[PATCH net] net: x25: Fix kernel crashes due to x25_disconnect releasing x25_neigh

[Xie He <xie.he.0141@gmail.com>]

The x25_disconnect function in x25_subr.c would decrease the refcount of
"x25->neighbour" (struct x25_neigh) and reset this pointer to NULL.

However:

1) When we receive a connection, the x25_rx_call_request function in
af_x25.c does not increase the refcount when it assigns the pointer.
When we disconnect, x25_disconnect is called and the struct's refcount
is decreased without being increased in the first place.

This causes frequent kernel crashes when using AF_X25 sockets.

2) When we initiate a connection but the connection is refused by the
remote side, x25_disconnect is called which decreases the refcount and
resets the pointer to NULL. But the x25_connect function in af_x25.c,
which is waiting for the connection to be established, notices the
failure and then tries to decrease the refcount again, resulting in a
NULL-pointer-dereference error.

This crashes the kernel every time a connection is refused by the remote
side.

Fixes: 4becb7ee5b3d ("net/x25: Fix x25_neigh refcnt leak when x25 disconnect")
Cc: Martin Schiller <ms@dev.tdt.de>
Signed-off-by: Xie He <xie.he.0141@gmail.com>
---
 net/x25/af_x25.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/net/x25/af_x25.c b/net/x25/af_x25.c
index 0bbb283f23c9..8e59f9ecbeab 100644
--- a/net/x25/af_x25.c
+++ b/net/x25/af_x25.c
@@ -826,10 +826,12 @@ static int x25_connect(struct socket *sock, struct sockaddr *uaddr,
 	rc = 0;
 out_put_neigh:
 	if (rc) {
-		read_lock_bh(&x25_list_lock);
-		x25_neigh_put(x25->neighbour);
-		x25->neighbour = NULL;
-		read_unlock_bh(&x25_list_lock);
+		if (x25->neighbour) {
+			read_lock_bh(&x25_list_lock);
+			x25_neigh_put(x25->neighbour);
+			x25->neighbour = NULL;
+			read_unlock_bh(&x25_list_lock);
+		}
 		x25->state = X25_STATE_0;
 	}
 out_put_route:
@@ -1050,6 +1052,7 @@ int x25_rx_call_request(struct sk_buff *skb, struct x25_neigh *nb,
 	makex25->lci           = lci;
 	makex25->dest_addr     = dest_addr;
 	makex25->source_addr   = source_addr;
+	x25_neigh_hold(nb);
 	makex25->neighbour     = nb;
 	makex25->facilities    = facilities;
 	makex25->dte_facilities= dte_facilities;
-- 
2.27.0