mptcp: reset fallback status gracefully at disconnect() time

mptcp_disconnect() clears the fallback bit unconditionally, without
touching the associated flags.

The bit clear is safe, as no fallback operation can race with that --
all subflow are already in TCP_CLOSE status thanks to the previous
FASTCLOSE -- but we need to consistently reset all the fallback related
status.

Also acquire the relevant lock, to avoid fouling static analyzers.

Fixes: b29fcfb54cd7 ("mptcp: full disconnect implementation")
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250714-net-mptcp-fallback-races-v1-3-391aff963322@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index bf92cee9b5cee39e2b0831b6f7e06ce013fb6913..6a817a13b1549c3397e8fa2e315448ce2770e195 100644 (file)
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -3142,7 +3142,16 @@ static int mptcp_disconnect(struct sock *sk, int flags)
         * subflow
         */
        mptcp_destroy_common(msk, MPTCP_CF_FASTCLOSE);
+
+       /* The first subflow is already in TCP_CLOSE status, the following
+        * can't overlap with a fallback anymore
+        */
+       spin_lock_bh(&msk->fallback_lock);
+       msk->allow_subflows = true;
+       msk->allow_infinite_fallback = true;
        WRITE_ONCE(msk->flags, 0);
+       spin_unlock_bh(&msk->fallback_lock);
+
        msk->cb_flags = 0;
        msk->recovery = false;
        WRITE_ONCE(msk->can_ack, false);