[PATCH net v2 3/5] mptcp: avoid a WARN on bad input.

[Paolo Abeni]

Syzcaller has found a way to trigger the WARN_ON_ONCE condition
in check_fully_established().

The root cause is a legit fallback to TCP scenario, so replace
the WARN with a plain message on a more strict condition.

Fixes: f296234c98a8 ("mptcp: Add handling of incoming MP_JOIN requests")
Signed-off-by: Paolo Abeni <pab...@redhat.com>
---
 net/mptcp/options.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index eadbd59586e4..ecf41d52d2fc 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -703,8 +703,6 @@ static bool check_fully_established(struct mptcp_sock *msk, 
struct sock *sk,
                goto fully_established;
        }
 
-       WARN_ON_ONCE(subflow->can_ack);
-
        /* If the first established packet does not contain MP_CAPABLE + data
         * then fallback to TCP
         */
@@ -714,6 +712,8 @@ static bool check_fully_established(struct mptcp_sock *msk, 
struct sock *sk,
                return false;
        }
 
+       if (unlikely(!READ_ONCE(msk->pm.server_side)))
+               pr_warn_once("bogus mpc option on established client sk");
        subflow->fully_established = 1;
        subflow->remote_key = mp_opt->sndr_key;
        subflow->can_ack = 1;
-- 
2.21.1