rxrpc: Fix NULL pointer deref due to call->conn being cleared on disconnect

When a call is disconnected, the connection pointer from the call is
cleared to make sure it isn't used again and to prevent further attempted
transmission for the call.  Unfortunately, there might be a daemon trying
to use it at the same time to transmit a packet.

Fix this by keeping call->conn set, but setting a flag on the call to
indicate disconnection instead.

Remove also the bits in the transmission functions where the conn pointer is
checked and a ref taken under spinlock as this is now redundant.

Fixes: 8d94aa38 ("rxrpc: Calls shouldn't hold socket refs")
Signed-off-by: David Howells <dhowells@redhat.com>

net/rxrpc/ar-internal.h

@@ -490,6 +490,7 @@ enum rxrpc_call_flag {
 	RXRPC_CALL_RX_HEARD,		/* The peer responded at least once to this call */
 	RXRPC_CALL_RX_UNDERRUN,		/* Got data underrun */
 	RXRPC_CALL_IS_INTR,		/* The call is interruptible */
+	RXRPC_CALL_DISCONNECTED,	/* The call has been disconnected */
 };
 
 /*

net/rxrpc/call_object.c

@@ -493,7 +493,7 @@ void rxrpc_release_call(struct rxrpc_sock *rx, struct rxrpc_call *call)
 
 	_debug("RELEASE CALL %p (%d CONN %p)", call, call->debug_id, conn);
 
-	if (conn)
+	if (conn && !test_bit(RXRPC_CALL_DISCONNECTED, &call->flags))
 		rxrpc_disconnect_call(call);
 	if (call->security)
 		call->security->free_call_crypto(call);
@@ -569,6 +569,7 @@ static void rxrpc_rcu_destroy_call(struct rcu_head *rcu)
 	struct rxrpc_call *call = container_of(rcu, struct rxrpc_call, rcu);
 	struct rxrpc_net *rxnet = call->rxnet;
 
+	rxrpc_put_connection(call->conn);
 	rxrpc_put_peer(call->peer);
 	kfree(call->rxtx_buffer);
 	kfree(call->rxtx_annotations);
@@ -590,7 +591,6 @@ void rxrpc_cleanup_call(struct rxrpc_call *call)
 
 	ASSERTCMP(call->state, ==, RXRPC_CALL_COMPLETE);
 	ASSERT(test_bit(RXRPC_CALL_RELEASED, &call->flags));
-	ASSERTCMP(call->conn, ==, NULL);
 
 	rxrpc_cleanup_ring(call);
 	rxrpc_free_skb(call->tx_pending, rxrpc_skb_cleaned);

net/rxrpc/conn_client.c

@@ -785,6 +785,7 @@ void rxrpc_disconnect_client_call(struct rxrpc_call *call)
 	u32 cid;
 
 	spin_lock(&conn->channel_lock);
+	set_bit(RXRPC_CALL_DISCONNECTED, &call->flags);
 
 	cid = call->cid;
 	if (cid) {
@@ -792,7 +793,6 @@ void rxrpc_disconnect_client_call(struct rxrpc_call *call)
 		chan = &conn->channels[channel];
 	}
 	trace_rxrpc_client(conn, channel, rxrpc_client_chan_disconnect);
-	call->conn = NULL;
 
 	/* Calls that have never actually been assigned a channel can simply be
 	 * discarded.  If the conn didn't get used either, it will follow
@@ -908,7 +908,6 @@ void rxrpc_disconnect_client_call(struct rxrpc_call *call)
 	spin_unlock(&rxnet->client_conn_cache_lock);
 out_2:
 	spin_unlock(&conn->channel_lock);
-	rxrpc_put_connection(conn);
 	_leave("");
 	return;
 

net/rxrpc/conn_object.c

@@ -171,6 +171,8 @@ void __rxrpc_disconnect_call(struct rxrpc_connection *conn,
 
 	_enter("%d,%x", conn->debug_id, call->cid);
 
