diff mbox series

[mptcp-next,3/3] mptcp: blackhole only if 1st SYN retrans w/o MPC is accepted

Message ID 20250114-mpc-no-blackhole-v1-3-994bd2a357fb@kernel.org (mailing list archive)
State New
Headers show
Series mptcp: blackhole: sysctl SYN retrans + fix conditions | expand

Checks

Context Check Description
matttbe/build success Build and static analysis OK
matttbe/checkpatch success total: 0 errors, 0 warnings, 0 checks, 11 lines checked
matttbe/shellcheck success MPTCP selftests files have not been modified
matttbe/KVM_Validation__normal success Success! ✅
matttbe/KVM_Validation__debug success Success! ✅
matttbe/KVM_Validation__btf-normal__only_bpftest_all_ success Success! ✅
matttbe/KVM_Validation__btf-debug__only_bpftest_all_ warning Unstable: 1 failed test(s): bpftest_test_progs_mptcp

Commit Message

Matthieu Baerts Jan. 14, 2025, 5:37 p.m. UTC 
The Fixes commit mentioned this:

> An MPTCP firewall blackhole can be detected if the following SYN
> retransmission after a fallback to "plain" TCP is accepted.

But in fact, this blackhole was detected if any following SYN
retransmissions after a fallback to TCP was accepted.

That's because 'mptcp_subflow_early_fallback()' will set 'request_mptcp'
to 0, and 'mpc_drop' will never be reset to 0 after.

This is an issue, because some not so unusual situations might cause the
kernel to detect a false-positive blackhole, e.g. a client trying to
connect to a server while the network is not ready yet, causing a few
SYN retransmissions, before reaching the end server.

Fixes: 27069e7cb3d1 ("mptcp: disable active MPTCP in case of blackhole")
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 net/mptcp/ctrl.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
diff mbox series

Patch

diff --git a/net/mptcp/ctrl.c b/net/mptcp/ctrl.c
index 3999e0ba2c35b50c36ce32277e0b8bfb24197946..2dd81e6c26bdb5220abed68e26d70d2dc3ab14fb 100644
--- a/net/mptcp/ctrl.c
+++ b/net/mptcp/ctrl.c
@@ -418,9 +418,9 @@  void mptcp_active_detect_blackhole(struct sock *ssk, bool expired)
 			MPTCP_INC_STATS(net, MPTCP_MIB_MPCAPABLEACTIVEDROP);
 			subflow->mpc_drop = 1;
 			mptcp_subflow_early_fallback(mptcp_sk(subflow->conn), subflow);
-		} else {
-			subflow->mpc_drop = 0;
 		}
+	} else if (ssk->sk_state == TCP_SYN_SENT) {
+		subflow->mpc_drop = 0;
 	}
 }