Subject: [PATCH AUTOSEL 5.13 146/219] tcp: enable data-less, empty-cookie SYN with TFO_SERVER_COOKIE_NOT_REQD

From: Sasha Levin <[email-masked]>
Date: Thu, 9 Sep 2021 07:45:22 -0400

From: Luke Hsiao <[email-masked]>

[ Upstream commit e3faa49bcecdfcc80e94dd75709d6acb1a5d89f6 ]

Since the original TFO server code was implemented in commit
168a8f58059a22feb9e9a2dcc1b8053dbbbc12ef ("tcp: TCP Fast Open Server -
main code path") the TFO server code has supported the sysctl bit flag
TFO_SERVER_COOKIE_NOT_REQD. Currently, when the TFO_SERVER_ENABLE and
TFO_SERVER_COOKIE_NOT_REQD sysctl bit flags are set, a server connection
will accept a SYN with N bytes of data (N > 0) that has no TFO cookie,
create a new fast open connection, process the incoming data in the SYN,
and make the connection ready for accepting. After accepting, the
connection is ready for read()/recvmsg() to read the N bytes of data in
the SYN, ready for write()/sendmsg() calls and data transmissions to
transmit data.

This commit changes an edge case in this feature by changing this
behavior to apply to (N >= 0) bytes of data in the SYN rather than only
(N > 0) bytes of data in the SYN. Now, a server will accept a data-less
SYN without a TFO cookie if TFO_SERVER_COOKIE_NOT_REQD is set.

Caveat! While this enables a new kind of TFO (data-less empty-cookie
SYN), some firewall rules setup may not work if they assume such packets
are not legit TFOs and will filter them.

Signed-off-by: Luke Hsiao <[email-masked]>
Acked-by: Neal Cardwell <[email-masked]>
Acked-by: Yuchung Cheng <[email-masked]>
Signed-off-by: Eric Dumazet <[email-masked]>
Link: https://lore.kernel.org/r/[email-masked]
Signed-off-by: Jakub Kicinski <[email-masked]>
Signed-off-by: Sasha Levin <[email-masked]>
---
 net/ipv4/tcp_fastopen.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_fastopen.c b/net/ipv4/tcp_fastopen.c
index d49709ba8e16..107111984384 100644
--- a/net/ipv4/tcp_fastopen.c
+++ b/net/ipv4/tcp_fastopen.c
@@ -379,8 +379,7 @@ struct sock *tcp_try_fastopen(struct sock *sk, struct sk_buff *skb,
 		return NULL;
 	}
 
-	if (syn_data &&
-	    tcp_fastopen_no_cookie(sk, dst, TFO_SERVER_COOKIE_NOT_REQD))
+	if (tcp_fastopen_no_cookie(sk, dst, TFO_SERVER_COOKIE_NOT_REQD))
 		goto fastopen;
 
 	if (foc->len == 0) {
-- 
2.30.2