+	set_bit(RXRPC_CALL_DISCONNECTED, &call->flags);
+
 	if (rcu_access_pointer(chan->call) == call) {
 		/* Save the result of the call so that we can repeat it if necessary
 		 * through the channel, whilst disposing of the actual call record.
@@ -223,9 +225,7 @@ void rxrpc_disconnect_call(struct rxrpc_call *call)
 	__rxrpc_disconnect_call(conn, call);
 	spin_unlock(&conn->channel_lock);
 
-	call->conn = NULL;
 	conn->idle_timestamp = jiffies;
-	rxrpc_put_connection(conn);
 }
 
 /*

net/rxrpc/output.c

@@ -129,7 +129,7 @@ static size_t rxrpc_fill_out_ack(struct rxrpc_connection *conn,
 int rxrpc_send_ack_packet(struct rxrpc_call *call, bool ping,
 			  rxrpc_serial_t *_serial)
 {
-	struct rxrpc_connection *conn = NULL;
+	struct rxrpc_connection *conn;
 	struct rxrpc_ack_buffer *pkt;
 	struct msghdr msg;
 	struct kvec iov[2];
@@ -139,18 +139,14 @@ int rxrpc_send_ack_packet(struct rxrpc_call *call, bool ping,
 	int ret;
 	u8 reason;
 
-	spin_lock_bh(&call->lock);
-	if (call->conn)
-		conn = rxrpc_get_connection_maybe(call->conn);
-	spin_unlock_bh(&call->lock);
-	if (!conn)
+	if (test_bit(RXRPC_CALL_DISCONNECTED, &call->flags))
 		return -ECONNRESET;
 
 	pkt = kzalloc(sizeof(*pkt), GFP_KERNEL);
-	if (!pkt) {
-		rxrpc_put_connection(conn);
+	if (!pkt)
 		return -ENOMEM;
-	}
+
+	conn = call->conn;
 
 	msg.msg_name	= &call->peer->srx.transport;
 	msg.msg_namelen	= call->peer->srx.transport_len;
@@ -244,7 +240,6 @@ int rxrpc_send_ack_packet(struct rxrpc_call *call, bool ping,
 	}
 
 out:
-	rxrpc_put_connection(conn);
 	kfree(pkt);
 	return ret;
 }
@@ -254,7 +249,7 @@ int rxrpc_send_ack_packet(struct rxrpc_call *call, bool ping,
  */
 int rxrpc_send_abort_packet(struct rxrpc_call *call)
 {
-	struct rxrpc_connection *conn = NULL;
+	struct rxrpc_connection *conn;
 	struct rxrpc_abort_buffer pkt;
 	struct msghdr msg;
 	struct kvec iov[1];
@@ -271,13 +266,11 @@ int rxrpc_send_abort_packet(struct rxrpc_call *call)
 	    test_bit(RXRPC_CALL_TX_LAST, &call->flags))
 		return 0;
 
-	spin_lock_bh(&call->lock);
-	if (call->conn)
-		conn = rxrpc_get_connection_maybe(call->conn);
-	spin_unlock_bh(&call->lock);
-	if (!conn)
+	if (test_bit(RXRPC_CALL_DISCONNECTED, &call->flags))
 		return -ECONNRESET;
 
+	conn = call->conn;
+
 	msg.msg_name	= &call->peer->srx.transport;
 	msg.msg_namelen	= call->peer->srx.transport_len;
 	msg.msg_control	= NULL;
@@ -312,8 +305,6 @@ int rxrpc_send_abort_packet(struct rxrpc_call *call)
 		trace_rxrpc_tx_packet(call->debug_id, &pkt.whdr,
 				      rxrpc_tx_point_call_abort);
 	rxrpc_tx_backoff(call, ret);
-
-	rxrpc_put_connection(conn);
 	return ret;
 }