyleft-next-0.3.1 license 2022-11-08 15:44:01 +01:00
mm 19 hotfixes, 8 of which are cc:stable. 2024-06-17 12:30:07 -07:00
net tcp: fix tcp_rcv_fastopen_synack() to enter TCP_CA_Loss for failed TFO 2024-06-25 17:22:49 -07:00
rust LoongArch changes for v6.10 2024-05-22 09:43:07 -07:00
samples tracing/treewide: Remove second parameter of __assign_str() 2024-05-22 20:14:47 -04:00
scripts Kbuild fixes for v6.10 (second) 2024-06-08 10:12:33 -07:00
security lsm/stable-6.10 PR 20240617 2024-06-17 18:35:12 -07:00
sound ALSA: hda: cs35l56: Select SERIAL_MULTI_INSTANTIATE 2024-06-20 09:58:25 +02:00
tools bpf-for-netdev 2024-06-24 18:15:22 -07:00
usr kbuild: use $(src) instead of $(srctree)/$(src) for source directory 2024-05-10 04:34:52 +09:00
virt The usual shower of singleton fixes and minor series all over MM, 2024-05-19 09:21:03 -07:00
.clang-format clang-format: Update with v6.7-rc4's for_each macro list 2023-12-08 23:54:38 +01:00
.cocciconfig
.editorconfig .editorconfig: remove trim_trailing_whitespace option 2024-06-13 16:47:52 +02:00
.get_maintainer.ignore Add Jeff Kirsher to .get_maintainer.ignore 2024-03-08 11:36:54 +00:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore kbuild: create a list of all built DTB files 2024-02-19 18:20:39 +09:00
.mailmap bpf-for-netdev 2024-06-14 17:57:10 -07:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Drop Gustavo Pimentel as PCI DWC Maintainer 2024-03-27 13:41:02 -05:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS bpf-for-netdev 2024-06-24 18:15:22 -07:00
Makefile Linux 6.10-rc4 2024-06-16 13:40:16 -07:00
README README: Fix spelling 2024-03-18 03:36:32 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.