[PATCH 5.9 14/46] mptcp: fix NULL ptr dereference on bad MPJ

[Greg Kroah-Hartman]

From: Paolo Abeni <pabeni@xxxxxxxxxx>

[ Upstream commit d3ab78858f1451351221061a1c365495df196500 ]

If an msk listener receives an MPJ carrying an invalid token, it
will zero the request socket msk entry. That should later
cause fallback and subflow reset - as per RFC - at
subflow_syn_recv_sock() time due to failing hmac validation.

Since commit 4cf8b7e48a09 ("subflow: introduce and use
mptcp_can_accept_new_subflow()"), we unconditionally dereference
- in mptcp_can_accept_new_subflow - the subflow request msk
before performing hmac validation. In the above scenario we
hit a NULL ptr dereference.

Address the issue doing the hmac validation earlier.

Fixes: 4cf8b7e48a09 ("subflow: introduce and use mptcp_can_accept_new_subflow()")
Tested-by: Davide Caratti <dcaratti@xxxxxxxxxx>
Signed-off-by: Paolo Abeni <pabeni@xxxxxxxxxx>
Reviewed-by: Matthieu Baerts <matthieu.baerts@xxxxxxxxxxxx>
Link: https://lore.kernel.org/r/03b2cfa3ac80d8fc18272edc6442a9ddf0b1e34e.1606400227.git.pabeni@xxxxxxxxxx
Signed-off-by: Jakub Kicinski <kuba@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 net/mptcp/subflow.c |    5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -542,9 +542,8 @@ create_msk:
 			fallback = true;
 	} else if (subflow_req->mp_join) {
 		mptcp_get_options(skb, &mp_opt);
-		if (!mp_opt.mp_join ||
-		    !mptcp_can_accept_new_subflow(subflow_req->msk) ||
-		    !subflow_hmac_valid(req, &mp_opt)) {
+		if (!mp_opt.mp_join || !subflow_hmac_valid(req, &mp_opt) ||
+		    !mptcp_can_accept_new_subflow(subflow_req->msk)) {
 			SUBFLOW_REQ_INC_STATS(req, MPTCP_MIB_JOINACKMAC);
 			fallback = true;
 		}